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Viranch Mistry Grade: 12

Qn.  When pressure is applied to the equilibrium system ‘Ice – Water’ which of the following phenomenon will happen?

A.     More ice will be formed

B.     Water will evapourate

C.     More water will be formed

D.    Equilibrium will not be established. 

     Sir shouldn't the answer be A- more ice will be formed. because when pressure is applied to this equilibrium, acc to Le chatelier principle, the reaction would proceed in such a way that it opposes the change. So the the reaction would try decreasing the pressure i.e. increasing the volume! And ice has more volume than water at equilibrium (0 degree C)?? But A is not the answer. Please explain.

6 years ago

Answers : (1)

Black Widow
36 Points

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 .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.

6 years ago
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