Freezing-point depression

Lore & Background

Freezing-point depression is the reduction of the temperature at which a substance freezes, caused by the addition of a smaller amount of another, non-volatile substance. The added substance is termed the solute, while the original substance in greater quantity is the solvent. This phenomenon occurs in both liquid solutions and solid-solid mixtures. The underlying cause is a lowering of the chemical potential of the solvent in the mixture compared to the pure solvent, a difference proportional to the natural logarithm of the mole fraction. This effect is entropy-driven: the increased randomness of the solution opposes the ordering process of freezing, so a lower temperature is required to achieve equilibrium between the liquid and solid phases. In the solution, the solvent is diluted by the solute, reducing the concentration of solvent molecules available to freeze; equilibrium is re-established at a lower temperature where the rates of freezing and liquefying become equal. The solute does not physically block the solvent from solidifying but simply reduces the probability of freezing events. A common example is the addition of salt to water, used in ice cream makers and for de-icing roads. Sodium chloride can depress water’s freezing point to about -21 °C; at lower road temperatures, other salts like calcium chloride or magnesium chloride are employed. In airports, less corrosive media such as sodium formate or potassium acetate are used instead. Freezing-point depression also explains why sea water remains liquid below 0 °C, why antifreeze (ethylene or propylene glycol in water) protects car radiators, and why solder made by adding copper to molten silver flows at a lower temperature. Some cold-adapted organisms, such as the rainbow smelt and the spring peeper frog, produce high concentrations of compounds like glycerol or glucose to lower their internal freezing point, preventing solidification in extreme cold. The effect is also exploited in cryoscopy to measure molar mass or degree of dissociation.

Reader's Guide

Freezing-point depression is a colligative property with wide-ranging practical significance. It is used in automobile antifreeze (ethylene or propylene glycol in water), road salting (sodium chloride, calcium chloride, or magnesium chloride), and in ice cream makers. Some organisms, such as the rainbow smelt and spring peeper frog, produce antifreeze compounds like glycerol or glucose to survive freezing conditions. In the laboratory, freezing-point depression is employed in cryoscopy to measure molar mass or degree of dissociation, and in differential scanning calorimetry for purity analysis.

Did You Know?

• Freezing-point depression causes sea water to remain liquid at temperatures below 0 °C (32 °F), the freezing point of pure water.
• Sodium chloride can depress the freezing point of water to about −21 °C (−6 °F).
• The spring peeper frog (Pseudacris crucifer) releases massive amounts of glucose into its blood in response to freezing temperatures.