Adding thermal energy by heat increases the temperature of a substance. But surprisingly, there are situations where adding energy does not change the temperature of a substance at all! Instead, the additional thermal energy acts to loosen bonds between molecules or atoms and causes a phase change. Because this energy enters or leaves a system during a phase change without causing a temperature change in the system, it is known as latent heat (latent means hidden).
As earlier discussed there are three phases of matter frequently encountered: solid, liquid and gas. Solid has the least energetic state, atoms in solids are in close contact, with forces between them that allow the particles to vibrate but not change position with neighbouring particles. (These forces can be thought of as springs that can be stretched or compressed, but not easily broken.) Liquid has a more energetic state, in which particles can slide smoothly past one another and change neighbours, although they are still held together by their mutual attraction. Gas has a more energetic state than liquid, in which particles are broken free of their bonds. Particles in gases are separated by distances that are large compared with the size of the particles.
During a phase change, matter changes from one phase to another, either through the addition of energy by heat and the transition to a more energetic state, or from the removal of energy by heat and the transition to a less energetic state.
Energy is required to melt a solid because the bonds between the particles in the solid must be broken. Since the energy involved in a phase change is used to break bonds, there is no increase in the kinetic energies of the particles, and therefore no rise in temperature. Similarly, energy is needed to vaporize a liquid to overcome the attractive forces between particles in the liquid. There is no temperature change until a phase change is completed. The temperature of a cup of soda and ice that is initially at 0°C stays at 0°C until all of the ice has melted. In the reverse of these processes (freezing and condensation) energy is released from the latent heat.