Sketch a qualitatively accurate graph of G vs. T for a pure substance as it changes from solid to liquid to gas at fixed pressure. Think carefully about the slope of the graph. Mark the points of the phase transformations and discuss the features of the graph briefly.

Write the expression for Gibbs free energy.

Here, G is Gibbs free energy, T is the absolute temperature, S is the entropy, P is the pressure and V is the volume.

A plot between G and T will simply be a line graph where the slope of the graph is the negative entropy -S. The entropy is lowest for the solid and since the slope is negative, the line for the solid has the highest value in the vertical axis. The substance then melts into liquid which results larger value of entropy than solid, making the slope of the graph more stepper and finally it evaporates into a gas, the value of entropy for which is largest. It makes the slope of the graph most stepper.

Draw a graph to show the variation of G and T for a pure substance.

The slope of the curve is most stepper for gaseous state than liquid state than solid state for a pure substance.