Question

According to the phase rule, which one of the following is the number of degrees of freedom for pure water at its triple point?

• A. \( -3 \)
• B. 0
• C. 1
• D. 3

Hint:

Apply F = C - P + 2 with C = 1 (pure water) and P = 3 (solid, liquid, vapour coexisting) at the triple point.

Answer 1

The Correct Option is B

On a pressure-temperature diagram for a pure substance, single-phase regions are two-dimensional areas (F=2). Two-phase coexistence is a one-dimensional line (F=1). The triple point, where solid, liquid and vapour all coexist, is a single isolated point, zero dimensions, so F must be zero, matching F = C - P + 2 = 1 - 3 + 2 = 0. \[ \boxed{F = 0} \]