We use the term phase to describe a specific state of matter, such as a solid, liquid, or gas. A phase of matter is when all the physical properties within a material are uniform. At a given phase, a material will have the same density and refractive index. The transitions from one state to another are accompanied with either absorption or liberation of heat and change of volume. Change of state is not accompanied with change in temperature. Which means during change of state the speed and hence the kinetic energy of the molecules also remains constant. The absorbed or liberated heat energy is used in breaking or forming the bond between the molecules which in turn affects the potential energy of the molecules.

Substances may exist in three states: solid, liquid and gas states. For Example, water exists in the solid state as ice, in the liquid state as water, and in the gaseous state as steam or vapor. A transition from one phase to another is called a phase change. A change from one phase to another takes place under conditions of phase equilibrium between the two phases, and for a given pressure this occurs at only one specific temperature.

Key term

• The latent heat of fusion of a substance is the energy involved in changing the state of unit mass of the substance at the melting/freezing point. Q = mL_f
• The latent heat of vaporization of a substance is the energy involved in changing the state of unit mass of the substance at the boiling point. Q = mL_V