[dars200]

Part of this question states: "supercooled water at 10degC freezes reversibly to ice at -10degC" But why did it freeze?

The whole question is: "One mole of supercooled water at -10 °C freezes reversibly to ice at -10 °C at a constant pressure of 1500 atm. The system is at chemical equilibrium. Assume that ΔHfusion is independent of temperature. Also assume that the densities of ice and water are independent of temperature."
Then it asks me to state whether all the thermo quantities are less than, greater than or equal to zero.

But why would the water freeze if system is at equilibrium? It would take additional energy to freeze the water woudnt it? Can someone rephrase the question, put it in another way?  Im not understanding whats happening in the system, so i therefore am having trouble answering consequent questions... Without any additional energy, the water wouldnt freeze right?  thanks for any help

[Borek]

It would take additional energy to freeze the water woudnt it?

Wouldn't. Melting requires energy, freezing is exothermic.

[rabolisk]

The system is in equilibrium between the solid phase and the liquid phase.

[Enthalpy]

My understanding is that heat is indeed removed from water to produce ice. Maybe "water is being frozen" would be clearer than "water freezes".

Freezing is to happen at -10°C because pressure is high (and with water, liquid is denser than solid, so pressure favours the liquid). But I didn't check if -10°C do correspond to 1500 atm.

There again, "supercooled" is misleading, as it suggests (to me) an unstable state. This water must be quite normal.

So it all looks like the normal freezing process of water by some chilling machine, except that it happens at -10°C because of the pressure.

Could that be it?