Chapter 4 of 8

All 8 chapters
  1. 1The particle model, readunderstood
  2. 2Evidence for particles, readunderstood
  3. 3Changes of state, readunderstood
  4. 4Diffusion and temperature, readunderstood
  5. 5Gas pressure, readunderstood
  6. 6Expanding and shrinking, readunderstood
  7. 7Dissolving and solutions, readunderstood
  8. 8Density, readunderstood

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

  • Compare how fast colour spreads in hot and in cold water
  • Explain why particles spread faster when they are warmer

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.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (865 KB). It opens on any phone.