A change of state occurs when matter is converted from one physical state to another. For example, when water is heated, it changes from a liquid to a gas—when cooled water will eventually freeze into a solid: ice. A change of state is usually accompanied by a change in temperature and/or pressure.

Matter commonly exists in one of three forms, or states: solid, liquid, or gas. One fundamental way in which these three states differ from each other is the energy of the particles of which they are made. The particles in a solid contain relatively little energy and move slowly. The particles in a liquid are at a higher energy level and move more rapidly. The particles in a gas are at an even higher energy level and move most rapidly.

The state in which matter occurs can be changed by...