# Why does increase in pressure favor formation of water in water-ice equilibrium (at 273K)?

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?

• First have a close look at the phase diagram. May 5 '17 at 10:25
• More ice will be formed when pressure increases May 5 '17 at 10:52
• @ Archis Welankar, are you sure? this is water/ice at constant temperature initially very close to the triple point. May 5 '17 at 13:41

To answer this question, it is best to look at a phase diagram of $\ce{H2O}$. 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.