μs(T, p) = μl(T, p)
One equation [μs(T, p) = μl(T, p)], two variables (T, p). This means that coexistence of two phases is described by T(p) or p(T). e.g. a line in the (T, p) phase diagram.
At the triple point, the chemical potential of all three phases are the same… solid, liquid and gas coexist.
μs(T, p) = μl(T, p) = μg(T, p)
Two equations, two variables. This defines a unique point (Tt, pt) in the (T, p) phase diagram.
For H2O, Tt = 273.16 K and pt = 0.006 bar
(Recall the definition of the temperature scale in Lecture # 2)