Level 3 · Science S3.5

Particles

Everything is made of tiny particles, and how they move explains a great deal.

Picture the particles in a solid, a liquid and a gas. Use them to explain smells, melting, pressure, rails that grow in the heat, dissolving and density.

You are ready for this book if

You can sort ten things at home into solids, liquids and gases.

Written and drawn by attnom. Free to read, print and share.

S3.5.1 Chapter 1

The particle model

Water pours and air spreads, but a stone stays put. What is different inside them?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

A stone keeps its shape. Water takes the shape of its cup. Air fills a whole room. Now we look inside them, at things far too small to see.

A small hatched square labelled grain of salt, with an arrow labelled zoom in pointing to a box of circles in neat rows, labelled its particles. Labels: grain of salt, zoom in, its particles.grain of saltzoom inits particles
A grain of salt, and the particles inside it. They are far too small to see.

Everything is made of tiny bits called particles. A grain of salt holds more of them than there are people on Earth.

The particle model is a way of picturing them. We draw each particle as a small circle. The model explains how solids, liquids and gases behave.

Draw it first

Draw the particles in three boxes

  1. Three boxes of circles, side by side. In the box labelled solid, the circles sit in neat rows and all touch. In the box labelled liquid, the circles touch but are jumbled, and they fill only the bottom, under a flat top. In the closed box labelled gas, a few circles are far apart, all through the box, each with short lines behind it. Labels: solid, liquid, gas.solid

    1 Draw a box. Fill it with circles in neat rows, all touching. Label it solid.

  2. Three boxes of circles, side by side. In the box labelled solid, the circles sit in neat rows and all touch. In the box labelled liquid, the circles touch but are jumbled, and they fill only the bottom, under a flat top. In the closed box labelled gas, a few circles are far apart, all through the box, each with short lines behind it. Labels: solid, liquid, gas.solidliquid

    2 Draw a second box with an open top. Fill the bottom with circles that touch but are jumbled. Add a flat top and label it liquid.

  3. Three boxes of circles, side by side. In the box labelled solid, the circles sit in neat rows and all touch. In the box labelled liquid, the circles touch but are jumbled, and they fill only the bottom, under a flat top. In the closed box labelled gas, a few circles are far apart, all through the box, each with short lines behind it. Labels: solid, liquid, gas.solidliquidgas

    3 Draw a third box with a lid. Put a few circles in it, far apart. Add short lines behind them. Label it gas.

In a solid, the particles touch and pull on each other. Each one is held in its place. It cannot move around, but it can vibrate. To vibrate is to shake to and fro in a tiny space.

A row of five circles that all touch, the middle one with two short arrows pointing left and right from above it, and the label vibrates in place under the row. Labels: vibrates in place.vibrates in place
A particle in a solid shakes to and fro. Its neighbours hold it in place.

In a liquid, the particles still touch and still pull on each other. But they are not held in place. They slide past each other, so a liquid can flow.

In a gas, the particles are far apart. They hardly pull on each other. They fly about in every direction. Between them there is nothing at all, only empty space.

The particles are the same in every state. A particle of water is the same in ice, in water and in steam. Only the way the particles are held, and the way they move, is different.

Three boxes of circles in a row, labelled ice, water and steam. The circles in ice touch in neat rows. The circles in water touch and are jumbled at the bottom. The circles in steam are far apart. Labels: ice, water, steam.icewatersteam
The same particles of water in three states.

Now we can explain what you see. A solid keeps its shape because its particles are held in place. A liquid pours because its particles slide past each other.

A gas spreads out because its particles fly about and nothing holds them. They keep going until they fill the whole container.

See it three ways

With things

Think of people. In a packed queue, each person keeps a spot and only shuffles. In a busy market, people slide past each other. In a big empty field, a few people walk in any direction.

As a picture

A table with the columns state, shape and volume. Solid keeps its shape and keeps its volume. Liquid changes its shape and keeps its volume. Gas changes its shape and changes its volume.stateshapevolumesolidkeepskeepsliquidchangeskeepsgaschangeschanges

In words

In a solid, the particles touch and vibrate in place. In a liquid, they touch and slide. In a gas, they are far apart and move freely.

Try it

Be the particles with beans

You need: a tray or a shallow box, about 30 dried beans or small stones.

  1. Pack the beans into the tray so that they all touch. Shake the tray very gently. The beans only wobble.
  2. Tip the beans into a loose heap at one end. Shake the tray a little more. The beans slide past each other.
  3. Take out all but 5 beans. Shake the tray hard. The beans scatter and fly about.

What you will see: Packed beans wobble in place, like a solid. Loose beans slide, like a liquid. A few beans fly about and fill the tray, like a gas.

If you cannot try it, this is what happens.
Three boxes of circles, labelled packed, loose and few. The circles in packed sit in rows and touch. The circles in loose touch and are jumbled at the bottom. The few circles in the box labelled few are far apart. Labels: packed, loose, few.packedloosefew

A wrong idea many people have

In a solid, the particles do not move at all.

Why it feels right: A stone sits still and keeps its shape.

The test that breaks it: Think of a steel rail in the sun. It gets a little longer as it warms. If its particles stood still, warming could not change its length. The shaking is too small to see or feel, but we feel it as warmth.

What is true: The particles in a solid are held in place, but they vibrate all the time. They never stand still.

Key words

particles
the tiny bits that everything is made of.
particle model
a picture that shows particles as circles and explains how matter behaves.
vibrate
shake to and fro in a tiny space.
flow
move along because the particles slide past each other.

Back to your guess

The answer is C: Inside a stone the bits are held in place. In water they slide. In air they fly about.

Every material is made of tiny particles. In a stone they are held in place. In water they slide past each other. In air they fly about.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. In which state are the particles far apart and flying about in every direction?

    Tap your answer.

    1. Not this one. In a solid the particles touch and are held in place. Go to S3.5.1. Try another.

    2. Yes, that is right. The particles of a gas are far apart. Almost nothing holds them, so they fly about and fill the container.

    3. Not this one. In a liquid the particles touch. They slide past each other, but they stay close. Go to S3.5.1. Try another.

    See the answer

    Gas

    The particles of a gas are far apart. Almost nothing holds them, so they fly about and fill the container.

    • If you chose A: In a solid the particles touch and are held in place. Go to S3.5.1.
    • If you chose C: In a liquid the particles touch. They slide past each other, but they stay close. Go to S3.5.1.

  2. You can pour water but not a stone. Which reason is right?

    Tap your answer.

    1. Not this one. The particles are the same in every state. Their size does not explain pouring. How they are held does. Go to S3.5.1. Try another.

    2. Not this one. In a liquid the particles do touch. They are free to slide, and that lets it pour. Go to S3.5.1. Try another.

    3. Yes, that is right. Sliding particles let a liquid flow. Particles held in place keep a solid in its shape.

    See the answer

    The particles of water slide past each other. The particles of a stone are held in place.

    Sliding particles let a liquid flow. Particles held in place keep a solid in its shape.

    • If you chose A: The particles are the same in every state. Their size does not explain pouring. How they are held does. Go to S3.5.1.
    • If you chose B: In a liquid the particles do touch. They are free to slide, and that lets it pour. Go to S3.5.1.

  3. Gas fills a closed jar. What is there between its particles?

    Tap your answer.

    1. Not this one. The particles are the gas itself. Between them there is empty space. Go to S3.5.1. Try another.

    2. Yes, that is right. The particles of a gas are far apart. The space between them has nothing in it.

    3. Not this one. A gas is not a liquid. Its particles are far apart with nothing between them. Go to S3.5.1. Try another.

    See the answer

    Nothing at all. It is empty space.

    The particles of a gas are far apart. The space between them has nothing in it.

    • If you chose A: The particles are the gas itself. Between them there is empty space. Go to S3.5.1.
    • If you chose C: A gas is not a liquid. Its particles are far apart with nothing between them. Go to S3.5.1.

  4. Tariq says, "The particles in a stone do not move." What should you tell him?

    Tap your answer.

    1. Yes, that is right. Held in place does not mean still. Each particle shakes to and fro where it is.

    2. Not this one. The particles of a solid vibrate. You feel this as warmth, and it grows when the solid is heated. Go to S3.5.1. Try another.

    3. Not this one. Gas particles fly about freely. Solid particles only vibrate in place. Go to S3.5.1. Try another.

    See the answer

    They vibrate in place. They cannot move around, but they never stand still.

    Held in place does not mean still. Each particle shakes to and fro where it is.

    • If you chose B: The particles of a solid vibrate. You feel this as warmth, and it grows when the solid is heated. Go to S3.5.1.
    • If you chose C: Gas particles fly about freely. Solid particles only vibrate in place. Go to S3.5.1.

  5. You draw a box of gas with 6 circles. Which drawing is right?

    Tap your answer.

    1. Yes, that is right. The particles of a gas are far apart and fill the box. The empty space between them stays empty.

    2. Not this one. That is how a solid is drawn. The particles of a gas are far apart. Go to S3.5.1. Try another.

    3. Not this one. That is how a liquid is drawn. A gas spreads out to fill the whole box. Go to S3.5.1. Try another.

    See the answer

    Six circles far apart, spread through the whole box.

    The particles of a gas are far apart and fill the box. The empty space between them stays empty.

    • If you chose B: That is how a solid is drawn. The particles of a gas are far apart. Go to S3.5.1.
    • If you chose C: That is how a liquid is drawn. A gas spreads out to fill the whole box. Go to S3.5.1.

Keep it

Copy this into your exercise book.

All things are made of tiny particles. In a solid they touch and vibrate in place. In a liquid they touch and slide past each other. In a gas they are far apart and fly about.

Draw: Draw three boxes. Put neat rows of circles in the first. Put a jumbled pool in the second. Put a few circles far apart in the third.

S3.5.2 Chapter 2

Evidence for particles

How can I smell food from across the room when I cannot see anything travel?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Nobody has ever seen a single particle with their own eyes. So how do we know that particles are there?

We use evidence. Evidence is what we see or measure that supports an idea. Scientists use the particle model because it explains a great deal of evidence.

Here is the first piece. Someone peels an orange at one end of a room. Soon people at the other end can smell it. Nothing you can see has crossed the room.

Draw it first

Draw a smell spreading through air

  1. Three closed boxes of gas, side by side, with a key below. Each box has 6 open circles for air and 3 dark dots for smell. In the box labelled start, the dark dots are bunched in one corner. In the box labelled soon, some dark dots have moved away from the corner. In the box labelled later, the dark dots are mixed through the whole box. Labels: start, soon, later, air, smell.startairsmell

    1 Draw a box of air with open circles. Put the smell, dark dots, in one corner.

  2. Three closed boxes of gas, side by side, with a key below. Each box has 6 open circles for air and 3 dark dots for smell. In the box labelled start, the dark dots are bunched in one corner. In the box labelled soon, some dark dots have moved away from the corner. In the box labelled later, the dark dots are mixed through the whole box. Labels: start, soon, later, air, smell.startsoonairsmell

    2 Draw the same box a little later. Some dark dots have moved away from the corner.

  3. Three closed boxes of gas, side by side, with a key below. Each box has 6 open circles for air and 3 dark dots for smell. In the box labelled start, the dark dots are bunched in one corner. In the box labelled soon, some dark dots have moved away from the corner. In the box labelled later, the dark dots are mixed through the whole box. Labels: start, soon, later, air, smell.startsoonlaterairsmell

    3 Draw it later still. The dark dots are mixed through the whole box.

The orange gives off particles of a gas. They mix with the air, which is a gas made of particles too. The smell particles fly about and keep bumping into air particles. Each bump sends them a new way, so they zigzag.

