According to Gibbs' Phase Rule, how many degrees of freedom does a pure liquid water system with one phase have?

According to Gibbs' Phase Rule, how many degrees of freedom does a pure liquid water system with one phase have?

• A. 0
• B. 2
• C. 1
• D. 3

Answer: (B)

Gibbs' phase rule tells us how many independent variables you can change without changing the number of phases: F = C − P + 2. For water as a pure substance, C = 1. If the system is in a single liquid phase, P = 1. Plugging in gives F = 1 − 1 + 2 = 2. So there are two degrees of freedom—typically temperature and pressure—you can vary and still stay within the same liquid phase. You can move around the liquid region on a P–T diagram by changing either variable, as long as you don’t cross into a phase boundary where more than one phase would be present (that would reduce F accordingly).

Gibbs' phase rule tells us how many independent variables you can change without changing the number of phases: F = C − P + 2. For water as a pure substance, C = 1. If the system is in a single liquid phase, P = 1. Plugging in gives F = 1 − 1 + 2 = 2. So there are two degrees of freedom—typically temperature and pressure—you can vary and still stay within the same liquid phase. You can move around the liquid region on a P–T diagram by changing either variable, as long as you don’t cross into a phase boundary where more than one phase would be present (that would reduce F accordingly).