# Solids

Property of solidsWhy they are like this
They have a fixed shape and cannot flowThe particles cannot move from place to place
They cannot be compressed or squashedThe particles are close together and have no space to move into

## Liquids

Property of liquidsWhy they are like this
They flow and take the shape of their containerThe particles can move around each other
They cannot be compressed or squashedThe particles are close together and have no space to move into

## Gases

Property of gasesWhy they are like this
They flow and completely fill their container The particles can move quickly in all directions
They can be compressed or squashedThe particles are far apart and have space to move into

## Pressure and temperature

Imagine a gas is trapped inside a container which has a fixed size (its volume cannot change).

If the gas is heated the particles will gain kinetic energy which will make them move faster. This means they will collide with the walls more frequently.

This causes the force on the walls of the container to increase and so the increases.

## Applications of kinetic theory

There are many places where we can use kinetic theory to describe what is happening to the gases involved.

### Scuba diving

A diver uses tanks of air to breathe under water. When the tanks are filled, the air is transferred from the large volume of the outside air into the restricted, small volume of an oxygen tank. This means that as the volume decreases, the pressure inside the tanks increases as the particles are forced into a small space.

### Tyre pressure

When a car's tyres (or a bicycle or motorbike etc) go flat, there is less air inside the tyres than we need there to be – the pressure inside the tyres is low.

To increase the air inside, we need to pump air in. As we pump air in, more particles are forced into a volume that stays the same and the pressure inside the tyres increases.

### Aircraft

When an aircraft flies at a high altitude, the atmospheric pressure outside the cabin is very low. In fact, the pressure is so low that it would lead to altitude sickness in all of the passengers and crew. To avoid this happening, air is pumped from outside into the cabin, increasing the pressure inside and simulating the pressure we experience at ground level.