Ocean water

Why does the water deep inside the oceans not freeze even when the temperature in the deep is extremely low?

ABHIJEET S DESAI

Mumbai

A liquid freezes when its body temperature falls below its freezing point and a solid melts when its body temperature rises above its melting point. Though the term, ‘melting point,’ means the temperature where the solid commences to melt and the term, ‘freezing point,’ means the temperature where the liquid commences to freeze, these two terms are numerically the same, for a given pure material. However, this value depends on the pressure whereat the change of state (phase) from liquid to solid or vice versa takes place. Handbooks tabulate data of melting points of various substances for a pressure of 1 atm.

There is a famous equation known in Thermodynamics as ‘Clausius-Clapeyron Equation’ after the nineteenth century European physicists, Rudolf Clausius and Paul Clapeyron that relates the temperatures of phase transformations (melting point is one among them) to the pressure, change of density (during the phase transformation) and the heat of such phase transformation (quantity of heat released or absorbed during the phase change).

According to this proven equation, the melting point of a solid increases when pressure is increased where the density of the solid is higher than that of its liquid. This is usually the case with many of the substances we see around. But the case of water is unusual; water has lower density in its solid state (ice form) than in its liquid state. That is why ice floats on water. For such anomalous substances, the ‘Clasius-Clapeyron Equation’ states that the melting point decreases with increased pressure. In other words, water can remain as liquid water without freezing (or ice can readily melt) even when the temperature is below 0 C if it is under higher pressure (than 1 atm).

The water deep inside the oceans is under the heavy weight of the water held above it and is, effectively, experiencing very high pressure. So the ‘ice’ that should have been as ‘frozen ice’ has its melting point reduced so much extremely low that it is ‘already molten’ as liquid water; or that water is ‘yet to freeze.’ This is also a reason why in certain glaciers, water is still in liquid form below the sheets of ice.

Put your palm on an ice block and press it. You will see ice melting under your palm though the temperature there is considerably lower than 0 C.

PROF. A. RAMACHANDRAIAH

Department of Chemistry

National Institute of Technology Warangal

Warangal, Andhra Pradesh