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Le Chatlier's Principle and the Melting Point of Water (Why does increasing pressure lower MP?)

  1. Oct 15, 2014 #1
    So at increased pressure (increased atmospheric), the boiling point of water Increases because the atmospheric pressure goes up.

    But at increased pressures, the melting point of a substance decreases!
    The explanation i'v seen is that it is due to le chatlier's principle.

    Ice is less dense than water is, so it has a greater volume.

    At increased pressures, the equilibrium between water and ice will shift to relieve the pressure and move in the direction of the lower pressure side. Now because ice is less dense than water it will have a greater volume and thus a greater pressure. So, the equilibrium will shift to the water side to relieve the increased pressure.

    What i'm having trouble with is understanding why greater volume equates to greater pressure.
    If we had a container of 10L of inert gas and then 1kg of water, and the container was sealed. There would be some pressure exerted by the inert gas. And the more "space" the water takes up, the greater the pressure will be. Intuitively this makes sense. And by the ideal gas law it makes sense as well (PV = nRT). If ice takes up more space, then the Volume will be lower. So pressure should be greater.

    But that's describing the pressure of the other gas. The solid ice itself doesnt really exert pressure. What pressure are they talking about when they say "to relieve the pressure the equilibrium shifts to the side of the lower pressure side).
     
  2. jcsd
  3. Oct 16, 2014 #2

    DrClaude

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    Staff: Mentor

    While the water in the container does not exert direct pressure on the walls of the container, the volume of water is directly related to the pressure inside the container. It is this overall pressure that is important.

    You could also see it as the water exerting a pressure on the gas, the same way a piston would. Or imagine a container with two chambers, and a movable wall between the two. One chamber contains a gas, and the other only water. Think of what happens if the pressure of the gas is increased.
     
  4. Oct 16, 2014 #3
    Ah I see. That makes sense. And in both cases/ways we consider it, to relieve the pressure, the reaction moves in the other direction.

    I think picturing the reaction in an enclosed/closed space helps picture it the most. Thanks!
     
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