# What is unusual about the phase diagram for water?

Jun 14, 2018

Where is the phase diagram of water? And what is the Clapeyron equation for phase transitions...?

#### Explanation:

The following graph represents the phase behaviour of water under differing regimes of pressure and temperature. Now for the transition ice to water...we consider $\frac{\mathrm{dP}}{\mathrm{dT}}$...and this slope CLEARLY is negative (even tho I think this diagram exaggerates the slope)....and consider the Clapeyron equation for phase transitions...

$\frac{\mathrm{dP}}{\mathrm{dT}} = \frac{\Delta S}{\Delta V}$ (this is derived from several state postulates, and at the moment I simply ask you to accept its formulation)…

And if $\frac{\mathrm{dP}}{\mathrm{dT}}$ is negative, $\frac{\Delta S}{\Delta V}$ IS ALSO NEGATIVE... Capisce? But $\Delta S$ for a solid to liquid transition IS CLEARLY POSITIVE. And thus $\Delta V$ is NEGATIVE...i.e. the SOLID PHASE, the ICE, OCCUPIES MORE VOLUME than the liquid phase. This is quite an unusual phenomenon, with the result that ice-cubes, and ice-bergs, FLOAT IN WATER in that they are less dense. And I think this is the result you are asked to consider....