Particles and their behaviour · MODEL

Gas pressure

What is actually pushing on the inside of a balloon?

Draft — not yet science-reviewed.

Start here

The marshmallow in the vacuum jar

You already know gas particles are far apart and moving fast. Here is what that lets them do. A marshmallow sits in a sealed jar. Air is pumped out of the jar and the marshmallow swells up. Air is let back in and it shrivels.

Nothing was added to the marshmallow. So why did it grow?

Your turn

Commit first. What causes the pressure inside a blown-up balloon?

Gas particles move fast in every direction. Each time one hits a wall it gives that wall a tiny push. There are so many hits each second that they add up to one steady push. That push is gas pressure. So the marshmallow has gas pushing out from inside. It has air pushing in from outside. Pump the outside air away and the inside push has nothing left to balance it. So the marshmallow swells.

Gas particles colliding with the walls of a container, with one collision arrowed.

Think again

The wrong idea, out loud: 'the particles are squashed up against each other, and that squashing is what pushes.' Predict before you run it — spread the same particles out into a bigger box. What happens to the pressure?

Make your prediction first — then the lab runs.

Every wall hit adds one to the counter. Watch the counter and the pressure gauge move together.

Words to know

pressure
How hard a force pushes on each bit of a surface.

You will meet pressure = force ÷ area as a calculation in Physics P5.

collision
When a moving particle hits something and bounces off.

Your turn

Now change something else instead. A sealed can of air is heated and the pressure inside rises. Why?

Think again

Name it: 'heating makes the particles themselves get bigger.' Now test it. If every particle really had grown, what would happen to the mass of gas in the sealed can?

Investigate

Here is the full instrument. Predict first: what happens to the pressure if you squash the gas into half the space?

Make your prediction first — then the lab runs.

Each wall hit is one push. More hits per second means more pressure. Change one control at a time.

Your turn

Three ways to raise the pressure, and one trap. Say why each one works before you tap it.

Marshmallow in a bell jar, before and after pumping.

Your turn

Back to the marshmallow, this time in full and in your own words. Explain why it swelled when the air was pumped out of the jar.

Check your answer
  • Says the gas inside the marshmallow pushes outwards
  • Says fewer particles outside means fewer collisions on the outside
  • Says the inside push is now greater than the outside push
  • Does not say the marshmallow 'gained air'

Mastery ladder

① Recall

What causes the pressure of a gas on the walls of its container?

② Apply

A sealed syringe of gas is squashed to half its volume. What happens to the pressure, and why?

③ Explain

Explain why the pressure inside a sealed can rises when it is heated.

Mark your answer against this list
  • Says heating gives particles more energy
  • Says the particles move faster
  • Says they hit the walls more often and harder
  • Says the number of particles has not changed

④ Produce

Predict what happens to a sealed, part-inflated balloon taken to the top of a mountain, and explain it.

Mark your answer against this list
  • Predicts the balloon expands
  • Says there are fewer air particles outside at altitude
  • Says fewer collisions on the outside surface
  • Says the inside pressure now exceeds the outside

Key note

Gas pressure is billions of particles hitting the walls every second. More particles, less space, or higher temperature all mean more collisions — and more pressure. The particles never change size.

Going further

Heating does two jobs at once. Most answers only mention one of them. Faster particles reach the walls more often. They also arrive harder, because they are moving faster when they land. Both of those raise the pressure. And not one extra particle was added to do it.

Your turn

Same two effects, somewhere they matter. Explain why an aerosol can carries the warning 'do not place on a fire'.

Check your answer
  • Heating makes the particles move faster
  • More and harder collisions with the walls
  • Pressure rises until the can may burst

Before this lesson

At GCSE this becomes

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