I've been staring at this phase diagram of water and I still can't figure it out. Why is the slope of the solid/liquid equilibrium line negative? (For most other substances like CO2, etc. the slope slants right, i.e. is positive.) I know the negative slope is supposed to explain a lot of peculiarities of water, such as why ice floats on top of water (density of the solid is less), and why water actually expands as it freezes. But, I can't for the life of me see how. This is from the chemguide website someone else suggested, referring to the equilibrium between ice(s) <-> water(l). [FONT=Helvetica, Arial] "If you have this equilibrium and increase the pressure on it, according to Le Chatelier's Principle the equilibrium will move to reduce the pressure again. That means that it will move to the side with the smaller volume. Liquid water is produced." .I don't get why the side with the smaller volume has less pressure? From PV = nRT, if V decreases, doesn't P increase? Also, I'm not sure what pressure they're referring to... the vapor pressure exerted by ice vs water ?