A closed box with open circles for air and one dark dot for a smell particle on the left. A line goes from the dark dot in straight stretches. It bends each time the dot touches an air particle, and it ends with an arrow. Labels: one smell particle.one smell particle
A smell particle flies straight until it hits an air particle. Then it goes a new way.

Slowly, the smell particles spread through the whole room. This is diffusion. In diffusion, particles spread out by themselves, from where there are many to where there are few.

In a real room, slow air currents carry most of the smell. Diffusion alone is slower, but it never stops, even in still air. Leave a drop of food colour in a glass of water. With no stirring, the colour still spreads, though it takes an hour or more.

Here is the second piece. Stir a spoon of sugar into water. Soon you cannot see it. Did the sugar vanish?

No. The water particles pull the sugar apart into separate particles. They are far too small to see. They spread out among the water particles, so the water stays clear. We say the sugar dissolves.

Three open cups of water, side by side, with a key below. Each cup has 20 open circles for water and 6 dark dots for sugar, and the circles all touch. In the cup labelled at first, the dark dots are bunched in the bottom left corner. In the cup labelled soon, some dark dots have moved away. In the cup labelled later, the dark dots are mixed through the water. Labels: at first, soon, later, water, sugar.at firstsoonlaterwatersugar
Sugar in water. The sugar particles are still there. They are spread through the water.

The sugar did not vanish. It is mixed in. Salt does the same. Leave salty water in a dish in the sun. The water goes into the air, and white salt is left on the dish.

See it three ways

With things

A drop of ink spreads through still water. Perfume spreads through a room. Sugar spreads through tea. In each one, something mixes through something else without a spoon or a wind.

As a picture

Two open cups of water, side by side, with a key below. Each cup has open circles for water and dark dots for ink. In the cup labelled start, the dark dots are bunched in one corner. In the cup labelled later, the dark dots are mixed through the water. Labels: start, later, water, ink.startlaterwaterink

In words

Particles of two materials mix by themselves. The particles move about, and they spread from where there are many to where there are few.

Try it

Time a smell across a room

You need: an orange or a lemon, a room with the door and windows shut, 3 friends, a clock, or a way to count seconds.

Be safe: Nobody should sniff the peel close up. Do not try this with anyone who is allergic to oranges or lemons.

  1. Ask your friends to stand in the room. One stands near the orange, one half way, and one at the far end. They must stay still.
  2. Peel the orange at one end of the room. Start counting at once.
  3. Ask each friend to put a hand up the moment they smell it. Write down each time.

What you will see: The friend nearest the orange smells it first. The friend at the far end smells it last. You saw nothing cross the room, but particles did. Air currents help carry them, so this test cannot show diffusion alone.

If you cannot try it, this is what happens.
A row of three people stand in a line away from an orange on the left. The nearest person smells it first, the one half way smells it second, and the one at the far end smells it third. Labels: orange, 1st, 2nd, 3rd.orange1st2nd3rd

A wrong idea many people have

A smell needs wind to travel.

Why it feels right: A breeze often brings the smell of cooking from the next house.

The test that breaks it: Leave a drop of food colour in a glass of still water. With no wind and no stirring, the colour still spreads out, though very slowly.

What is true: Wind and air currents carry a smell faster. But the particles also spread out by themselves, even in still air, only slowly. This is diffusion.

Key words

evidence
what we see or measure that supports an idea.
diffusion
particles spreading out by themselves, from where there are many to where there are few.
dissolves
mixes into a liquid so completely that the particles cannot be seen.

Back to your guess

The answer is C: Tiny particles from the food spread out through the air until some reach my nose.

Particles from the food mix into the air and spread out by themselves. Air currents carry them faster.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Someone opens a bottle of perfume at one end of a still room. Why can a person at the other end smell it after a while?

    Tap your answer.

    1. Not this one. The particles zigzag as they bump into air particles. They spread out, and nothing pushes them in a line. Go to S3.5.2. Try another.

    2. Not this one. Wind helps, but particles spread by themselves, even in still air. Go to S3.5.2. Try another.

    3. Yes, that is right. The particles spread out. Air currents help, and diffusion alone is slower, but it needs no wind.

    See the answer

    Perfume particles fly about, bump into air particles and spread through the room.

    The particles spread out. Air currents help, and diffusion alone is slower, but it needs no wind.

    • If you chose A: The particles zigzag as they bump into air particles. They spread out, and nothing pushes them in a line. Go to S3.5.2.
    • If you chose B: Wind helps, but particles spread by themselves, even in still air. Go to S3.5.2.

  2. Why can you not see the sugar after you stir it into water?

    Tap your answer.

    1. Not this one. The sugar does not change into water. Its particles are still sugar, spread through the water. Go to S3.5.2. Try another.

    2. Yes, that is right. The sugar dissolves. Its particles are mixed among the water particles, so the water stays clear.

    3. Not this one. Nothing is destroyed. Leave the water to dry and the sugar is still there. Go to S3.5.2. Try another.

    See the answer

    The sugar breaks into particles too small to see, and they spread through the water.

    The sugar dissolves. Its particles are mixed among the water particles, so the water stays clear.

    • If you chose A: The sugar does not change into water. Its particles are still sugar, spread through the water. Go to S3.5.2.
    • If you chose C: Nothing is destroyed. Leave the water to dry and the sugar is still there. Go to S3.5.2.

  3. The picture shows 6 air particles and 3 smell particles in a box. How many particles are in the box in all?

    A closed box of gas with 6 open circles for air and 3 dark dots for smell, and a key below. Labels: air, smell.airsmell
    Tap your answer.

    1. Yes, that is right. Add the air particles and the smell particles. 6 + 3 = 9.

    2. Not this one. A smell is made of particles too. Count the dark dots as well as the open circles. Go to S3.5.2. Try another.

    3. Not this one. The air is made of particles too. Count the open circles as well as the dark dots. Go to S3.5.2. Try another.

    See the answer

    9 particles in all

    Add the air particles and the smell particles. 6 + 3 = 9.

    • If you chose B: A smell is made of particles too. Count the dark dots as well as the open circles. Go to S3.5.2.
    • If you chose C: The air is made of particles too. Count the open circles as well as the dark dots. Go to S3.5.2.

  4. In one orange test, a friend near the orange smelled it after 20 seconds. A friend at the far end smelled it after 90 seconds. How many seconds longer did the far friend wait?

    Tap your answer.

    1. Yes, that is right. The smell took longer to reach the far end. 90 − 20 = 70.

    2. Not this one. That is the far friend's whole time. The question asks how much longer than the near friend. Go to S3.5.2. Try another.

    3. Not this one. To find how much longer, take the shorter time away. Do not add the times. Go to S3.5.2. Try another.

    See the answer

    70 seconds longer

    The smell took longer to reach the far end. 90 − 20 = 70.

    • If you chose B: That is the far friend's whole time. The question asks how much longer than the near friend. Go to S3.5.2.
    • If you chose C: To find how much longer, take the shorter time away. Do not add the times. Go to S3.5.2.

  5. You stir salt into water and can no longer see it. How could you show the salt is still there?

    Tap your answer.

    1. Not this one. The salt has not gone. Its particles are still in the water, so nothing has been lost. Go to S3.5.2. Try another.

    2. Yes, that is right. The water goes into the air as a gas, and the salt particles stay behind. That shows the salt was dissolved, not destroyed.

    3. Not this one. Salt does not turn into water. It dissolves, and its particles are spread among the water particles. Go to S3.5.2. Try another.

    See the answer

    Leave the salty water in a dish in the sun. The water dries away, and white salt is left.

    The water goes into the air as a gas, and the salt particles stay behind. That shows the salt was dissolved, not destroyed.

    • If you chose A: The salt has not gone. Its particles are still in the water, so nothing has been lost. Go to S3.5.2.
    • If you chose C: Salt does not turn into water. It dissolves, and its particles are spread among the water particles. Go to S3.5.2.

  6. From an earlier chapter

    In which state do the particles touch each other but slide past each other?

    Tap your answer.

    1. Not this one. In a solid the particles touch but are held in place. Go to S3.5.1. Try another.

    2. Yes, that is right. In a liquid the particles touch, but they are free to slide. That is why a liquid flows.

    3. Not this one. In a gas the particles are far apart. Go to S3.5.1. Try another.

    See the answer

    Liquid

    In a liquid the particles touch, but they are free to slide. That is why a liquid flows.

    • If you chose A: In a solid the particles touch but are held in place. Go to S3.5.1.
    • If you chose C: In a gas the particles are far apart. Go to S3.5.1.

Keep it

Copy this into your exercise book.

We cannot see particles, so we use evidence. A smell spreads because its particles fly about and mix with air. This is diffusion. Dissolved sugar is not gone. It is spread out.

Draw: Draw two boxes of open circles. Put dark dots in one corner of the first. Spread them through the second.

S3.5.3 Chapter 3

Changes of state

Why does ice melt on a hot day, and steam turn to drops on a cold lid?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Ice melts on a hot day. Steam turns back to drops on a cold lid. In both, a material turns from one state to another. This is a change of state.

Heat gives particles energy. Energy is what makes particles move. The more energy they have, the faster they move.

Heat a solid. Its particles vibrate harder and harder. At last they shake free of their places and begin to slide. The solid has melted. The temperature where this happens is the melting point.

Keep heating the liquid. Its particles move faster and faster. At the boiling point they have enough energy to break free of each other. Bubbles of gas form all through the liquid. It boils.

Cooling does the opposite. Energy leaves the material. A gas cools, and its particles slow down. They are pulled together, and the gas condenses into a liquid. Cool the liquid more. The particles slow until they lock in place. It freezes.

Draw it first

Draw the changes of state

  1. Three boxes of particles in a row, labelled solid, liquid and gas, with solid on the left. Two arrows above the boxes point to the right. The arrow from solid to liquid is labelled melting. The arrow from liquid to gas is labelled boiling. Both are labelled energy in. Two arrows below the boxes point to the left. The arrow from gas to liquid is labelled condensing. The arrow from liquid to solid is labelled freezing. Both are labelled energy out.solidliquidgas

    1 Draw three boxes of particles in a row: solid, liquid and gas.

  2. Three boxes of particles in a row, labelled solid, liquid and gas, with solid on the left. Two arrows above the boxes point to the right. The arrow from solid to liquid is labelled melting. The arrow from liquid to gas is labelled boiling. Both are labelled energy in. Two arrows below the boxes point to the left. The arrow from gas to liquid is labelled condensing. The arrow from liquid to solid is labelled freezing. Both are labelled energy out.solidliquidgasmeltingenergy inboilingenergy in

    2 Draw arrows above them, pointing right. Heating puts energy in. Write melting and boiling.

  3. Three boxes of particles in a row, labelled solid, liquid and gas, with solid on the left. Two arrows above the boxes point to the right. The arrow from solid to liquid is labelled melting. The arrow from liquid to gas is labelled boiling. Both are labelled energy in. Two arrows below the boxes point to the left. The arrow from gas to liquid is labelled condensing. The arrow from liquid to solid is labelled freezing. Both are labelled energy out.solidliquidgasmeltingenergy inboilingenergy incondensingenergy outfreezingenergy out

    3 Draw arrows below them, pointing left. Cooling takes energy out. Write condensing and freezing.

While a pure solid melts, its temperature stays the same. Ice stays at 0 °C until all of it has gone. The energy goes into freeing the particles, not into making them hotter. The same is true while a liquid boils.

