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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.