The Phase Diagram for Water

The Phase Diagram for Water

There is only one difference between the phase diagram for water and the other phase diagrams discussed. The solid-liquid equilibrium line (the melting point curve) slopes backwards rather than forwards.

For water, the melting point gets lower at higher pressures. This is because solid ice is less dense than liquid water. This phenomenon is caused by the crystal structure of the solid phase. In the solid forms of water and some other substances, the molecules crystallize in a lattice with greater average space between molecules, thus resulting in a solid occupying a larger volume and consequently with a lower density than the liquid. When it melts, the liquid water formed occupies a smaller volume

For water, the melting point gets lower at higher pressures. This is because solid ice is less dense than liquid water. This phenomenon is caused by the crystal structure of the solid phase. In the solid forms of water and some other substances, the molecules crystallize in a lattice with greater average space between molecules, thus resulting in a solid occupying a larger volume and consequently with a lower density than the liquid. When it melts, the liquid water formed occupies a smaller volume

An increase in pressure will move the above equilibrium to the side with the smaller volume. Liquid water is produced. To make the liquid water freeze again at this higher pressure, the temperature should be reduced. Higher pressures mean lower melting (freezing) points.

An increase in pressure will move the above equilibrium to the side with the smaller volume. Liquid water is produced. To make the liquid water freeze again at this higher pressure, the temperature should be reduced. Higher pressures mean lower melting (freezing) points.

Identifying data from the phase diagram of water

Identifying data from the phase diagram of water

Notice that the triple point for water occurs at a very low pressure, 0.006 atm and at 273.2 K temperature. Also notice that the critical temperature is 647 K (374°C). It would be impossible to convert water from a gas to a liquid by compressing it above this temperature. The critical pressure is 218 atm.