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4.3 Pressure in gases

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4.3 Pressure in gases

Summary: Look at Figure 1. Have you everblown a balloon up until it bursts? Tags: science oxford-science student-book-7 physics Created: 2026-05-19 Last Updated: 2026-05-19

After this topic, you will be able to:

  • describe how volume and température affect gas Pressure
  • describe atmospheric pressure
  • describe how atmospheric pressure changes with height.

Think back

  1. Name the force that slows down objects moving through the air.
  2. What is an unbalanced force?
  3. Write down what happens to the Speed of gas Molecules as you heat the gas.

Key idea Gas or air pressure is produced when particles hit (or collide with) a surface. The more collisions per second, the higher the pressure.

Key words gas pressure, compressed, atmospheric pressure, density

Look at Figure 1**. Have you everblown a balloon up until it bursts?**

A man blowing an inflated balloon.

Figure 1There is lots of gaspressure inside a balloon.

What is gas pressure?

When you blow up a balloon,there are millions of air moleculeshitting the inside of the balloon. The collisions (hits) between the airmolecules and the balloon producegas pressure (air pressure). Lots ofcollisions per second make a high gas pressure. This is because there is a big force over a small area. Gas pressure is exerted (applied) in all directions.

How do temperature and volume affect pressure?

When a gas cools down, its molecules move more slowly. If the container does not change shape, the pressure goes down. There are fewer collisions with the sides of the container.

Two jars, which are the same size and have the same number of molecules inside. One jar is labelled as hot gas and shows faster-moving molecules, more collisions and higher pressure. The other jar is labelled as cold gas and shows slower-moving molecules, fewer collisions and lower pressure.

Figure 2Hot gas and cool gas.

A Suggest what happens to a balloon if you warm it up.

If you squash (compress) a gas into a smaller volume, there willbe more collisions between the gas molecules and the walls of thecontainer. The pressure increases.

Two jars, which are different sizes but have the same number of molecules inside. The jar with a large volume shows fewer collisions and smaller pressure. The jar with a smaller volume shows more collisions and bigger pressure.

Figure 3The effect of volume on gas pressure.

What is a compressed gas?

When you blow up a bicycle tyre, you increase the gas pressure. Asyou blow more gas into a container, the gas becomes compressed. There are more molecules in the same space, so there are morecollisions. The pressure is bigger. You need a strong container tohold a compressed gas.

B Write down what happens to the force on the inside of the tyreas you blow in more gas.

What is atmospheric pressure? How does it change with height?

There is air all around you. The airexerts (applies) a pressure on yourbody all the time. This is calledatmospheric pressure. You do not feel the pressure. It is cancelled outby the pressure of the gases andliquids in your body pushing out. However, you can see the effects ofatmospheric pressure in Figure 4.

Two conical flasks containing marshmallows. In the flask on the left, the marshmallows are a normal size. In the flask on the right, the marshmallows have expanded and are much bigger.

Figure 4Marshmallows contain pockets of air that expand when you remove the air around them.

Take a look at Figure 5. Theatmospheric pressure at sea level is bigger than the atmospheric pressure high up a mountain. Gravitypulls the air particles towards Earth. Where the particles are closer,there are more collisions. The pressure is higher. Density is howclose the particles are in a substance. Gas has a higher density atsea level.

A diagram showing the atmospheric pressure on the ground and on the top of mountain.

Figure 5Atmospheric pressure. Full text alternative to the content

Mountain climbers may find it hard to get enough oxygen. They may take oxygen tanks (containers) when they climb. The tanks contain oxygen gas that is compressed into a small volume.

C Compare the mass of gas in 1 m3 at the top and at the bottom of a mountain.

Summary questions

  1. Copy and complete the sentences. A gas exerts a pressure on the walls of its container because the particles collide with/ stick to the walls. If the gas gets hotter, the pressure will be bigger/smaller. If the volume gets bigger, the pressure will be bigger/ smaller. As you go up a mountain, the air pressure is bigger/smaller because there are fewer/more gas particles.
  2. A climber climbs a mountain. Explain why they might take a tank of oxygen with them.Explain why the oxygen needs to be compressed.
  3. A teacher heats some water in a drinks can until it is boiling. Steam comes out of the can. The teacher quickly turns the can over and puts it into some cold water. Describe and explain what happens to the pressure inside the can when it enters the cold water.Explain why the can collapses.

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