What does a positive or negative liquid-solid slope indicate in a phase diagram?

1 Answer
anor277
Jun 9, 2018

Let us consider the phase diagram of water, which is HIGHLY unusual...

Explanation:

serc.carleton.eduserc.carleton.edu

Consider the transition "ice " rarr" water"ice → water...now the slope (DeltaP)/(DeltaT) is CLEARLY negative....(mind you the graph exaggerates this slope). And thus (DeltaP)/(DeltaT)="a negative slope"...

And now we go to the Clapeyron equation for phase change...

(DeltaP)/(DeltaT)-=(DeltaS)/(DeltaV)...now given the stated transition, CLEARLY DeltaS is POSITIVE (the liquid state is more disordered than the solid state)....because we go from a SOLID phase, ice, to a liquid phase....and ACCORDINGLY, DeltaV MUST be negative to account for the slope. And thus the VOLUME of a given mass of ICE is GREATER than the volume of a given mass of WATER under equivalent conditions. The result is that ice floats....and this is an highly unusual condition for substances.

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