At intersection point ∆G=0, which represents the equilibrium point and sudden increase in slope is due to melting or boiling point of the metal oxide. According to Gibb's free rule, ∆G=∆H−T∆S where, ∆H= change in enthalpy ∆G= change in Gibb's free energy ∆S= change in entropy We can see that ∆G is dependent on ∆S. So, when the phase changes (s→I or I→ g) the entropy increases and hence, slope increases.