The freezing point is the temperature at which a liquid begins to solidify; it is not always well-defined or equal to the melting point. The freezing point usually rises with pressure, solids being slightly denser than liquids, though water is a notable exception; it is lowered by solutes in the liquid, the amount providing an accurate means of determining relative molecular mass.

The solid separating from a solution usually has a different composition from the liquid, and repeated freezing can be used to separate substances. Pure substances can often be "supercooled" below their freezing point for limited periods, as the formation of the solid crystal requires enucleation by rough surfaces in contact with or particles suspended in the liquid.