How do you read phase diagrams?

1 Answer
Jan 2, 2018



Let us look at a phase diagram of WATER and see if we make some sense from it....typically pressure is plotted against temperature....and we can read the resultant phase given the two coordinates.

The scale of the #x# and #y# axes, representing temperature, and pressure respectively are fairly arbitrary....but they are useful. We see under conditions of high pressure we get a condensed phase, whereas under conditions of low pressure, and high temperature a gas is favoured. At #P=218*atm# and #T=374*K# we reach the critical point...and importantly at this point the densities of the water phase, and the gaseous phase become equal and we can no longer distinguish between the two phases, hence beyond this there is no designation of phase.

At lower temperatures, say at #100# #""^@C# liquid water expresses a vapour pressure of #1*atm#...and thus we define the normal boiling point of water. At high pressures, and low temperatures we typically encounter a solid, and the graph reflects this.

At much lower pressures, we find the triple point, the conditions of temperature and pressure at which ALL three phases are in equilibrium...

Of special significance for the water molecule is that the SLOPE of the line differentiating the solid phase, and the liquid phase is NEGATIVE. This is very unusual....but we need further information from the Clapeyron equation to determine its put it in a nutshell, this means that the density of the solid phase is LESS than the density of the liquid phase, and so ice-bergs float....

Anyway, we are flying blind a bit here. If there is a specific issue or query raise it, and someone will address it. I would also consult your text for its account of phase diagrams.