Phase Transitions: Melting and Freezing

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.