The heat of melting of water has a maximum at -17 degrees Celsius

Scientific Explanation

The heat of melting (enthalpy of fusion) tells us how much energy is needed to convert ice into liquid water. At the normal melting point of 0 degrees Celsius, this value is about 6.0 kilojoules per mole. Surprisingly, this is not the maximum: under pressure, when water melts at lower temperatures, the enthalpy of fusion rises and reaches its peak at approximately minus 17 degrees Celsius.

This behavior arises from the growing structural difference between ice and supercooled liquid water. At minus 17 degrees, the ice crystal lattice is denser and more stable, while the supercooled liquid adopts an increasingly open, ice-like local order. The energy gap between the two phases is at its greatest at this temperature.

Below minus 17 degrees, the structures of the liquid and solid phases converge again — the liquid water becomes ever more ice-like. Consequently, the enthalpy of fusion decreases once more and would eventually approach zero at the temperature of the hypothetical second critical point.

Everyday Relevance

This maximum has practical implications for freezing technology and food preservation. During flash-freezing of food, ice crystals pass through exactly this temperature range. The elevated heat of melting at minus 17 degrees means that particularly large amounts of energy must be removed in this range — a factor that engineers account for when designing cooling systems.