Phase diagram of water v/s other substances

Question

Why for water increase in pressure decreases solidification temperature?
The fusion curve of water is not same as that of other substances and for other substances specific volume increases during this period (when they start melting) but not for water (ice) why...?

Answer 1

It's because normal ice, ice Ih, is less dense than liquid water. Ice Ih forms hexagonal crystals. The bonds in that crystalline structure make the water molecules slightly further apart than they are in the liquid form at the same pressure. That water expands on freezing makes water resist freezing as pressure increases. This in turn makes the fusion point decrease with increasing pressure, so long as the pressure isn't so very high that something other than ice Ih results from the freezing process.

There are several other forms of ice besides ice Ih. For example, liquid water freezes to form ice VII instead of ice Ih at extremely high pressures. Ice VII has a cubic crystalline structure rather than than the hexagonal structure of ice Ih. The cubic bonds in ice VII in draw the water molecules closer together in the solid form as opposed to the liquid form. In this extreme high pressure regime, increasing pressure even more raises the fusion temperature. In other words, water behaves like most other substances at very high pressures.