The air that you breathe, the helium in a balloon, and the neon
inside the tube in a neon light are gases. A gas is a substance with no
definite volume and no definite shape. Solids and liquids have
volumes that do not change easily. A gas, on the other hand, has a volume that changes to match the volume of its container.
Gas Composition
The molecules in a gas are very far apart compared with the molecules
in a solid or a liquid. The amount of space between the molecules in a
gas can change easily. If a rigid container—one that cannot change its
shape—has a certain amount of air and more air is pumped in, the
volume of the gas does not change. However, there is less space between
the molecules than there was before. If the container is opened, the
molecules spread out and mix with the air in the atmosphere.
As you saw, gas molecules in a container can be compared to a
group of people in a room. If the room is small, there is less space
among people. If the room is large, people can spread out so that
there is more space among them. When people leave the room, they
go in all different directions and mix with all of the other people in
the surrounding area