water phase diagram

[Oh_Gee]

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why does the melting point of water decrease if external pressure is applied to it. i'm looking at the phase diagram but can't figure it out

 

[techfan]

Water is special in that it expands when it becomes a solid (ice is less dense because the hydrogen bonds of the water hold it in a lattice as a solid and they can't move around and get as close as they can when water is a liquid) , and since you are increasing the external pressure the volume wants to decrease and water takes up less volume in the liquid form than as ice.