The specific heat capacity Cp has a maximum at approximately -45 degrees Celsius

Scientific Explanation

When liquid water is cooled below the freezing point without crystallizing (supercooling), the specific heat capacity Cp rises dramatically. At about minus 45 degrees Celsius, Cp reaches a sharp maximum before declining at still lower temperatures. This divergence of Cp is one of the strongest pieces of evidence for the existence of a second critical point in water.

Near a critical point, fluctuations between different structural states become maximal. In water, this means molecules switch ever more vigorously between a low-density, open configuration (LDL) and a high-density, compact configuration (HDL). Each switch absorbs or releases energy — and it is precisely this that drives Cp upward.

Experimental verification is extremely challenging because water crystallizes almost instantaneously at these temperatures. Only with ultrafast laser techniques that heat and measure water within nanoseconds has it been possible to trace the Cp rise close to the maximum.

Everyday Relevance

This Cp maximum lies in a temperature range that is hardly accessible in everyday life, but its effects reach into our temperature range. The anomalously high Cp of water at room temperature is, in a sense, an echo of this maximum: the structural fluctuations that peak at minus 45 degrees are still visible at 20 degrees Celsius — weaker, but still stronger than in any normal liquid.