Chemical changes within matter always use energy; physical changes sometimes use energy, though they use much less energy than chemical changes. Physical changes use energy to change the physical state of a substance. Chemical changes either release or absorb energy when changing a substance into a completely new one.
In a physical change, the particles of a substance are not broken up. These particles may be rearranged to change the form of a substance, but the substance itself is never changed into another substance. Physical changes are easily reversed. When water is boiled and changes from a liquid to a gas, the molecules rearrange themselves but the hydrogen and oxygen in the water molecules are not separated. Mass is always conserved in a physical change. Energy may be released or absorbed when a substance changes from one physical state to another.
In a chemical change, a chemical reaction yields a completely new substance. A substance's particles are changed during a chemical reaction. In a chemical reaction that causes the decomposition of a substance, energy is released. Energy is absorbed in a chemical reaction that results in the formation of a new substance. Chemical changes are not easily reversed. Mass is conserved in a chemical reaction, though the molecule count may change.