Supercooling is the process of cooling water below its freezing point without it freezing. A liquid below its freezing point will crystallize by using a “crystal seed” or a nucleus to form the crystal around, but if the water is lacking a nucleus, the liquid will stay in that state even past its freezing point.
The process of supercooling can be rather simple, but it comes with many terms to know and understand. The first to know are phase changes. It's something quite elementary; we all learn about it at a young age. You have a solid, like ice, a liquid, like water, and a gas, like steam. You may think, “Oh, that's an easy one. Water freezes at 32 degrees Fahrenheit and water boils at 212 degrees Fahrenheit.” Yes, that is true, but a lot of energy goes into making these phase changes. It takes 80 cal/gm (calories per gram) to transition from ice to water, and 640 cal/gm to go from that water to steam. That's a good amount of energy.
The energy used to make these transitions are called the latent heat of fusion and the latent heat of vaporization. Heat of fusion is the energy required to change a gram of a substance from a solid to a liquid without changing its temperature. This energy will break down the solid bonds, but leaves energy to convert to a liquid state. Heat of vaporization is the energy required to change a gram of liquid into a gas. The energy breaks down the attractive forces and provides the extra energy necessary to expand the gas.
The phase change used in supercooling is freezing. Most liquids freeze by crystallization. This is a first-order thermodynamic phase transition. This means that, as long as solid and liquid coexist, the equilibrium temperature of the system remains constant and equal to the melting point. Crystallization occurs through two different steps: nucleation and crystal growth. Nucleation is the step when the molecules start to gather into clusters by arranging in a defined and periodic manner to define the crystal structure in which the crystal growth begins.
The terms above help to understand the concept of supercooling. Freezing point depression is the actual process of lowering the freezing point temperature of the liquid. The freezing point is lowered by adding another compound to it. The solution will have a lower freezing point than that of the pure solvent. An example of freezing point depression is the freezing point of seawater. It is lower than that of pure water. Because of the way freezing point works, it depends on the number of particles present, not the type of the particles present or their mass.
