The density of supercooled water has a minimum

Scientific Explanation

Under certain conditions, water can remain liquid well below its normal freezing point — a state known as supercooling. In this supercooled state, a remarkable trend continues: the density, which already decreases upon cooling below 4 degrees Celsius, drops further and reaches a minimum around minus 30 to minus 35 degrees Celsius.

Below this minimum, density begins to increase again. This behavior points to a fundamental transformation in the structure of liquid water. In the supercooled region, increasingly larger domains of ice-like tetrahedral order form. At the lowest accessible temperatures (before spontaneous crystallization sets in), the fraction of these ordered structures grows so large that the density passes through a minimum.

This density minimum is discussed as evidence for a possible second critical point in the phase diagram of water, where two distinct liquid phases — a high-density and a low-density form — could be distinguished. This hypothetical critical point lies deep in the supercooled region and remains extremely difficult to access experimentally.

Everyday Relevance

Supercooled water is more common than most people realize. In clouds, tiny droplets can remain liquid down to minus 30 degrees Celsius or below. This supercooled water plays a central role in rain and hail formation. Freezing rain occurs when supercooled droplets instantly crystallize upon impact with cold surfaces.

The density minimum in this temperature range affects atmospheric flow dynamics and provides clues to the fundamental nature of water as a liquid — a topic that remains under active investigation.