The four states of matter are solid, liquid, gas and plasma. The state of matter of a substance can be changed by adding or removing heat.
A solid, such as a steel bar, has a defined shape and volume. A liquid, such as water, has a defined volume but takes the shape of its container A gas has neither a defined volume nor a defined shape. Examples of gases include oxygen, helium and air. Plasma also does not have a defined volume or shape. Plasma has distinct properties differing it from gas, as its electrons have been separated from its nuclei. Plasma can be seen in neon signs and fluorescent lights. The stars are also made of plasma.