Two cups of hot water side by side. In the cup labelled boiling, bubbles of gas are all through the water, and the label under it says 100 °C. In the cup labelled evaporating, there are no bubbles, and arrows rise from the surface only. The label under it says below 100 °C. Labels: boiling, 100 °C, evaporating, below 100 °C.boiling100 °Cevaporatingbelow 100 °C
Boiling makes bubbles all through the liquid. Evaporation happens only at the surface.

Evaporation is different. It happens at the surface of a liquid, at any temperature below the boiling point. A few particles near the surface have more energy than the rest, and they escape as a gas. Boiling happens all through the liquid, and only at the boiling point.

A pure material, such as water or iron, has its own melting point and boiling point. They tell you its state at any temperature. Below the melting point it is a solid. Between the two points it is a liquid. Above the boiling point it is a gas. These points are for normal air pressure, near sea level.

A table of five materials with the columns name, melts at and boils at, in °C. oxygen melts at −219 and boils at −183. mercury melts at −39 and boils at 357. water melts at 0 and boils at 100. salt melts at 801 and boils at 1465. iron melts at 1538 and boils at 2862. Labels: name, melts at, boils at, oxygen, mercury, water, salt, iron, −219, −183, −39, 357, 0, 100, 801, 1465, 1538, 2862.namemelts atboils atoxygen−219−183mercury−39357water0100salt8011465iron15382862
Melting points and boiling points in °C, at normal air pressure near sea level. High up, water boils at a lower temperature. Mercury is a poisonous metal. Never touch it.

Room temperature is about 20 °C. Oxygen melts at −219 °C and boils at −183 °C. 20 °C is far above both, so oxygen is a gas in a room. Iron needs 1538 °C to melt, so it is a solid.

See it three ways

As a picture

A tall temperature scale for water in degrees Celsius, from minus 20 to 120. A band beside it is hatched and labelled solid up to the melting point, dotted and labelled liquid between the melting point and the boiling point, and crossed and labelled gas above the boiling point. The melting point is marked at 0 °C and the boiling point at 100 °C. A dashed mark shows room temperature at 20 °C, in the liquid part. Labels: solid, liquid, gas, melting point, boiling point, room temperature.solidliquidgas−20020406080100120boiling point100 °Croom temperature20 °Cmelting point0 °C

In words

Below the melting point, a material is a solid. Between its melting point and its boiling point, it is a liquid. Above its boiling point, it is a gas.

In symbols

At sea level, water is solid below 0 °C. It is liquid from 0 °C to 100 °C. It is gas above 100 °C.

Try it

Turn steam back into drops

You need: a pot with a little water, a grown up to heat it, a cold metal plate or lid, an oven glove.

Be safe: A grown up must do the heating. Steam and hot water burn badly. Keep your face and hands away from the pot.

  1. A grown up heats the water until bubbles rise all through it. Then they turn the heat off.
  2. The grown up holds the cold plate, tilted, about 20 cm above the pot, wearing the oven glove.
  3. Wait a few seconds. Look at the underside of the plate. Do not touch the pot or the steam.

What you will see: Drops of water form on the underside of the cold plate. The vapour gave energy to the plate and turned back into a liquid. The plate also gets warm.

If you cannot try it, this is what happens.
A pot of water with arrows rising from it to a cold plate above. Three drops hang under the plate. Labels: cold plate, drops, water vapour, pot.cold platedropswater vapourpot

A wrong idea many people have

When ice melts, the particles themselves turn runny.

Why it feels right: The ice turns soft and runny, so it seems each little bit has softened.

The test that breaks it: Freeze the water again and you get ice again, the same as before. If the particles had changed into something else, the ice could not come back.

What is true: The particles do not change. Heat gives them more energy, so they move more and break free of their places.

Key words

change of state
a material turning from a solid, liquid or gas into another of the three.
energy
what makes particles move. More energy means faster particles.
melting point
the temperature at which a solid melts. It is also where the liquid freezes.
boiling point
the temperature at which a liquid boils, with bubbles of gas all through it.

Back to your guess

The answer is A: Heat gives the particles energy. They move more and break free of their places.

The particles do not change. Heat gives them energy, so they vibrate harder and slide free. Cooling takes the energy away again.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Ice is heated. What happens to its particles as it melts?

    Tap your answer.

    1. Not this one. The particles stay the same size. Only their energy and the way they are held change. Go to S3.5.3. Try another.

    2. Yes, that is right. Heat gives the particles energy. They vibrate harder until they slide free, and the solid is a liquid.

    3. Not this one. Heating makes particles move more, not less. Go to S3.5.3. Try another.

    See the answer

    They gain energy and break free of their fixed places.

    Heat gives the particles energy. They vibrate harder until they slide free, and the solid is a liquid.

    • If you chose A: The particles stay the same size. Only their energy and the way they are held change. Go to S3.5.3.
    • If you chose C: Heating makes particles move more, not less. Go to S3.5.3.

  2. Bubbles of gas form all through a pan of water. What is happening?

    Tap your answer.

    1. Yes, that is right. Boiling makes bubbles all through the liquid, at the boiling point. Evaporation happens at the surface, at any lower temperature.

    2. Not this one. Evaporation happens only at the surface. Bubbles all through the water mean it is boiling. Go to S2.4.4. Try another.

    3. Not this one. Melting turns a solid into a liquid. This water is already a liquid. Go to S3.5.3. Try another.

    See the answer

    The water is boiling.

    Boiling makes bubbles all through the liquid, at the boiling point. Evaporation happens at the surface, at any lower temperature.

    • If you chose B: Evaporation happens only at the surface. Bubbles all through the water mean it is boiling. Go to S2.4.4.
    • If you chose C: Melting turns a solid into a liquid. This water is already a liquid. Go to S3.5.3.

  3. The scale is for iron. Is iron a solid, a liquid or a gas at room temperature, about 20 °C?

    A tall temperature scale for iron in degrees Celsius, from 0 to 3500. A band beside it is hatched and labelled solid up to the melting point at 1538 °C, dotted and labelled liquid up to the boiling point at 2862 °C, and crossed and labelled gas above that. A dashed mark shows room temperature at 20 °C. Labels: solid, liquid, gas, melting point, boiling point, room temperature.solidliquidgas0500100015002000250030003500boiling point2862 °Cmelting point1538 °Croom temperature20 °C
    Tap your answer.

    1. Not this one. Zero degrees is the melting point of water. Iron has its own melting point, and it is much higher. Go to S3.5.3. Try another.

    2. Not this one. Metals do not melt easily. Iron needs 1538 °C to melt, and a room is far cooler than that. Go to S3.5.3. Try another.

    3. Yes, that is right. Room temperature is far below the melting point, so the iron is a solid. It would need 1538 °C to melt.

    See the answer

    A solid. 20 °C is below the melting point of 1538 °C.

    Room temperature is far below the melting point, so the iron is a solid. It would need 1538 °C to melt.

    • If you chose A: Zero degrees is the melting point of water. Iron has its own melting point, and it is much higher. Go to S3.5.3.
    • If you chose B: Metals do not melt easily. Iron needs 1538 °C to melt, and a room is far cooler than that. Go to S3.5.3.

  4. Oxygen melts at −219 °C and boils at −183 °C. What state is oxygen at room temperature, about 20 °C?

    Tap your answer.

    1. Not this one. It has melted, but it is also far above its boiling point of −183 °C. So it has boiled and is a gas. Go to S3.5.3. Try another.

    2. Yes, that is right. 20 °C is above the boiling point, so the particles have broken free of each other. Oxygen is a gas.

    3. Not this one. A solid melts when it gets warmer than its melting point. At 20 °C oxygen is far warmer than −219 °C. Go to S3.5.3. Try another.

    See the answer

    A gas. 20 °C is above the boiling point of −183 °C.

    20 °C is above the boiling point, so the particles have broken free of each other. Oxygen is a gas.

    • If you chose A: It has melted, but it is also far above its boiling point of −183 °C. So it has boiled and is a gas. Go to S3.5.3.
    • If you chose C: A solid melts when it gets warmer than its melting point. At 20 °C oxygen is far warmer than −219 °C. Go to S3.5.3.

  5. Steam touches a cold window and turns into drops of water. What happens to the particles?

    Tap your answer.

    1. Not this one. That is what happens when a liquid boils. Steam turning into drops is the opposite. Go to S3.5.3. Try another.

    2. Not this one. The particles stay water particles. Only their energy and the way they are held change. Go to S3.5.3. Try another.

    3. Yes, that is right. The cold glass takes energy from the vapour. The particles slow down until they are held close together. That is a liquid, so the vapour condenses.

    See the answer

    They lose energy, slow down and are pulled together.

    The cold glass takes energy from the vapour. The particles slow down until they are held close together. That is a liquid, so the vapour condenses.

    • If you chose A: That is what happens when a liquid boils. Steam turning into drops is the opposite. Go to S3.5.3.
    • If you chose B: The particles stay water particles. Only their energy and the way they are held change. Go to S3.5.3.

  6. From an earlier chapter

    What is it called when particles spread out by themselves?

    Tap your answer.

    1. Not this one. Freezing is a liquid turning into a solid. Spreading out by themselves has another name. Go to S3.5.2. Try another.

    2. Yes, that is right. Diffusion is the spreading of particles by their own movement, such as a smell crossing a room.

    3. Not this one. Melting is a solid turning into a liquid. Spreading out by themselves has another name. Go to S3.5.2. Try another.

    See the answer

    Diffusion

    Diffusion is the spreading of particles by their own movement, such as a smell crossing a room.

    • If you chose A: Freezing is a liquid turning into a solid. Spreading out by themselves has another name. Go to S3.5.2.
    • If you chose C: Melting is a solid turning into a liquid. Spreading out by themselves has another name. Go to S3.5.2.

Keep it

Copy this into your exercise book.

Heat gives particles energy. At the melting point a solid melts. At the boiling point a liquid boils. Cooling takes energy away: gas condenses, liquid freezes.

Draw: Draw solid, liquid and gas boxes in a row. Add arrows above, pointing right, and arrows below, pointing left. Name each one.

S3.5.4 Chapter 4

Diffusion and temperature

Why does a drop of colour spread faster in hot water than in cold water?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

A smell spreads by diffusion. Does it spread faster when it is warm? Think of a drop of food colour in water.

Temperature tells you how hot something is. The hotter it is, the faster its particles move, on average. In hot water the particles move fast. In cold water they move slowly.

A tall temperature scale for water in degrees Celsius, from minus 20 to 120. A band beside it is hatched and labelled solid up to the melting point, dotted and labelled liquid between the melting point and the boiling point, and crossed and labelled gas above the boiling point. The melting point is marked at 0 °C and the boiling point at 100 °C. Dashed marks show cool water at 20 °C and hot water at 70 °C, both in the liquid part. Labels: solid, liquid, gas, melting point, boiling point, cool water, hot water.solidliquidgas−20020406080100120boiling point100 °Chot water70 °Ccool water20 °Cmelting point0 °C
Cool water and hot water are both liquid. The hotter water has the faster particles.

