The particles of a solid tightly pack together through attractive forces and vibrate at fixed positions without disrupting the lattice. 

The addition of heat causes the particles’ thermal energy to rise, and they vibrate faster. The particles move about and rearrange by partially overcoming the intermolecular forces. Subsequently, the lattice collapses, and the solid melts.

This transition from solid to liquid is called melting or fusion, and the temperature at which it occurs is called the melting point or the fusion point. 

The change in enthalpy that is required to completely melt 1 mole of a solid at its melting point is called its molar heat of fusion or its molar enthalpy of fusion. Since melting nearly always requires energy, it is an endothermic process with a positive enthalpy value — with a few exceptions.

For example, when a mole of ice absorbs 6.02 kilojoules of heat energy from its surroundings, its temperature increases. When the temperature hits 0 °C, it begins to melt.

For any substance, the heat of fusion is lower than the heat of vaporization. For instance, while melting a mole of ice requires merely 6.02 kilojoules of energy, vaporizing a mole of water requires 40.8 kilojoules of energy. 

This is because vaporization involves the complete separation of molecules by breaking free from nearly all intermolecular forces. In comparison, melting involves only partially overcoming the attractive forces while the molecules continue to stay in close contact.

The reverse of fusion, that is, the transition from liquid to solid, is called freezing or solidification. When liquid-phase molecules lose energy, their thermal motion decreases and the molecules pack close enough to re-establish the intermolecular forces.

Eventually, the liquid converts into its solid form. Freezing is an exothermic process, and its enthalpy value is negative — with a few exceptions.

Substances typically freeze at about the same temperature at which they melt. Although the enthalpy of freezing is negative, its magnitude is the same as the enthalpy of fusion.

When a substance is held at its melting point or freezing point, the solid and liquid phases coexist.