Freezing

Freezing, or solidification, is a phase transition in which a liquid turns into a solid when its temperature is lowered below its freezing point

For most substances, the melting and freezing points are the same temperature; however, certain substances possess differing solid–liquid transition temperatures. For example, agar displays a hysteresis in its melting and freezing temperatures. It melts at 85 °C (185 °F) and solidifies from 31 °C to 40 °C (89.6 °F to 104 °F).

Crystallization

Most liquids freeze by crystallization, formation of crystalline solid from the uniform liquid. This is a first-order thermodynamic phase transition, which means that, as long as solid and liquid coexist, the temperature of the whole system remains very nearly equal to the melting point due to slow removal of heat when in contact with air, which is a poor heat conductor. Because of the latent heat of fusion, the freezing is greatly slown down and the temperature will not drop anymore once the freezing starts but will continue dropping once it finishes. Crystallization consists of two major events, nucleation and crystal growth. Nucleation is the step wherein the molecules start to gather into clusters, on the nanometer scale, arranging in a defined and periodic manner that defines the crystal structure. The crystal growth is the subsequent growth of the nuclei that succeed in achieving the critical cluster size.