Draw it first

Draw colour spreading in a cold cup and a hot cup

  1. Two pictures of the same two open cups of water, one shown after the other, with a key above. The cups hold open circles for water and dark dots for colour. Every particle has an arrow inside it, and the arrows in the hot cup are longer than in the cold cup. The first picture, labelled cold and hot, shows the start. In each cup the dark dots are bunched in one corner. The second picture, labelled cold later and hot later, shows the same cups a little later. In the cold cup the dark dots have spread a little. In the hot cup they have spread much further. Labels: water, colour, cold, hot, cold later, hot later.watercolourcoldhot

    1 Draw two cups of water. Put the colour, dark dots, in one corner of each. Give every particle an arrow, short in the cold cup and long in the hot cup.

  2. Two pictures of the same two open cups of water, one shown after the other, with a key above. The cups hold open circles for water and dark dots for colour. Every particle has an arrow inside it, and the arrows in the hot cup are longer than in the cold cup. The first picture, labelled cold and hot, shows the start. In each cup the dark dots are bunched in one corner. The second picture, labelled cold later and hot later, shows the same cups a little later. In the cold cup the dark dots have spread a little. In the hot cup they have spread much further. Labels: water, colour, cold, hot, cold later, hot later.watercolourcold laterhot later

    2 Draw the same cups later. In the hot cup the dark dots have spread much further than in the cold cup.

The colour particles are knocked about by the water particles around them. Fast water particles knock them harder and more often. So the colour spreads out sooner in hot water.

Cold water is not still. Its particles move too, only more slowly. So colour does spread in cold water, just slowly.

In a real cup, small currents help the colour spread too, and hot water makes more of them. So the hot cup is quicker for two reasons. To see diffusion alone, the water would have to be perfectly still.

The same is true of a smell. The smell of food reaches you sooner in a warm kitchen than in a cold one.

Two closed boxes of air, one above the other. Each has 9 air particles and one dark dot for a smell particle on the left. In the box labelled cold air, the path of the dot bends 3 times. In the box labelled warm air, the path of the dot bends 6 times in the same time, so it is longer and goes farther. Labels: cold air, warm air.cold airwarm air
In air, a faster particle bumps more often in the same time, so it goes farther.

To compare hot and cold fairly, change only one thing. Use the same amount of water and the same colour. Stir neither cup, and do not move them. This is a fair test. If you stir one cup, you cannot tell what made the difference.

Two pairs of cups. Under the title fair test, the cold cup and the hot cup each hold 200 mL of water. Under the title not fair, the cold cup holds 300 mL and the hot cup holds 100 mL. Labels: fair test, cold, 200 mL, hot, 200 mL, not fair, cold, 300 mL, hot, 100 mL.fair testcold200 mLhot200 mLnot faircold300 mLhot100 mL
In a fair test only the temperature differs. The amounts of water are the same.

See it three ways

With things

Add a drop of food colour to a cup of hot water. Add one to a cup of cold water. Leave both alone and watch. The hot cup turns one even colour first.

As a picture

Two cups of water with a drop of colour added at the top of each, shown after the same time. In the cup labelled cold, the hatched colour has spread only a little way down from the top. In the cup labelled hot, the hatched colour has spread all through the water. Labels: cold, hot.coldhot

In words

In hot water the particles move faster. They bump about and spread out faster. So the colour spreads in less time.

Try it

Hot cup, cold cup

You need: 2 clear cups or glasses, food colour or ink, cold water, hot water from a grown up, a clock, or a way to count minutes.

Be safe: A grown up must pour the hot water. Use hot, not boiling, water. Do not drink it. Keep the cups away from the table edge.

  1. Pour the same amount of cold water into one cup. A grown up pours the same amount of hot, not boiling, water into the other cup.
  2. Wait until both cups are still. Add one drop of colour to each cup, as gently as you can. Do not stir.
  3. Look every few minutes. Write down how long each cup takes to become one even colour.

What you will see: The hot cup becomes one even colour sooner. Currents help in the hot cup too, so this test shows the pattern but not diffusion alone.

If you cannot try it, this is what happens.
A table with the columns cup and after a while. The cold cup is still patchy. The hot cup is one even colour. Labels: cup, after a while, cold, hot, still patchy, one even colour.cupafter a whilecoldstill patchyhotone even colour

A wrong idea many people have

The particles in cold water stop moving.

Why it feels right: Cold water looks perfectly still.

The test that breaks it: Look at the cold cup in the colour test. The colour still spreads, only slowly. If the particles stood still, nothing could spread.

What is true: Particles in cold water move more slowly than in hot water, but they never stand still.

Key words

temperature
how hot something is. The hotter it is, the faster its particles move.
fair test
a test that changes only one thing and keeps everything else the same.

Back to your guess

The answer is A: In hot water the particles move faster, so the colour spreads out sooner.

Particles move faster when they are hotter. Fast particles knock the colour particles about harder and more often, so the colour spreads sooner.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Why does a drop of food colour spread faster in hot water?

    Tap your answer.

    1. Not this one. A cup of hot water and a cup of cold water hold about the same number of particles. What differs is how fast they move. Go to S3.5.4. Try another.

    2. Not this one. The colour particles do not change. It is the movement of the water particles that is different. Go to S3.5.4. Try another.

    3. Yes, that is right. Heat gives particles energy and they move faster. They knock the colour about harder and more often.

    See the answer

    The particles move faster, so they spread out sooner.

    Heat gives particles energy and they move faster. They knock the colour about harder and more often.

    • If you chose A: A cup of hot water and a cup of cold water hold about the same number of particles. What differs is how fast they move. Go to S3.5.4.
    • If you chose B: The colour particles do not change. It is the movement of the water particles that is different. Go to S3.5.4.

  2. In Ana's test, food colour took 15 minutes to spread evenly in the hot cup. It took 60 minutes in the cold cup. How many times longer did the cold cup take?

    Tap your answer.

    1. Not this one. You took the shorter time away. That gives how many minutes longer. To find how many times longer, divide. Go to S3.5.4. Try another.

    2. Not this one. Adding the two times does not compare them. Divide the longer time by the shorter time. Go to S3.5.4. Try another.

    3. Yes, that is right. Divide the longer time by the shorter time. 60 ÷ 15 = 4.

    See the answer

    4 times longer

    Divide the longer time by the shorter time. 60 ÷ 15 = 4.

    • If you chose A: You took the shorter time away. That gives how many minutes longer. To find how many times longer, divide. Go to S3.5.4.
    • If you chose B: Adding the two times does not compare them. Divide the longer time by the shorter time. Go to S3.5.4.

  3. Kofi tests hot and cold water. He uses a big cup of cold water and a small cup of hot water. What is wrong with his test?

    Tap your answer.

    1. Yes, that is right. A fair test changes only the temperature. The amount of water, and the colour, must be the same in both cups.

    2. Not this one. The temperature should be the only thing that changes. The amount of water must stay the same. Go to S3.5.4. Try another.

    3. Not this one. Stirring would add another change. In a fair test only one thing changes. Go to S3.5.4. Try another.

    See the answer

    The amounts of water are different, so the test is not fair.

    A fair test changes only the temperature. The amount of water, and the colour, must be the same in both cups.

    • If you chose B: The temperature should be the only thing that changes. The amount of water must stay the same. Go to S3.5.4.
    • If you chose C: Stirring would add another change. In a fair test only one thing changes. Go to S3.5.4.

  4. What does the temperature of a material tell you about its particles?

    Tap your answer.

    1. Not this one. The size of particles does not change with temperature. Go to S3.5.4. Try another.

    2. Not this one. Heating a material does not add particles. It makes them move faster. Go to S3.5.4. Try another.

    3. Yes, that is right. The hotter a material is, the faster its particles move, on average.

    See the answer

    How fast they move, on average.

    The hotter a material is, the faster its particles move, on average.

    • If you chose A: The size of particles does not change with temperature. Go to S3.5.4.
    • If you chose B: Heating a material does not add particles. It makes them move faster. Go to S3.5.4.

  5. After five minutes the colour has hardly spread in the cold cup. Sam says, "Cold particles stop moving." What should you tell Sam?

    Tap your answer.

    1. Not this one. If the particles stood still, the colour could not spread at all. It does spread, only slowly. Go to S3.5.4. Try another.

    2. Yes, that is right. Temperature tells how fast the particles move, on average. Cold particles move slowly but never stand still.

    3. Not this one. Cold water is not thicker in a way that stops the particles. They move, but more slowly than in hot water. Go to S3.5.4. Try another.

    See the answer

    Cold particles still move, only more slowly, so the colour spreads more slowly.

    Temperature tells how fast the particles move, on average. Cold particles move slowly but never stand still.

    • If you chose A: If the particles stood still, the colour could not spread at all. It does spread, only slowly. Go to S3.5.4.
    • If you chose C: Cold water is not thicker in a way that stops the particles. They move, but more slowly than in hot water. Go to S3.5.4.

  6. From an earlier chapter

    Which change happens only at the surface of a liquid, at any temperature below its boiling point?

    Tap your answer.

    1. Yes, that is right. A few particles at the surface have more energy than the rest, and they escape as a gas. This can happen at any temperature.

    2. Not this one. Boiling makes bubbles all through the liquid, and only at the boiling point. Go to S3.5.3. Try another.

    3. Not this one. Melting turns a solid into a liquid. Here the liquid is turning into a gas. Go to S3.5.3. Try another.

    See the answer

    Evaporation

    A few particles at the surface have more energy than the rest, and they escape as a gas. This can happen at any temperature.

    • If you chose B: Boiling makes bubbles all through the liquid, and only at the boiling point. Go to S3.5.3.
    • If you chose C: Melting turns a solid into a liquid. Here the liquid is turning into a gas. Go to S3.5.3.

Keep it

Copy this into your exercise book.

The hotter something is, the faster its particles move. Fast particles spread out sooner, so diffusion is quicker when warm. Cold particles still move, only slowly.

Draw: Draw a cold cup and a hot cup of water with dark dots in one corner. Draw the dots later, spread further in the hot cup.

S3.5.5 Chapter 5

Gas pressure

Why does a bicycle tyre go hard when I pump air into it?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Pump up a bicycle tyre and it goes hard. There is only air inside. Why does air push so hard?

The particles of a gas fly about in every direction. They keep hitting the walls of the container. Each hit gives the wall a tiny push.

A tyre holds an enormous number of particles. They make an enormous number of hits every second. Together they make a steady push. Gas pressure is the push of a gas on the walls of its container.

Draw it first

Draw a gas pushing on its box

  1. Two closed boxes of gas, side by side. Each box has 8 particles. In the box labelled cool, 4 particles have short arrows pointing at the walls, and a thick mark on the wall where each lands. In the box labelled hot, 6 particles have longer arrows pointing at the walls, with a thick mark where each lands. The long arrows are the fast particles. Labels: cool, hot.cool

    1 Draw a closed box with 8 gas particles. Draw 4 short arrows from particles to the walls. A short arrow is a slow particle. Mark each wall where an arrow lands.

  2. Two closed boxes of gas, side by side. Each box has 8 particles. In the box labelled cool, 4 particles have short arrows pointing at the walls, and a thick mark on the wall where each lands. In the box labelled hot, 6 particles have longer arrows pointing at the walls, with a thick mark where each lands. The long arrows are the fast particles. Labels: cool, hot.coolhot

    2 Draw a second box with 8 particles moving faster. Draw 6 longer arrows. Faster particles hit harder and more often.

A hotter gas has faster particles. They hit the walls harder, and they hit more often. So the pressure goes up.

