Energy can affect the physical properties of a substance in various ways. For example, heating a substance can cause it to expand, whereas cooling it can cause it to contract. The state of matter of a substance can also be changed by adding or removing energy. For instance, adding energy to a solid can cause it to melt into a liquid, and further adding energy can cause the liquid to vaporize into a gas. The amount of energy required to change the state of a substance is referred to as its heat of transformation.