Gibbs phase rule for a non-reacting system with C components and P phases is F = C − P + 2. True or false?

• A. True
• B. False
• C. F = C − P
• D. F = P − C + 2

Answer: (A)

Gibbs phase rule describes how many independent variables you can change while keeping the number of phases in equilibrium. In a non-reacting system with C components and P phases, you have two base variables you can vary freely—temperature and pressure—plus one degree of freedom contributed by each additional component, minus one for each extra phase. Putting those pieces together gives F = C − P + 2. This means, for example, a single-component system with two coexisting phases has one degree of freedom (you can adjust T or P along the coexistence line), while the same system with three coexisting phases has zero degrees of freedom (the triple point is fixed). If you have two components in two phases, you have two degrees of freedom, often corresponding to varying temperature and one of the compositions (or pressure). The statement is true for non-reacting systems; if reactions were present, the number of independent variables would be reduced by the number of independent reactions.