Thermal properties of materials refer to the response of materials to changes in their temperature and to the application of heat. As a solid absorbs energy in the form of heat, its temperature rises, and its dimensions increase. But different materials react to the application of heat differently.
Heat capacity, thermal expansion, and thermal conductivity are properties that are often critical in the practical use of solids.
Latent Heat of Fusion of Materials
In the case of solid to liquid phase change, the change in enthalpy required to change its state is known as the enthalpy of fusion (symbol ∆Hfus; unit: J), also known as the (latent) heat of fusion. Latent heat is the amount of heat added to or removed from a substance to produce a phase change. This energy breaks down the attractive intermolecular forces and must provide the energy necessary to expand the system (the pΔV work).
The liquid phase has higher internal energy than the solid phase. This means energy must be supplied to a solid to melt it, and energy is released from a liquid when it freezes because the molecules in the liquid experience weaker intermolecular forces and so have higher potential energy (a kind of bond-dissociation energy for intermolecular forces).
The temperature at which the phase transition occurs is the melting point. The melting point also defines a condition in which the solid and liquid can exist in equilibrium. Adding heat will convert the solid into a liquid with no temperature change. At the melting point, the two phases of a substance, liquid, and vapor, have identical free energies and therefore are equally likely to exist. Below the melting point, the solid is the more stable state of the two, whereas above the liquid form is preferred. The melting point of a substance depends on pressure and is usually specified at standard pressure. When considered as the temperature of the reverse change from liquid to solid, it is called the freezing point or crystallization point.
Heat of fusion in the periodic table