There are two more ways to raise the pressure. One is to put more particles into the same box. The other is to compress the gas. To compress a gas is to squeeze it into a smaller volume. Volume is the space a thing takes up.

Two closed boxes of gas, side by side. Each has 8 particles moving at the same speed. The box labelled big is tall, and 4 particles are about to hit its walls. The box labelled squeezed is much shorter, and 6 particles are about to hit its walls. Labels: big, squeezed.bigsqueezed
The same particles in a smaller space hit the walls more often. The pressure goes up.

On a hot day, the air inside a ball or a tyre is warmer. Its particles move faster. They hit the inside harder and more often, so the ball feels harder.

See it three ways

With things

Pump air into a bicycle tyre. The more air you add, the harder the tyre gets.

As a picture

Two closed boxes of gas of the same size, side by side. The box labelled few has 4 particles, and 2 of them are about to hit a wall. The box labelled many has 10 particles, and 5 of them are about to hit a wall. Labels: few, many.fewmany

In words

More particles in the same box means more hits on the walls each second. More hits means more pressure.

Try it

Feel the push of the air

You need: a balloon.

Be safe: Squeeze gently and keep the balloon away from your face. It may burst.

  1. Blow up the balloon until it is round and not tight. Tie it.
  2. Hold it in both hands and squeeze it gently. Feel it push back.
  3. Squeeze a little harder. Let go. The balloon goes back to its round shape.

What you will see: The balloon pushes back on your hands, from every side. The harder you squeeze, the harder it pushes back.

If you cannot try it, this is what happens.
A round balloon with a hand arrow on each side pointing in, both labelled squeeze. Below the balloon is the label air pushes back. Labels: squeeze, air pushes back.squeezesqueezeair pushes back
A closed box of gas with 10 particles. Two particles are about to hit each of the four walls, top, bottom, left and right, each with an arrow pointing at the wall and a thick mark where it lands. Labels: every wall is hit.every wall is hit
The hits land on every wall, not only the bottom. The push is the same all round.

A wrong idea many people have

A gas only pushes down, because air has weight.

Why it feels right: Things you can feel pressing down, such as a heavy lid, push down.

The test that breaks it: Squeeze a balloon from the top, the bottom and both sides. It pushes back on your hands each time. A balloon is round, not flat at the bottom.

What is true: Gas particles fly in every direction. In a small container, a gas pushes equally on every wall: up, down and sideways.

Key words

pressure
the push of a gas on the walls of its container. It comes from particles hitting the walls.
compress
squeeze into a smaller volume.
volume
the space a thing takes up.

Back to your guess

The answer is A: Air particles keep hitting the inside of the tyre, which pushes its walls out.

The particles of the air fly about and hit the wall of the tyre again and again. All those hits add up to a push on every part of the wall.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. What causes the push of a gas on the walls of its container?

    Tap your answer.

    1. Not this one. A gas pushes on every wall, not only the bottom. It is not the weight. It is the hits. Go to S3.5.5. Try another.

    2. Yes, that is right. Each hit is a tiny push. More hits every second than anyone could count make a steady pressure.

    3. Not this one. The particles do not stick. They hit the wall and bounce away. Go to S3.5.5. Try another.

    See the answer

    Gas particles hitting the walls.

    Each hit is a tiny push. More hits every second than anyone could count make a steady pressure.

    • If you chose A: A gas pushes on every wall, not only the bottom. It is not the weight. It is the hits. Go to S3.5.5.
    • If you chose C: The particles do not stick. They hit the wall and bounce away. Go to S3.5.5.

  2. The cool box shows 4 hits. The hot box shows 6 hits. How many more hits does the hot box show?

    Two closed boxes of gas, side by side. The box labelled cool has 4 short arrows pointing at its walls. The box labelled hot has 6 longer arrows pointing at its walls. Labels: cool, hot.coolhot
    Tap your answer.

    1. Not this one. You added the two numbers. To find how many more, take the smaller number from the larger. Go to S3.5.5. Try another.

    2. Not this one. That is the number of hits in the hot box on its own. Compare it with the cool box. Go to S3.5.5. Try another.

    3. Yes, that is right. Subtract the hits in the cool box from the hits in the hot box. 6 − 4 = 2. The hot gas also hits harder.

    See the answer

    2 more hits

    Subtract the hits in the cool box from the hits in the hot box. 6 − 4 = 2. The hot gas also hits harder.

    • If you chose A: You added the two numbers. To find how many more, take the smaller number from the larger. Go to S3.5.5.
    • If you chose B: That is the number of hits in the hot box on its own. Compare it with the cool box. Go to S3.5.5.

  3. Why does a football feel harder on a hot day?

    Tap your answer.

    1. Yes, that is right. Warm particles move faster. Faster particles hit the wall harder and more often, so the pressure rises.

    2. Not this one. The air does not get heavier. Its particles move faster, so they hit the walls harder and more often. Go to S3.5.5. Try another.

    3. Not this one. The ball is sealed. The same air is inside. It is the particles that move faster. Go to S3.5.5. Try another.

    See the answer

    The air inside is warmer, so its particles hit the inside harder and more often.

    Warm particles move faster. Faster particles hit the wall harder and more often, so the pressure rises.

    • If you chose B: The air does not get heavier. Its particles move faster, so they hit the walls harder and more often. Go to S3.5.5.
    • If you chose C: The ball is sealed. The same air is inside. It is the particles that move faster. Go to S3.5.5.

  4. The end of a bicycle pump is blocked. You push in the handle, so the air has a smaller volume. What happens to the pressure?

    Tap your answer.

    1. Not this one. Closer together means more hits on the walls, not fewer. The pressure goes up. Go to S3.5.5. Try another.

    2. Yes, that is right. The same particles now have less room. They reach a wall sooner and hit it more often.

    3. Not this one. The same particles in a smaller space hit the walls more often. The pressure changes. Go to S3.5.5. Try another.

    See the answer

    It goes up, because the particles hit the walls more often.

    The same particles now have less room. They reach a wall sooner and hit it more often.

    • If you chose A: Closer together means more hits on the walls, not fewer. The pressure goes up. Go to S3.5.5.
    • If you chose C: The same particles in a smaller space hit the walls more often. The pressure changes. Go to S3.5.5.

  5. From an earlier chapter

    In a fair test, how many things do you change?

    Tap your answer.

    1. Not this one. Then you could not tell which change made the difference. Go to S3.5.4. Try another.

    2. Yes, that is right. A fair test changes one thing and keeps everything else the same.

    3. Not this one. You change one thing, so you can see what it does. Go to S3.5.4. Try another.

    See the answer

    One.

    A fair test changes one thing and keeps everything else the same.

    • If you chose A: Then you could not tell which change made the difference. Go to S3.5.4.
    • If you chose C: You change one thing, so you can see what it does. Go to S3.5.4.

  6. Why can a sealed can burst if it gets very hot?

    Tap your answer.

    1. Yes, that is right. Heat gives the particles energy. More energy means harder hits. A sealed can has no way to let the gas out.

    2. Not this one. Heating does not make the gas heavier. It makes the particles move faster, and a gas pushes on every wall. Go to S3.5.5. Try another.

    3. Not this one. The particles stay the same size. They move faster, so they hit the walls harder and more often. Go to S3.5.5. Try another.

    See the answer

    The gas inside gets hot. Its particles hit the walls harder and more often, and the pressure rises until the can bursts.

    Heat gives the particles energy. More energy means harder hits. A sealed can has no way to let the gas out.

    • If you chose B: Heating does not make the gas heavier. It makes the particles move faster, and a gas pushes on every wall. Go to S3.5.5.
    • If you chose C: The particles stay the same size. They move faster, so they hit the walls harder and more often. Go to S3.5.5.

Keep it

Copy this into your exercise book.

Gas pressure is the push of particles hitting the walls. Faster particles, more particles, or a smaller volume give more hits, so more pressure.

Draw: Draw a closed box with 8 particles. Add arrows from some of them to the walls, and mark where they land.

S3.5.6 Chapter 6

Expanding and shrinking

Why do railway lines have small gaps between them?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Some railway lines have small gaps between the rails. Many bridges have gaps in their road. Why leave a gap in something that should be joined?

Most solids, liquids and gases get bigger when they are heated. They expand. When they cool, they get smaller. They contract.

Draw it first

Draw two rails, cold and hot

  1. Two pictures of the same two rails, side by side. Each rail is a block of circles in two rows, fixed to a thick post at the outer end. In the picture labelled cold, the circles touch, and there is a wide gap between the rails. In the picture labelled hot, the circles are the same in number but a little further apart, each rail is longer, and the gap is smaller. Labels: gap, cold, gap, hot. The picture is drawn much bigger than real life.gapcold

    1 Draw two rails end to end, fixed at the outer ends. Make each rail two rows of touching circles. Leave a gap between them.

  2. Two pictures of the same two rails, side by side. Each rail is a block of circles in two rows, fixed to a thick post at the outer end. In the picture labelled cold, the circles touch, and there is a wide gap between the rails. In the picture labelled hot, the circles are the same in number but a little further apart, each rail is longer, and the gap is smaller. Labels: gap, cold, gap, hot. The picture is drawn much bigger than real life.gapcoldgaphot

    2 Draw them again, hot. Use the same number of circles, but set them a little further apart. Each rail is longer, so the gap is smaller.

The picture is drawn much bigger than real life, so that you can see it. In a real hot rail the particles are still almost touching. They vibrate harder and sit a tiny bit further apart.

Heat gives the particles energy. In a solid, each particle vibrates harder. It shakes a little further from its place and pushes on its neighbours. So the average space between particles grows, and the whole rail gets longer.

The particles do not get bigger. The same particles are just a little further apart. Cool the rail and they come back together.

Liquids expand too. Gases expand much more than solids, because their particles are free to fly further apart.

Two liquid thermometers side by side with a scale in degrees Celsius from 0 to 100. In the one labelled cold water, the dark liquid stands at 10 °C. In the one labelled hot water, the dark liquid stands higher, at 70 °C. Labels: cold water, hot water, 0, 20, 40, 60, 80, 100, °C.020406080100°Ccold water020406080100°Chot water
The liquid in a thermometer expands as it warms, so it climbs the tube.
A long hatched bar for a steel rail, with a line above it marked 10 m. At its right end a small dark piece shows how much it grows, labelled grows 6 mm. The label not to scale says the growth is drawn far too big. Labels: 10 m, grows 6 mm, not to scale.10 mgrows 6 mmnot to scale
The growth is small. It is drawn much too big, so that you can see it.

The change is small. A steel rail 10 m long grows about 6 mm when it warms by 50 °C. That is about the width of a pencil.

A longer rail has more particles in a row, so it grows more. Double the length and the growth doubles.

Long cables between poles expand too. In the heat a cable is longer, so it hangs lower.

Two pairs of poles with a cable between each pair. On the cold day the cable sags a little. On the hot day the same cable is longer and sags more. Labels: cold day, hot day. The sag is drawn bigger than in real life.cold dayhot day
A longer cable between two fixed poles must hang lower. The sag is drawn bigger than in real life.

Engineers leave gaps so that hot rails do not push on each other and bend. The gaps in a bridge do the same job. Some modern lines have no gaps. Their rails are welded and held firmly in place.

See it three ways

With things

A metal lid is stuck on a jar. Hot water poured over the lid makes it a little bigger, and it comes loose.

