Chapter 5 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

S3.5.5 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

  • Explain gas pressure as particles hitting the walls of a container
  • Explain why a ball or a tyre feels harder on a hot day

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.

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.