The GATE Grind

GATE 2026 CH – Question 16

Process Calculations · Gibbs phase rule and degree of freedom analysis · 1 mark · Multiple choice

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

  1. −3
  2. 0
  3. 1
  4. 3

Practise this question in The GATE Grind →

Show answer and explanation

Correct answer: (B) 0

Explanation

The Gibbs phase rule for a non-reacting system is $F = C - \pi + 2$. Pure water has $C = 1$ and at the triple point three phases (ice, liquid and vapour) coexist, so $\pi = 3$ and $F = 1 - 3 + 2 = 0$. The triple point is fixed.