As a picture

A bar chart of how much a steel rail grows when it warms by 50 degrees, in millimetres. The bars for 10 m, 20 m and 30 m reach 6, 12 and 18. Labels: 10 m, 20 m, 30 m, growth in mm.0510152010 m20 m30 mgrowth in mm

In words

A longer rail has more particles in a row. Each pair moves a little further apart, so the rail grows by more.

In symbols

10 m grows 6 mm. 20 m grows 12 mm. 30 m grows 18 mm.

Try it

Warm the air in a bottle

You need: a plastic bottle, a balloon, two bowls, cold water, hot water from a grown up.

Be safe: A grown up must pour the hot water. Use hot, not boiling, water, because it can soften the bottle. Use a plastic bottle, never glass.

  1. Stretch the balloon over the neck of the empty bottle. The bottle holds air.
  2. Stand the bottle in a bowl of cold water. Wait a minute. Look at the balloon.
  3. Move the bottle into a bowl of hot, not boiling, water that a grown up pours. Wait a minute. Look at the balloon.

What you will see: In the hot water, the balloon stands up and swells a little. The air in the bottle got warmer and expanded. Put the bottle back in cold water and the balloon goes limp again.

If you cannot try it, this is what happens.
Two plastic bottles, each standing in a bowl of water, with a balloon over its neck. In the bowl labelled cold water, the balloon is limp. In the bowl labelled hot water, the balloon stands up and is a little bigger. Labels: cold water, hot water.cold waterhot water

A wrong idea many people have

Things expand because their particles get bigger.

Why it feels right: The object gets bigger, so it seems each part of it must have grown.

The test that breaks it: If growing particles made things expand, a solid would grow most, because its particles touch. In fact the air in the bottle grows far more than a steel rail does. So it is the space between the particles that grows.

What is true: The particles stay the same size. They move or vibrate more, so the average space between them grows. A gas has much more empty space to gain, so it expands far more.

Key words

expand
get bigger when heated, because the particles move further apart.
contract
get smaller when cooled, because the particles move closer together.

Back to your guess

The answer is C: Rails get longer when they are hot, so they need room to grow.

Heat makes the particles vibrate harder and sit further apart, so the rail gets longer. The gap gives it room to grow.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Why do railway lines have small gaps between them?

    Tap your answer.

    1. Yes, that is right. Heat makes the particles vibrate harder and sit further apart. The rails expand and need room.

    2. Not this one. Rain is not the reason. The gaps are for the rails to grow into when they are hot. Go to S3.5.6. Try another.

    3. Not this one. Heat makes things expand, not shrink. The gaps give the rails room to grow. Go to S3.5.6. Try another.

    See the answer

    The rails grow longer when hot, and the gaps give them room.

    Heat makes the particles vibrate harder and sit further apart. The rails expand and need room.

    • If you chose B: Rain is not the reason. The gaps are for the rails to grow into when they are hot. Go to S3.5.6.
    • If you chose C: Heat makes things expand, not shrink. The gaps give the rails room to grow. Go to S3.5.6.

  2. A metal lid is stuck on a glass jar. A grown up pours hot water over the lid. Why does the lid come loose?

    Tap your answer.

    1. Not this one. Heat makes a lid expand, not contract. Cooling makes it contract. Go to S3.5.6. Try another.

    2. Not this one. Metal does not dissolve in water. The lid only gets a little bigger. Go to S3.5.6. Try another.

    3. Yes, that is right. The metal warms first. Its particles move further apart, so the lid is a little wider.

    See the answer

    The hot lid expands, so it fits more loosely.

    The metal warms first. Its particles move further apart, so the lid is a little wider.

    • If you chose A: Heat makes a lid expand, not contract. Cooling makes it contract. Go to S3.5.6.
    • If you chose B: Metal does not dissolve in water. The lid only gets a little bigger. Go to S3.5.6.

  3. A steel rail 10 m long grows 6 mm when it warms by 50 °C. How much does a rail 50 m long grow in the same warming?

    Tap your answer.

    1. Not this one. That is the growth of the 10 m rail. A longer rail has more particles in a row, so it grows more. Go to S3.5.6. Try another.

    2. Not this one. You multiplied by the length. The rail is 5 times as long, so it grows 5 times as much. Go to S3.5.6. Try another.

    3. Yes, that is right. The rail is 5 times as long, so it grows 5 times as much. 6 × 5 = 30.

    See the answer

    30 mm

    The rail is 5 times as long, so it grows 5 times as much. 6 × 5 = 30.

    • If you chose A: That is the growth of the 10 m rail. A longer rail has more particles in a row, so it grows more. Go to S3.5.6.
    • If you chose B: You multiplied by the length. The rail is 5 times as long, so it grows 5 times as much. Go to S3.5.6.

  4. Two steel rails, each 10 m long, meet at a gap. Their far ends are fixed. On a hot day each rail grows 6 mm toward the gap. How wide must the gap be, at the least, so that the rails do not touch?

    Tap your answer.

    1. Yes, that is right. Each rail grows 6 mm, and there are two rails. 6 + 6 = 12. The gap must be at least that wide.

    2. Not this one. You counted only one rail. There are two, and each one grows toward the gap. Go to S3.5.6. Try another.

    3. Not this one. The growth is in millimetres, not metres. The rails grow by a few millimetres only. Go to S3.5.6. Try another.

    See the answer

    12 mm

    Each rail grows 6 mm, and there are two rails. 6 + 6 = 12. The gap must be at least that wide.

    • If you chose B: You counted only one rail. There are two, and each one grows toward the gap. Go to S3.5.6.
    • If you chose C: The growth is in millimetres, not metres. The rails grow by a few millimetres only. Go to S3.5.6.

  5. Why do power cables hang lower on a hot day?

    Tap your answer.

    1. Not this one. The cables do not gain any matter. They expand, so they get longer. Go to S3.5.6. Try another.

    2. Yes, that is right. A longer cable between two fixed poles has to hang lower. Bridges have gaps for the same reason: they give the road room to grow.

    3. Not this one. The poles stay put. It is the cables that get longer when they are hot. Go to S3.5.6. Try another.

    See the answer

    The cables expand, so they are longer and sag more.

    A longer cable between two fixed poles has to hang lower. Bridges have gaps for the same reason: they give the road room to grow.

    • If you chose A: The cables do not gain any matter. They expand, so they get longer. Go to S3.5.6.
    • If you chose C: The poles stay put. It is the cables that get longer when they are hot. Go to S3.5.6.

  6. From an earlier chapter

    Sealed air in a box is heated. What happens to its pressure?

    Tap your answer.

    1. Not this one. The box is sealed, so the particles are no further apart. They move faster, so the pressure rises. Go to S3.5.5. Try another.

    2. Not this one. Faster particles hit harder and more often. The pressure changes. Go to S3.5.5. Try another.

    3. Yes, that is right. Heat gives the particles energy, so they move faster. Each hit is harder, and there are more hits.

    See the answer

    It goes up, because faster particles hit the walls harder and more often.

    Heat gives the particles energy, so they move faster. Each hit is harder, and there are more hits.

    • If you chose A: The box is sealed, so the particles are no further apart. They move faster, so the pressure rises. Go to S3.5.5.
    • If you chose B: Faster particles hit harder and more often. The pressure changes. Go to S3.5.5.

Keep it

Copy this into your exercise book.

Heating gives particles energy. They move further apart, so the material expands. The particles stay the same size. Cooling makes it contract. Gaps give rails room to grow.

Draw: Draw two rails with a wide gap and cold circles touching. Draw them hot, with circles further apart and a smaller gap.

S3.5.7 Chapter 7

Dissolving and solutions

Where does the salt go when I stir it into water, and does the cup weigh less?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Sugar disappears when you stir it into tea. Salt disappears in soup. Where do they go? Does the cup weigh less afterwards?

Draw it first

Draw sugar mixing into water

  1. Three open cups of water, side by side, with a key above them. Each cup has 20 open circles for water and 6 dark dots for sugar, and the circles all touch. In the cup labelled lump, the dark dots are bunched together at the bottom left. In the cup labelled mixing, some dark dots have been pulled away from the lump. In the cup labelled mixed, the dark dots are spread through the water. Labels: lump, mixing, mixed, water, sugar.watersugarlump

    1 Draw a cup of water with open circles. Put the sugar, dark dots, in a lump at the bottom.

  2. Three open cups of water, side by side, with a key above them. Each cup has 20 open circles for water and 6 dark dots for sugar, and the circles all touch. In the cup labelled lump, the dark dots are bunched together at the bottom left. In the cup labelled mixing, some dark dots have been pulled away from the lump. In the cup labelled mixed, the dark dots are spread through the water. Labels: lump, mixing, mixed, water, sugar.watersugarlumpmixing

    2 Draw it again a little later. The water particles pull some dark dots away from the lump.

  3. Three open cups of water, side by side, with a key above them. Each cup has 20 open circles for water and 6 dark dots for sugar, and the circles all touch. In the cup labelled lump, the dark dots are bunched together at the bottom left. In the cup labelled mixing, some dark dots have been pulled away from the lump. In the cup labelled mixed, the dark dots are spread through the water. Labels: lump, mixing, mixed, water, sugar.watersugarlumpmixingmixed

    3 Draw it later still. The dark dots are mixed through the water.

Water particles pull on the sugar particles and drag them away from the lump. The sugar particles spread out among the water particles. They are too small to see, so the water stays clear.

When a solid mixes into a liquid and can no longer be seen, we say it dissolves. The solid that dissolves is the solute. The liquid that does the dissolving is the solvent. Together they make a solution.

An open cup of water with open circles and dark dots mixed through it, and the label solution under the cup. A key above names the open circles solvent and the dark dots solute. Labels: solvent, solute, solution.solventsolutesolution
Salt in water. The water is the solvent. The salt is the solute. The mixture is the solution.

In salty water, the salt is the solute and the water is the solvent. The salty water is the solution. A solution is clear. You can see through it, though it may have a colour.

Not everything dissolves in water. Sand sinks to the bottom and stays as grains you can see, however long you stir. Sugar is soluble in water, which means it can dissolve. Sand is insoluble, which means it cannot.

Two open cups of water side by side. In the cup labelled sugar in water, open circles and dark dots are mixed through the water. In the cup labelled sand in water, hatched grains lie in a pile at the bottom and the water above is clear. Labels: sugar in water, sand in water.sugar in watersand in water
Sugar spreads through the water as particles too small to see. Sand stays as grains at the bottom.

Dissolving is not melting. Melting is a solid turning into a liquid when it is heated. Dissolving is a solid mixing into a liquid, and it needs no heat.

Nothing is lost when a solid dissolves. The sugar particles are still in the cup. So the mass of the solution is the mass of the water plus the mass of the solute. Mass is how much matter something has, and we measure it in grams.

See it three ways

With things

Put a cup of water and a heap of salt on a kitchen scale. Read the scale. Then stir the salt into the water, and read the scale again.

As a picture

Two kitchen scales, one above the other. In the picture labelled before, a cup of water and a heap of salt stand on the plate, and the display shows 210 g. In the picture labelled after, a cup of salty water stands on the plate, and the display shows 210 g. Labels: before, water, salt, 210 g, after, salty water.beforewatersalt210 gaftersalty water210 g

In words

The salt is still in the cup, spread through the water. Nothing has been lost, so the scale shows the same mass.

