This article talks about the melting and freezing point depression due to mixtures of compounds. For depression due to small particle size, see melting point depression.

Freezing-point depression describes the process in which adding a solute to a solvent decreases the freezing point of the solvent. Examples include salt in water, alcohol in water, or the mixing of two solids such as impurities in a finely powdered drug. In the last case, the added compound is the solute, and the original solid is thought of as the solvent. The resulting solution or solid-solid mixture has a lower freezing point than the pure solvent or solid. This phenomenon is what causes sea water, (a mixture of salt (and other things) in water) to remain liquid at temperatures below 0 °C (32 °F), the freezing point of pure water.