Skip to content

Particles in a gas

Heat and matter · level 5, lesson 3 of 6

A question to start

The air in a balloon pushes outwards in every direction and keeps it round. There is no spring inside. What is doing the pushing?

The idea

A gas is made of particles flying about at high speed, in straight lines, until they hit each other or the walls. Each time a particle bounces off a wall, it gives the wall a tiny push. Billions upon billions of pushes every second add up to a steady force: the gas's pressure.

pushgas

This particle model explains a lot:

  • More particles in the same space: more hits each second, more pressure.
  • Hotter gas: the particles move faster, hit harder and more often, more pressure.
  • Bigger container: the particles have further to go between hits, so fewer hits each second, less pressure.

Mars's air has about a hundredth as many particles in each cubic metre as Earth's, which is why its pressure is so low.

Worked examples

1. Pumping a tyre. Each push of the pump forces more particles in. More particles means more hits on the inside of the tyre: the pressure rises.

2. A balloon in the sun. The air inside warms; its particles move faster and push harder. The balloon swells.

3. Speeds. At 20 °C, the particles in air move at about 500 m/s on average, faster than a jet airliner.

Try it

  1. What causes the pressure of a gas?
  2. Why does pumping more air into a ball raise its pressure?
  3. Why does a gas's pressure rise when it is heated in a closed bottle?
  4. Why is Mars's air pressure so low?
Answers
  1. Particles hitting the walls.
  2. More particles means more hits on the inside each second.
  3. The particles move faster, so they hit the walls harder and more often.
  4. There are far fewer particles in each cubic metre to hit things.

Common mistake

"The particles in a gas get bigger when it is heated." The particles stay the same size. They move faster, and if the gas can expand, they spread further apart.

Where it leads