Why does increased pressure lower the melting point of ice? Can we use the First Law of Thermodynamics to explain?

Asked by: Shawn Tee

Answer

For most substances, increasing the pressure when a system is in equilibrium between liquid and solid phases will increase the phase transition temperature. Water is one of a few special substances for which the pressure lowers the temperature of transition. The basic reason is that water actually expands when it goes from the liquid to solid phase. In textbooks you will find the explanation for these properties by using the Clapeyron-Clausius formula, but it is perhaps most readily explained using LeChatelier's principle.

This principle states that when a system is in equilibrium, any external changes that try to take it out of equilibrium (like applying pressure to ice) will cause the system to adjust in a way to counteract that change. This is a general property of what we mean by 'equilibrium' so it probably derives more from the Second Law of Thermodynamics ('equilibrium is the state in which entropy is maximized') than the First Law -- though perhaps a more creative individual could find a good way to explain your question using that law too. In this case, if you increase the pressure on the ice the ice-water system wants to try to lower it again. It can do that by making itself fit into a smaller volume. But since water fills a smaller volume when it's liquid, rather than solid, it will go to a lower melting point -- allowing more solid to become liquid.
Answered by: Brent Nelson, M.A. Physics, Ph.D. Student, UC Berkeley