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Drakkith
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Nov13-12, 12:26 AM
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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.