Chapter 7 of 8

All 8 chapters
  1. 1Signs of life, readunderstood
  2. 2Seeing small things, readunderstood
  3. 3Animal and plant cells, readunderstood
  4. 4What each part does, readunderstood
  5. 5Cells with special jobs, readunderstood
  6. 6Cells, tissues and organs, readunderstood
  7. 7Diffusion, readunderstood
  8. 8Living things with one cell, readunderstood

S3.1.7 Chapter 7

Diffusion

Why can you smell cooking from across a room?

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

  • Explain diffusion as spreading from a crowded place to a less crowded one
  • Show diffusion with a drop of ink or tea in still water

Someone cooks in one corner of a room. After a little while, you can smell the food at the other side. Nobody carried the smell across. How did it get there?

Everything is made of tiny particles. In a gas the particles are far apart, and they are always moving. A smell is made of particles too.

Moving air carries a smell much of the way. But even in still air, the smell particles spread on their own. That spreading is what this chapter is about.

Draw it first

Draw a smell spreading

  1. A box of air shown three times, with a key above. Open circles are air and dark dots are smell. At the start, all the dark dots are crowded in one corner. A little later they have spread part of the way. Much later they are mixed all through the box. Labels: air, smell, start, a little later, much later.airsmellstart

    1 Draw a box of air. Put the smell, dark dots, close together in one corner.

  2. A box of air shown three times, with a key above. Open circles are air and dark dots are smell. At the start, all the dark dots are crowded in one corner. A little later they have spread part of the way. Much later they are mixed all through the box. Labels: air, smell, start, a little later, much later.airsmella little later

    2 Draw the same box a little later. The dark dots have moved out into the empty places.

  3. A box of air shown three times, with a key above. Open circles are air and dark dots are smell. At the start, all the dark dots are crowded in one corner. A little later they have spread part of the way. Much later they are mixed all through the box. Labels: air, smell, start, a little later, much later.airsmellmuch later

    3 Draw it much later. The dark dots are spread through the whole box.

The smell particles bump about, and they move in every direction. Near the pot there are many of them, so many bump into their neighbours and move away. Few come back. So the smell spreads out into the less crowded air.

This spreading is called diffusion. Particles spread from a place where they are crowded to a place where they are less crowded. They do it by their own movement, with no stirring.

In the end the particles are spread evenly. They do not stop moving. Just as many go one way as the other, so you see no more change.

Diffusion happens in liquids too, but it is much slower. Do not drop colour in from the top. Colour is heavier than water, so it sinks in streaks. That is not diffusion. Lay a little colour at the bottom of still water instead, and watch it.

Three cups of still water, each with the same colour laid at the bottom. The colour is hatched. At the start it is a small layer at the bottom. Later it reaches a little way up. Much later, after many days, it reaches all through the water, but paler, so it is shown with dots. Labels: start, later, much later.startlatermuch later
Colour spreads up through still water, from where it is crowded.

In still water, diffusion is slow. The edge of the colour may creep up only a centimetre or two in a day. A small shake or a warm spot makes currents that mix it faster. So keep the cup still to see diffusion alone.

See it three ways

With things

Lay a little colour at the bottom of a jar of still water. Look after an hour and after a day. The colour has spread a little further up each time.

As a picture

Two cups of water seen close up, with a key above. Open circles are water and dark dots are ink particles. At the start the dark dots are bunched in the bottom left corner. Later they have spread up and across the cup. Labels: water, ink, start, later.waterinkstartlater

In words

Ink particles move among the water particles. They spread from where there are many ink particles to where there are few. This goes on until they are mixed evenly.

Your cells use diffusion. A cell uses up oxygen, so there is less oxygen inside than outside. Oxygen spreads in through the membrane. The cell makes carbon dioxide, so there is more inside than outside. It spreads out.

An animal cell. An arrow comes in from the left, labelled oxygen in, and goes into the cytoplasm. A second arrow leaves the cytoplasm to the right, labelled carbon dioxide out. Labels: oxygen in, CO₂ out.oxygen inCO₂ out
A cell takes in oxygen and lets out carbon dioxide by diffusion.

