Cryoscopic Constant of Water

Dissolve anything in water and the freezing point drops in proportion to the number of dissolved particles, not their identity: ΔT_f = i·K_f·m, with K_f = 1.86 K·kg/mol. Sodium chloride splits into two ions, so its van 't Hoff factor is close to 2 and a 1 molal solution freezes near −3.7 °C, which is exactly why road salt works and why it stops working below about −10 °C, where the practical limit of saturated brine is reached.

François-Marie Raoult established the proportionality in the 1880s, and it became the standard way of measuring molar masses for the next forty years — dissolve a known mass, measure the freezing point drop, and the molar mass falls out. The technique still survives as osmometry in clinical laboratories, where freezing-point depression measures the osmolality of blood and urine to a few milliosmoles. It is a colligative property in the strict sense: 1 mole of sugar and 1 mole of urea depress the freezing point identically.