# Eqilibrium between ice and water

## Main Question or Discussion Point

When ice is @ eqilibrium with water, the pressure is increased, what happens to the eqilibrium?

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DaveC426913
Gold Member
I'm not an expert in this area, but I do know that when you apply pressure to ice, it will begin to liquify. This is the physics behind most winter sports - from skating to curling.

Last edited:
vanesch
Staff Emeritus
Gold Member
chound said:
When ice is @ eqilibrium with water, the pressure is increased, what happens to the eqilibrium?
It shifts towards the water, and that is because the density of ice is smaller than the density of water (which is quite an exceptional feat : most materials are denser in their solid phase than in their liquid phase).

It is a consequence of a general law of thermodynamical equilibrium (which you can derive more formally, but I cannot recall its derivation off the top of my head): "any action on a system in equilibrium will shift its point of equilibrium in such a way as to oppose the change you want to apply"

So, when you increase pressure, the system can try to relieve pressure by shrinking in volume (hence shifting towards the most dense phase).

vanesch said:
It shifts towards the water, and that is because the density of ice is smaller than the density of water (which is quite an exceptional feat : most materials are denser in their solid phase than in their liquid phase).
It is a consequence of a general law of thermodynamical equilibrium (which you can derive more formally, but I cannot recall its derivation off the top of my head): "any action on a system in equilibrium will shift its point of equilibrium in such a way as to oppose the change you want to apply"
So, when you increase pressure, the system can try to relieve pressure by shrinking in volume (hence shifting towards the most dense phase).
I thought pressure was inversely propotional to volume. So if u increase pressure volume should also increase to decrease the pressure

vanesch
Staff Emeritus
Gold Member
chound said:
I thought pressure was inversely propotional to volume. So if u increase pressure volume should also increase to decrease the pressure
Eh, your remark makes me see how badly I formulated my proposition.

Imagine you have 2 phases F1 and F2, and imagine that the specific volume of phase F1, as a function of pressure p, is v1(p), and the specific volume of phase F2 is v2(p) ; now, assume that v1(p) is much smaller than v2(p).

The total volume of the system is V = N1 v1(p) + N2 v2(p) for a given distribution of the substance over the two phases N1 in phase F1 and N2 in phase N2, which happens to be a certain equilibrium point.

Now, when pressure is increased from p to p', one can show that in general, N1 will increase, and N2 will decrease, so that the phase with the smallest specific volume will gain in importance, hence lowering the overall volume MORE than if N1 and N2 remained constant. As such, the pressure needed to put the system in volume V' (with the changing N1 and N2), is LESS than the pressure that is needed to put the system in volume V' WITHOUT shifting the equilibrium. So it is loosely said that the increase in pressure is "resisted" by the system by shifting its own degree of freedom (the point of equilibrium N1/N2). So the shifting equilibrium will 'give in more' than you would expect if the ratios remained constant (if the equilibrium would not shift).

If you assume water and ice are both incompressible, pressure will not decrease the volume of either. In general, extra pressure means extra heat (like putting air in a tire) so you might argue that the temperature of the mix will increase and that would cause some ice to melt. You could also argue that water is denser, so increased pressure will compress the ice more...that would also tend to increase the temperature of the ice, so either way it looks like ice loses!!

vanesch
Staff Emeritus