define equilibrium between two solid phases and a

Question

Define Equilibrium between two solid phases and a gas phase?

Consider the following reaction equilibrium:

3 CuO(s) + 2NH₃ (g) ↔ 3 Cu(s) + 3H₂O (g) + N₂ (g)

According to the species approach, there are five species, one relation (for reaction equilib- rium), and three phases. The phase rule gives

F = 2 + s - r - P = 3

It is more difficult to apply the components approach to this example. As components, we might choose CuO and Cu (from which we could prepare the solid phases) and also NH₃ and H₂O. Then to obtain the N₂ needed to prepare the gas phase, we could use CuO and NH₃ as reactants in the reaction 3 CuO + 2NH₃ → 3H₂O +N₂ and remove the products Cu and H₂O. In the components approach, we are allowed to remove substances from the system provided they are counted as components.