Air, helium and chlorine are gases at room temperature. Water vapour is water as a gas. The particles in a gas are:

  • far apart
  • arranged in a random way
Gas particles are far apart in a random pattern, and can move freely and quickly.Particles in a gas

The particles in a gas can:

  • move quickly in all directions

There are no bonds between the particles in a gas, so they are free to move in any direction.

The table shows some of the properties of gases and why they are like this:

They flow and completely fill their containerThe particles can move quickly in all directions
They can be compressed (squashed)The particles are far apart and have space to move into

Gas pressure

The particles in a gas move quickly in all directions, but they do not get far before they bump into each other or the walls of their container. When gas particles hit the walls of their container they cause pressure. If the temperature is increased, the particles in a gas move faster, so they hit the walls of the container more often. This causes the pressure to rise. This is also why the pressure of a gas also increases when the volume of its container is decreased.

Shows gas particles moving around in a container. Some bounce off the sidesParticles in a gas colliding with their container wall

Particle arrangements in a solid, a liquid and a gas