In symbols

200 g + 10 g = 210 g

Three things make a solid dissolve faster. Warm the water, so the particles move faster. Stir, so fresh water reaches the solid. Use smaller pieces, such as grains instead of a lump, so more of the solid touches the water.

Water can only hold so much. Keep adding salt, and at last some stays on the bottom.

Try it

Race the sugar

You need: 3 clear cups, sugar, a spoon, cold water, warm water from a grown up, a clock, or a way to count seconds.

Be safe: A grown up must pour the warm water. Use warm water, never boiling. Do not taste anything.

  1. Pour the same amount of water into three cups. Cups A and B get cold water. Cup C gets warm water poured by a grown up.
  2. Add one level spoon of sugar to each cup at the same moment. Stir cup B only, gently and steadily.
  3. Count the seconds until no sugar can be seen in each cup. Cup A can take several minutes. Write the three times down.

What you will see: The sugar in cups B and C disappears much sooner than in cup A. Your times will differ, but the pattern will be the same. Warmth and stirring both speed up dissolving.

If you cannot try it, this is what happens.
A bar chart of the seconds the sugar took to disappear in three cups. The bar for A reaches 120, the bar for B reaches 40 and the bar for C reaches 60. Labels: A, B, C, seconds.020406080100120140ABCseconds

A wrong idea many people have

Dissolving is the same as melting.

Why it feels right: Both turn a solid into something runny, and warmth helps both.

The test that breaks it: Salt melts at 801 °C, far hotter than any cup of water. Yet salt dissolves in cold water. Leave salty water in the sun. The water goes into the air, and the salt is left behind.

What is true: Melting is a solid turning into a liquid when it is heated. Dissolving is a solid mixing into a liquid, with its particles spread among the liquid particles.

Key words

dissolves
mixes into a liquid so completely that it cannot be seen.
solute
the solid that dissolves.
solvent
the liquid that does the dissolving.
solution
the clear mixture of a solute dissolved in a solvent.
soluble
able to dissolve in a liquid.

Back to your guess

The answer is A: The salt is still there, spread through the water, so the total weighs the same.

The salt particles are still in the cup, mixed among the water particles. Nothing is lost, so the mass is the same.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Salt is stirred into water until it has dissolved. Which names are right?

    Tap your answer.

    1. Not this one. The solute is the solid that dissolves, however little of it there is. The liquid that does the dissolving is the solvent. Go to S3.5.7. Try another.

    2. Yes, that is right. The solute is the solid that dissolves. The solvent is the liquid that does the dissolving. Together they make the solution.

    3. Not this one. The water alone is the solvent. The solution is the whole mixture of salt and water together. Go to S3.5.7. Try another.

    See the answer

    The salt is the solute. The water is the solvent. The salty water is the solution.

    The solute is the solid that dissolves. The solvent is the liquid that does the dissolving. Together they make the solution.

    • If you chose A: The solute is the solid that dissolves, however little of it there is. The liquid that does the dissolving is the solvent. Go to S3.5.7.
    • If you chose C: The water alone is the solvent. The solution is the whole mixture of salt and water together. Go to S3.5.7.

  2. A cup of water has a mass of 200 g. You stir in 10 g of salt until it dissolves. What mass does the scale show now?

    Tap your answer.

    1. Yes, that is right. Nothing is lost when salt dissolves. Add the two masses. 200 + 10 = 210.

    2. Not this one. The salt is not gone. Its particles are still in the cup, so their mass counts. Go to S3.5.7. Try another.

    3. Not this one. Dissolving does not take mass away. The salt is still there, spread through the water. Go to S3.5.7. Try another.

    See the answer

    210 g

    Nothing is lost when salt dissolves. Add the two masses. 200 + 10 = 210.

    • If you chose B: The salt is not gone. Its particles are still in the cup, so their mass counts. Go to S3.5.7.
    • If you chose C: Dissolving does not take mass away. The salt is still there, spread through the water. Go to S3.5.7.

  3. A cup of salty water has a mass of 250 g. It holds 20 g of dissolved salt. What was the mass of the cup of water before the salt went in?

    Tap your answer.

    1. Not this one. The 20 g of salt is part of the total. Take it away to find the water alone. Go to S3.5.7. Try another.

    2. Yes, that is right. The mass of the solution is the water plus the salt. So take the salt away from the whole mass. 250 − 20 = 230.

    3. Not this one. You added the salt. The total already holds the salt, so take it away. Go to S3.5.7. Try another.

    See the answer

    230 g

    The mass of the solution is the water plus the salt. So take the salt away from the whole mass. 250 − 20 = 230.

    • If you chose A: The 20 g of salt is part of the total. Take it away to find the water alone. Go to S3.5.7.
    • If you chose C: You added the salt. The total already holds the salt, so take it away. Go to S3.5.7.

  4. The chart shows the seconds the sugar took to disappear. Cup A was cold and not stirred. Cup B was cold and stirred. How many seconds sooner did the sugar disappear in cup B?

    A bar chart of the seconds the sugar took to disappear in three cups. The bar for A reaches 120, the bar for B reaches 40 and the bar for C reaches 60. Labels: A, B, C, seconds.020406080100120140ABCseconds
    Tap your answer.

    1. Not this one. That is the time for cup B on its own. The question asks how much sooner than cup A, so take it away from the time for A. Go to S3.5.7. Try another.

    2. Yes, that is right. Stirring brings fresh water to the sugar. Take the shorter time from the longer time. 120 − 40 = 80.

    3. Not this one. That is the time for cup A on its own. Compare the two cups: take the shorter time from the longer. Go to S3.5.7. Try another.

    See the answer

    80 seconds sooner

    Stirring brings fresh water to the sugar. Take the shorter time from the longer time. 120 − 40 = 80.

    • If you chose A: That is the time for cup B on its own. The question asks how much sooner than cup A, so take it away from the time for A. Go to S3.5.7.
    • If you chose C: That is the time for cup A on its own. Compare the two cups: take the shorter time from the longer. Go to S3.5.7.

  5. Sand is stirred into water. It sinks to the bottom and stays as grains you can see, however long you stir. Is sand soluble or insoluble in water?

    Tap your answer.

    1. Not this one. Not every solid dissolves. A soluble solid disappears and leaves a clear solution. Sand does not. Go to S3.5.7. Try another.

    2. Not this one. Stirring only moves the grains about. They settle again, so they have not dissolved. Go to S3.5.7. Try another.

    3. Yes, that is right. The water particles cannot pull sand apart. The grains stay as they are, and sink.

    See the answer

    Insoluble. The grains stay as grains and sink.

    The water particles cannot pull sand apart. The grains stay as they are, and sink.

    • If you chose A: Not every solid dissolves. A soluble solid disappears and leaves a clear solution. Sand does not. Go to S3.5.7.
    • If you chose B: Stirring only moves the grains about. They settle again, so they have not dissolved. Go to S3.5.7.

  6. From an earlier chapter

    Why does sugar dissolve faster in warm water than in cold water?

    Tap your answer.

    1. Yes, that is right. Faster particles hit the sugar harder and more often, and they carry it away sooner.

    2. Not this one. Warm and cold water in a cup hold about the same number of particles. Warm particles move faster. Go to S3.5.4. Try another.

    3. Not this one. Sugar is not melting. It is dissolving, and warmth only makes the particles move faster. Go to S3.5.2. Try another.

    See the answer

    Warm particles move faster, so they pull sugar apart and spread it sooner.

    Faster particles hit the sugar harder and more often, and they carry it away sooner.

    • If you chose B: Warm and cold water in a cup hold about the same number of particles. Warm particles move faster. Go to S3.5.4.
    • If you chose C: Sugar is not melting. It is dissolving, and warmth only makes the particles move faster. Go to S3.5.2.

Keep it

Copy this into your exercise book.

A solute dissolves in a solvent to make a solution. Its particles spread among the solvent particles. Nothing is lost, so the mass stays the same. Warm water and stirring dissolve faster.

Draw: Draw a cup of water with sugar in a lump. Draw it again with the dark dots spread through the water.

S3.5.8 Chapter 8

Density

Why does oil float on water, and why does a stone sink?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

A stone sinks in water, but a big log floats. A little oil floats on water. So heavy things do not always sink. What decides it?

The mass of a thing is how much matter it holds. Its volume is the space it takes up. We measure mass in grams, and volume in cubic centimetres, written cm³. A cm³ is the space of a cube 1 cm wide.

Two boxes of the same size, side by side, each packed full with the same number of touching particles. In the box labelled heavy, the particles are solid dark dots, which are heavier. In the box labelled light, the particles are open circles, which are lighter. Labels: heavy, light.heavylight
The same space and the same number of particles. The box of heavy particles holds more mass.

Both boxes take up the same space. The box of heavy particles holds more mass in that space. We say it is more dense.

The same volume of different materials can have very different masses. 1 cm³ of water has a mass of 1 g. 1 cm³ of iron has a mass of about 8 g.

Density tells you the mass in each cm³. A dense material packs a lot of mass into a small space.

See it three ways

With things

Fill one cup with dry sand and an identical cup with cotton wool. Lift them. The cup of sand is far heavier. Sand is denser than cotton wool.

As a picture

A table of the density of eight materials, in grams for each cm³. air is about 0.001. cork is about 0.2. pine is about 0.5. cooking oil is about 0.9. water is about 1. stone is about 2.7. iron is about 7.8. gold is about 19. Labels: material, g/cm³, air, cork, pine, cooking oil, water, stone, iron, gold, 0.001, 0.2, 0.5, 0.9, 1, 2.7, 7.8, 19.materialg/cm³air0.001cork0.2pine0.5cooking oil0.9water1stone2.7iron7.8gold19

In words

A dense material has a lot of mass in each cm³. A material with a low density has little. The table shows about how many grams are in 1 cm³. Real samples differ a little.

In symbols

density = mass ÷ volume. The unit is g/cm³.

Heavier particles make a material denser. A gas has very few particles in each cm³, because they are far apart. So a gas has a very low density. Water is hundreds of times denser than air. Cotton wool and cork are light because they hold pockets of air.

A block of wood drawn as a hatched rectangle 10 cm wide and 8 cm high, with a note that it is 5 cm deep, and the working 10 × 8 × 5 = 400 cm³ under it. Below that is a kitchen scale with the block on it, and its display shows 200 g. Labels: 10 cm, 8 cm, 5 cm deep, 10 × 8 × 5 = 400 cm³, wood, 200 g.10 cm8 cm5 cm deep10 × 8 × 5 = 400 cm³wood200 g
A block of wood. Its volume is 10 × 8 × 5 = 400 cm³, and its mass is 200 g.

To find a density, divide the mass by the volume. Here is a worked example with this block.

Worked example

A block of wood has a mass of 200 g and a volume of 400 cm³. What is its density?

  1. Write the rule: density = mass ÷ volume.
  2. Put in the numbers: 200 ÷ 400.
  3. 200 ÷ 400 = 0.5.

Answer: 0.5 g/cm³. This is less than the density of water, so the block floats.

Now the rule for floating. A thing floats in a liquid if it is less dense than the liquid. It sinks if it is denser. If the two are equal, it stays wherever you put it, neither rising nor sinking.

Oil floats on water because oil is less dense than water. Oil and water do not mix, so they stay in two layers. A ship is mostly air inside, so the whole ship is less dense than water. That is why it floats.

