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

##### 1 Answer

Aug 27, 2017

When you already know the

Consider the phase diagram of water.

The liquid-solid coexistence curve is **Clapeyron equation**:

#(dP)/(dT) = (DeltabarH)/(TDeltabarV)#

Consider

- We know that the enthalpy of fusion for
water is*melting*#DeltabarH_"fus" = "6.02 kJ/mol"# , or#"60.20 L"cdot"bar/mol"# , a positive quantity. - We also know that the temperature
#T# in#"K"# must be positive.

Since the slope of

#(-) = ((+))/((+)(?))#

Thus, the **change in molar volume** *negative*.

**In other words, ice contracts when it melts and water expands when it freezes.**