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

1 Answer
Jun 9, 2018

Answer:

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

Explanation:

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Consider the transition #"ice " rarr" 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.