This Demonstration shows the effect of dissolved sodium chloride on the freezing point of water. Pure water, the solvent, freezes and forms a crystal lattice at zero degrees Celsius. Adding a solute, such as sodium chloride, disrupts the crystal lattice formation, resulting in a decreased freezing point. This phenomenon is represented by the equation
is the freezing point of the solution,
is the cryoscopic constant of the solvent,
is the molality,
is the van 't Hoff factor, and
is the freezing temperature of the pure solvent. This linear relation is approximately valid for moderate solute concentrations. For higher concentrations, the dependence is more complex, depending on the activities
of the solutions, as described, for example, by the Debye–Hückel theory.