Primarily there are three different phases of matter: solid, liquid, and gas. On changing the temperature, one state changes into another. This phenomenon is called phase change, which is reversible in nature. On heating, a solid changes to liquid, this process is called melting, while vice-versa is called freezing. The heat absorbed or released in this transition for a material of mass 'm' can be evaluated using the heat of fusion, which is the energy per unit mass required or released in phase change. Similarly, if a liquid is heated, it converts to a gas, and the process is called vaporization or boiling. On cooling, the change of a gas into a liquid is called condensation. Energy per unit mass required or released in phase change between liquid and gas is called heat of vaporization, where Q is the heat absorbed or released. Some of the materials like dry ice can directly have a transition from solid to gas, which is called sublimation.