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P-T graph of water

by Outrageous
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Nov12-12, 11:43 PM
P: 375
refer to
say: This indicates that the liquid phase is more dense than the solid phase.

From that graph, how do we know the density and water expand when freezing from the graph?

Thank you
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Nov13-12, 12:26 AM
Drakkith's Avatar
P: 12,010
It explains it in the link a bit further down.

Normally the solid/liquid phase line slopes positively to the right (as in the diagram for carbon dioxide above). However for other substances, notably water, the line slopes to the left as the diagram for water shows. This indicates that the liquid phase is more dense than the solid phase. This phenomenon is caused by the crystal structure of the solid phase. In the solid forms of water and some other substances, the molecules crystalize in a lattice with greater average space between molecules, thus resulting in a solid with a lower density than the liquid. Because of this phenomenon, one is able to melt ice simply by applying pressure and not by adding heat.
Nov13-12, 03:38 AM
P: 375
So can I say at higher pressure what is more likely to form will be denser?
One thing I don't understand is let's say carbon dioxide, when we apply higher pressure, the freezing curve of CO2 slopes positively to the right , that means higher the pressure ,higher the freezing point, more difficult to form solid. Is this correct?
But I always think that we need high pressure so that we can easily change from gas to liquid then to solid.

Nov13-12, 04:06 AM
Sci Advisor
P: 2,470
P-T graph of water

Not exactly. Try to think of it in terms of Le Chatelier's principle. Imagine I take a block of ice just bellow melting point. Then I compress it. In other words, I raise pressure. Le Chatelier's principle tells me that system will try to resist the change. In other words, it will try to do something to reduce pressure. For example, it can go into a denser state if one is available. You can also see what happens based on PV diagram. If I increase pressure, temperature just bellow freezing becomes just above freezing. Ice melts. Combined with above it tells me that substance found a denser state, which happens to be liquid.

Higher freezing point means solid will form at higher temperature. It will be easier to make solid CO2 at higher pressure. What's more important is that if you start with a block of frozen CO2 at just bellow the freezing point and compress it, increasing pressure, you stay bellow freezing point. On the other hand, if you start with liquid CO2 just above freezing point, compressing it will put you bellow freezing point, meaning that in this case the solid is denser.

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