Here is the membrane, with oxygen on both sides. The arrow shows the way that more of the oxygen goes.

A cell membrane drawn as a dashed line with gaps, with the outside on the left and the inside on the right. There are 14 dark dots of oxygen outside and 4 inside. An arrow below points from the outside to the inside, labelled more go this way. Labels: outside, inside, more go this way, oxygen.outsideinsidemore go this wayoxygen
More oxygen is outside the cell, so more goes in than comes out.

Try it

Colour at the bottom of still water

You need: a clear glass or jar, water, ink or food colour, a thin straw or a hollow grass stem, a ruler, a place where the glass will not be knocked.

Be safe: Ink and food colour stain. Cover the table. Keep them away from your eyes and mouth. Do not drink the water. Wash your hands after.

  1. Fill the glass with water. Leave it for ten minutes on a table, so the water is still.
  2. Dip the straw a little way into the colour. Put a finger over the top end of the straw, to trap the colour inside.
  3. Lower the straw slowly to the bottom of the glass. Lift your finger so the colour runs out. Lift the straw out gently. Do not stir.
  4. Look at once. Measure with the ruler how high the colour reaches, in centimetres. Look again after an hour, and the next day.

What you will see: The colour stays low, because it is heavier than water. Its top edge slowly turns blurred and creeps upward. This is diffusion, and it is slow. It takes a day to move a centimetre or two.

If you cannot try it, this is what happens.
A table of what you see in the glass. At once, a dark patch at the bottom. After one hour, the top edge looks blurred. The next day, the colour has crept higher. Labels: when, you see, at once, 1 hour, next day, dark at bottom, edge blurred, creeps higher.whenyou seeat oncedark at bottom1 houredge blurrednext daycreeps higher

A wrong idea many people have

When the ink is spread evenly, the particles have stopped moving.

Why it feels right: Nothing seems to change any more, so it seems that nothing moves.

The test that breaks it: Particles never stop moving. In an even mixture, just as many ink particles move one way as the other. Lay a little more colour at the bottom and you see it spread again.

What is true: The particles keep moving. The mixture only looks still because there is no more net change.

Key words

particle
one of the tiny pieces that everything is made of.
diffusion
the spreading of particles from where they are crowded to where they are less crowded.

Back to your guess

The answer is B: Smell particles spread out by themselves, from where they are crowded.

Smell particles move all the time. They spread from the crowded place near the pot into the emptier air, until they reach you. This spreading is called diffusion.

Check yourself

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

  1. In diffusion, which way do particles spread?

    Tap your answer.

    1. Not this one. That is the wrong way round. More particles leave the crowded place, so the spreading goes away from it. Go to S3.1.7. Try another.

    2. Not this one. Particles move in every direction. Diffusion goes from crowded to less crowded, up or down. Go to S3.1.7. Try another.

    3. Yes, that is right. More particles move out of the crowded place than into it. So they spread out until they are even.

    See the answer

    From a crowded place to a less crowded place.

    More particles move out of the crowded place than into it. So they spread out until they are even.

    • If you chose A: That is the wrong way round. More particles leave the crowded place, so the spreading goes away from it. Go to S3.1.7.
    • If you chose B: Particles move in every direction. Diffusion goes from crowded to less crowded, up or down. Go to S3.1.7.

  2. Why can you smell cooking from across a room?

    Tap your answer.

    1. Yes, that is right. Smell particles move in all directions. They spread from the crowded place near the pot into the rest of the room.

    2. Not this one. Your nose does not pull. The smell particles move out from the pot on their own. Go to S3.1.7. Try another.

    3. Not this one. Moving air helps a lot, but even in still air the particles spread by themselves, only more slowly. Go to S3.1.7. Try another.

    See the answer

    The smell particles spread out by diffusion.

    Smell particles move in all directions. They spread from the crowded place near the pot into the rest of the room.

