Gases are identified as materials that
- have no definite shape
- have no definite volume
- can be easily compressed
- can flow
In a gas the particles move in all directions and the bounce off one another, causing the particles to spread out further and further.
If the gas is held in a container the particles will bounce off the walls of the container as well as each other.
Water vapour is an example of a gas. Water vapour usually is invisible.