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The question:

Ice and water are placed in a closed container at a pressure of 1 atm and a temperature of 273 K. If the pressure of the system is increased to 2 atm while keeping temperature constant, why does the system's volume decrease?

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  • $\begingroup$ First have a close look at the phase diagram. $\endgroup$ – porphyrin May 5 '17 at 10:25
  • $\begingroup$ More ice will be formed when pressure increases $\endgroup$ – Archis Welankar May 5 '17 at 10:52
  • $\begingroup$ @ Archis Welankar, are you sure? this is water/ice at constant temperature initially very close to the triple point. $\endgroup$ – porphyrin May 5 '17 at 13:41
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To answer this question, it is best to look at a phase diagram of $\ce{H2O}$. Phase diagram of water Note the axes are kilopascal and celsius, but we know that $0^{\circ}\text{C}=273\text{K}$ and $101\text{kPa}=1\text{atm}$. We can see that if the pressure goes up at a constant temperature, the system will move from the phase boundary into the liquid phase.

The generally cited reason for why this occurs is that hydrogen bond network of ice makes it less dense than liquid water, and since higher pressure will favor the more dense state, the ice will melt to become liquid. This leads to a lower overall system volume.

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