    • If you chose B: Your nose does not pull. The smell particles move out from the pot on their own. Go to S3.1.7.
    • If you chose C: Moving air helps a lot, but even in still air the particles spread by themselves, only more slowly. Go to S3.1.7.

  3. A little colour is laid at the bottom of still water. You leave it for a day. What happens?

    Tap your answer.

    1. Yes, that is right. The colour particles move among the water particles and spread out. In still water this takes a long time.

    2. Not this one. The colour particles keep moving, so they spread. It is slow, but it does happen. Go to S3.1.7. Try another.

    3. Not this one. Stirring spreads it in seconds. In still water, diffusion is slow. It takes days. Go to S3.1.7. Try another.

    See the answer

    The colour slowly spreads up through the water on its own.

    The colour particles move among the water particles and spread out. In still water this takes a long time.

    • If you chose B: The colour particles keep moving, so they spread. It is slow, but it does happen. Go to S3.1.7.
    • If you chose C: Stirring spreads it in seconds. In still water, diffusion is slow. It takes days. Go to S3.1.7.

  4. Look at the picture of the membrane. Which way do most of the oxygen particles move?

    A cell membrane with 14 dark dots of oxygen outside on the left and 4 inside on the right. An arrow points from the outside to the inside. Labels: outside, inside, more go this way, oxygen.outsideinsidemore go this wayoxygen
    Tap your answer.

    1. Not this one. Out is from the less crowded side. Diffusion goes from the crowded side, which is outside. Go to S3.1.7. Try another.

    2. Yes, that is right. There are more oxygen particles outside, so more of them move in than out.

    3. Not this one. Particles are always moving. Here more move in than out. Go to S3.1.7. Try another.

    See the answer

    Into the cell.

    There are more oxygen particles outside, so more of them move in than out.

    • If you chose A: Out is from the less crowded side. Diffusion goes from the crowded side, which is outside. Go to S3.1.7.
    • If you chose C: Particles are always moving. Here more move in than out. Go to S3.1.7.

  5. The colour has spread evenly through the water. What are the colour particles doing?

    Tap your answer.

    1. Not this one. Particles never stop. The mixture only looks still because just as many go one way as the other. Go to S3.1.7. Try another.

    2. Not this one. Particles move in every direction. If they all went one way, the colour would not stay spread out. Go to S3.1.7. Try another.

    3. Yes, that is right. Particles keep moving. Evenly spread means that just as many go each way, so you see no more change.

    See the answer

    They are still moving about in every direction.

    Particles keep moving. Evenly spread means that just as many go each way, so you see no more change.

    • If you chose A: Particles never stop. The mixture only looks still because just as many go one way as the other. Go to S3.1.7.
    • If you chose B: Particles move in every direction. If they all went one way, the colour would not stay spread out. Go to S3.1.7.

  6. From an earlier chapter

    Which part of a cell decides what goes in and out?

    Tap your answer.

    1. Not this one. The wall only holds the shape. Water goes through it easily. The membrane decides. Go to S3.1.4. Try another.

    2. Not this one. The nucleus controls the cell from the inside. It is not at the edge. Go to S3.1.4. Try another.

    3. Yes, that is right. The thin membrane lets in what the cell needs and lets out waste.

    See the answer

    The cell membrane

    The thin membrane lets in what the cell needs and lets out waste.

    • If you chose A: The wall only holds the shape. Water goes through it easily. The membrane decides. Go to S3.1.4.
    • If you chose B: The nucleus controls the cell from the inside. It is not at the edge. Go to S3.1.4.

  7. In the picture, 14 oxygen dots are outside the cell and 4 are inside. How many more are outside than inside?

    See the answer

    10 more

    Take the smaller count from the larger count. 14 − 4 = 10.

Keep it

Copy this into your exercise book.

Diffusion is the spreading of particles from where they are crowded to where they are less crowded. It needs no stirring. In still water it is slow. Cells take in oxygen by diffusion.

Draw: Draw a box with dark dots crowded in one corner. Draw it again later with the dots spread out.

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 (754 KB). It opens on any phone.