Gen Chem Question

[mut9ya]

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Why does the freezing point of water decrease when its system pressure is increased?
Ans: Molecules expand when water freezes, a condition counteracted by increase of system pressure.

Can someone please explain this?
Thanks a lot.

 

[HeatingHomer]

For most liquids, the density increases as it turns into a solid. So increasing the pressure facilitates this and “freezing” can happen at a higher temperature. Water is actual more dense than ice (as anyone who has ice skated on a frozen pond can attest to). So as it freezes it tries to expand. By increasing the pressure the water can’t expand and a lower temperature would be required to freeze it. But for all liquids besides water increasing the pressure raises the freezing temperature.

 

[tresbon]

For most liquids, the density increases as it turns into a solid. So increasing the pressure facilitates this and "freezing" can happen at a higher temperature. Water is actual more dense than ice (as anyone who has ice skated on a frozen pond can attest to). So as it freezes it tries to expand. By increasing the pressure the water can't expand and a lower temperature would be required to freeze it. But for all liquids besides water increasing the pressure raises the freezing temperature.

I believe that's what Achiever explains in their solution for this question too. You may also click on the following keyword and pay attention to the solid/liquid line that slants slight left from the triple point:


Phase Diagram

Note how freezing point of water is decreased as you trace that line from the point upward where system pressure is being increased at the same time. This is one of DAT's favorite, and wouldn't want to miss this easily.