Most woods float, but a few very dense woods, such as ebony, sink. Most metals are denser than water, so a lump of iron sinks. Always compare the density of the thing with the density of the liquid.

A tall jar with three layers of liquid. From the top they are oil at 0.9 g/cm³ with no pattern, water at 1 g/cm³ dotted, and honey at 1.4 g/cm³ hatched. A piece of cork and a piece of pine float on the oil, the cork higher up. An egg floats where the water meets the honey. A stone lies on the bottom. Labels: oil, water, honey, cork, pine, egg, stone.oil0.9 g/cm³water1 g/cm³honey1.4 g/cm³corkpineeggstone
The densest liquid is at the bottom. Each thing rests on the first layer that is denser than it.

How do you find the volume of a stone? A stone pushes water out of its way. We say it displaces water. The water level rises by exactly the volume of the stone. Remember that 1 mL is the same as 1 cm³.

Draw it first

Find the density of a stone

  1. A kitchen scale with a stone on it, and the display shows 60 g. Below it are two measuring cylinders. The one labelled before has water up to 50 mL. The one labelled after has water up to 70 mL, with the stone lying at the bottom. Labels: stone, 60 g, before, after.stone60 g

    1 Put the stone on a scale. Read its mass in grams.

  2. A kitchen scale with a stone on it, and the display shows 60 g. Below it are two measuring cylinders. The one labelled before has water up to 50 mL. The one labelled after has water up to 70 mL, with the stone lying at the bottom. Labels: stone, 60 g, before, after.stone60 g0102030405060708090100mLbefore

    2 Pour water into a measuring cylinder. Read the level in mL.

  3. A kitchen scale with a stone on it, and the display shows 60 g. Below it are two measuring cylinders. The one labelled before has water up to 50 mL. The one labelled after has water up to 70 mL, with the stone lying at the bottom. Labels: stone, 60 g, before, after.stone60 g0102030405060708090100mLbefore0102030405060708090100mLafter

    3 Lower the stone in. Read the new level. The rise in the level is the volume of the stone.

Take the first level from the second to get the volume. Then divide the mass by the volume to get the density.

Try it

Sink or float

You need: a bowl of water, a stone, a piece of wood or cork, a spoon of cooking oil.

Be safe: Wipe up spilled oil, because it is slippery. Do not drink the water.

  1. Write down which of the stone and the wood you think will float.
  2. Put each into the water, one at a time. See which float and which sink.
  3. Pour the spoon of oil onto the water. Watch where the oil goes.

What you will see: The stone sinks. The wood floats. The oil spreads in a thin layer on top of the water and stays there. The picture draws the oil layer thicker so that you can see it.

If you cannot try it, this is what happens.
A jar with a layer of oil floating on a layer of water, drawn thicker than in real life. A piece of wood floats on top of the oil. A stone lies on the bottom, under the water. Labels: oil, water, wood, stone.oil0.9 g/cm³water1 g/cm³woodstone

A wrong idea many people have

Heavy things sink and light things float.

Why it feels right: A stone is heavy and sinks. A feather is light and floats.

The test that breaks it: A big log floats on a river. A tiny steel pin sinks in a cup. The log has far more mass than the pin.

What is true: Mass alone does not decide it. What counts is density, the mass in each cm³. A thing less dense than the liquid floats. A denser thing sinks.

Key words

mass
how much matter a thing holds. We measure it in grams.
volume
the space a thing takes up. We measure it in cm³ or mL.
density
the mass in each cm³: mass ÷ volume.
displaces
pushes water out of its way, so the level rises by the volume of the thing.

Back to your guess

The answer is B: A thing floats when it packs less mass into each cm³ than the liquid does.

What counts is how much mass is packed into each cm³. That is density. A thing less dense than the liquid floats. A denser thing sinks.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. A lump of metal has a mass of 800 g and a volume of 100 cm³. What is its density?

    Tap your answer.

    1. Not this one. You divided the volume by the mass. Density is the mass for each cm³, so divide the mass by the volume. Go to S3.5.8. Try another.

    2. Not this one. You multiplied. Density is how much mass is in each cm³, so you divide the mass by the volume. Go to S3.5.8. Try another.

    3. Yes, that is right. Divide the mass by the volume. 800 ÷ 100 = 8.

    See the answer

    8 g/cm³

    Divide the mass by the volume. 800 ÷ 100 = 8.

    • If you chose A: You divided the volume by the mass. Density is the mass for each cm³, so divide the mass by the volume. Go to S3.5.8.
    • If you chose B: You multiplied. Density is how much mass is in each cm³, so you divide the mass by the volume. Go to S3.5.8.

  2. A block has a mass of 90 g and a volume of 100 cm³. Water has a density of 1 g/cm³. Does the block float on water?

    Tap your answer.

    1. Not this one. Mass alone does not decide it. A big log has a lot of mass and floats. Work out the density and compare. Go to S3.5.8. Try another.

    2. Not this one. Size does not decide it. Compare the density of the block with the density of the water. Go to S3.5.8. Try another.

    3. Yes, that is right. 90 ÷ 100 = 0.9. A thing less dense than the water floats.

    See the answer

    Yes. Its density is 0.9 g/cm³, which is less than 1 g/cm³.

    90 ÷ 100 = 0.9. A thing less dense than the water floats.

    • If you chose A: Mass alone does not decide it. A big log has a lot of mass and floats. Work out the density and compare. Go to S3.5.8.
    • If you chose B: Size does not decide it. Compare the density of the block with the density of the water. Go to S3.5.8.

  3. A stone is put on a scale. Then it goes into a measuring cylinder of water. Use the pictures to find the density of the stone.

    A kitchen scale with a stone on it, and the display shows 75 g. Below it are two measuring cylinders. The one labelled before has water up to 20 mL. The one labelled after has water up to 50 mL, with the stone lying at the bottom. Labels: stone, 75 g, before, after.stone75 g0102030405060708090100mLbefore0102030405060708090100mLafter
    Tap your answer.

    1. Yes, that is right. The water rose from 20 to 50, so the volume is 30 cm³. The mass is 75 g. 75 ÷ 30 = 2.5. This is more than the density of water, so the stone sinks.

    2. Not this one. You used the water level after the stone went in as the volume. The volume of the stone is how far the level rose. Go to S3.5.8. Try another.

    3. Not this one. You used the water level before the stone went in as the volume. The volume of the stone is how far the level rose. Go to S3.5.8. Try another.

    See the answer

    2.5 g/cm³

    The water rose from 20 to 50, so the volume is 30 cm³. The mass is 75 g. 75 ÷ 30 = 2.5. This is more than the density of water, so the stone sinks.

    • If you chose B: You used the water level after the stone went in as the volume. The volume of the stone is how far the level rose. Go to S3.5.8.
    • If you chose C: You used the water level before the stone went in as the volume. The volume of the stone is how far the level rose. Go to S3.5.8.

  4. Why does oil float on water?

    Tap your answer.

    1. Not this one. A big bottle of oil has more mass than a spoon of water, yet it still floats. Compare equal volumes. That is density. Go to S3.5.8. Try another.

    2. Not this one. The size of the particles is not the rule. What counts is the mass in each cm³. Go to S3.5.8. Try another.

    3. Yes, that is right. Each cm³ of oil has less mass than each cm³ of water. So the water stays under the oil.

    See the answer

    Oil is less dense than water.

    Each cm³ of oil has less mass than each cm³ of water. So the water stays under the oil.

    • If you chose A: A big bottle of oil has more mass than a spoon of water, yet it still floats. Compare equal volumes. That is density. Go to S3.5.8.
    • If you chose B: The size of the particles is not the rule. What counts is the mass in each cm³. Go to S3.5.8.

  5. Why does a big log float while a tiny steel pin sinks?

    Tap your answer.

    1. Not this one. A big stone sinks in water. Size does not decide it. Density does. Go to S3.5.8. Try another.

    2. Not this one. The log has far more mass than the pin. It floats because it is less dense than water. Go to S3.5.8. Try another.

    3. Yes, that is right. Mass alone does not decide it. Each cm³ of wood has less mass than a cm³ of water. Each cm³ of steel has more.

    See the answer

    The wood is less dense than water, but the steel is denser than water.

    Mass alone does not decide it. Each cm³ of wood has less mass than a cm³ of water. Each cm³ of steel has more.

    • If you chose A: A big stone sinks in water. Size does not decide it. Density does. Go to S3.5.8.
    • If you chose B: The log has far more mass than the pin. It floats because it is less dense than water. Go to S3.5.8.

  6. From an earlier chapter

    A cup of water has a mass of 300 g. You stir in 20 g of sugar until it dissolves. What mass does a scale show now?

    Tap your answer.

    1. Not this one. The sugar is not gone. Its particles are still in the cup, so their mass counts. Go to S3.5.7. Try another.

    2. Yes, that is right. Nothing is lost when sugar dissolves. Add the masses. 300 + 20 = 320.

    3. Not this one. Dissolving does not take mass away. The sugar is still there, spread through the water. Go to S3.5.7. Try another.

    See the answer

    320 g

    Nothing is lost when sugar dissolves. Add the masses. 300 + 20 = 320.

    • If you chose A: The sugar is not gone. Its particles are still in the cup, so their mass counts. Go to S3.5.7.
    • If you chose C: Dissolving does not take mass away. The sugar is still there, spread through the water. Go to S3.5.7.

Keep it

Copy this into your exercise book.

Density is mass divided by volume, in g/cm³. A thing floats if it is less dense than the liquid and sinks if it is denser. Water is about 1 g/cm³.

Draw: Draw a jar with oil on top, water in the middle and honey below. Put a cork at the top and a stone at the bottom.

Key words, A to Z

Every key word in this book, what it means, and the chapter that teaches it.

boiling point
the temperature at which a liquid boils, with bubbles of gas all through it. S3.5.3
change of state
a material turning from a solid, liquid or gas into another of the three. S3.5.3
compress
squeeze into a smaller volume. S3.5.5
contract
get smaller when cooled, because the particles move closer together. S3.5.6
density
the mass in each cm³: mass ÷ volume. S3.5.8
diffusion
particles spreading out by themselves, from where there are many to where there are few. S3.5.2
displaces
pushes water out of its way, so the level rises by the volume of the thing. S3.5.8
dissolves
mixes into a liquid so completely that the particles cannot be seen. S3.5.2, S3.5.7
energy
what makes particles move. More energy means faster particles. S3.5.3
evidence
what we see or measure that supports an idea. S3.5.2
expand
get bigger when heated, because the particles move further apart. S3.5.6
fair test
a test that changes only one thing and keeps everything else the same. S3.5.4
flow
move along because the particles slide past each other. S3.5.1
mass
how much matter a thing holds. We measure it in grams. S3.5.8
melting point
the temperature at which a solid melts. It is also where the liquid freezes. S3.5.3
particle model
a picture that shows particles as circles and explains how matter behaves. S3.5.1
particles
the tiny bits that everything is made of. S3.5.1
pressure
the push of a gas on the walls of its container. It comes from particles hitting the walls. S3.5.5
soluble
able to dissolve in a liquid. S3.5.7
solute
the solid that dissolves. S3.5.7
solution
the clear mixture of a solute dissolved in a solvent. S3.5.7
solvent
the liquid that does the dissolving. S3.5.7
temperature
how hot something is. The hotter it is, the faster its particles move. S3.5.4
vibrate
shake to and fro in a tiny space. S3.5.1
volume
the space a thing takes up. S3.5.5, S3.5.8