Solids, liquids and gases are three states of matter. In solids, the particles are tightly packed together. In liquids, the particles have more movement, while in gases, they are spread out.
A gas has no definite shape or volume. If unconfined, the particles of a gas will spread out indefinitely; if confined, the gas will expand to fill its container. When a gas is put under pressure by reducing the volume of the container, the space between particles is reduced and the gas is compressed.
