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?
According to the phase rule, which one of the following is the number of degrees of freedom for pure water at its triple point?
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Apply F = C - P + 2 with C = 1 (pure water) and P = 3 (solid, liquid, vapour coexisting) at the triple point.
Updated On: Aug 10, 2026
- \( -3 \)
- 0
- 1
- 3
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The Correct Option is B
Solution and Explanation
Step 1: State the Gibbs phase rule.
The Gibbs phase rule for a non-reacting system relates degrees of freedom F to the number of components C and phases P: F = C - P + 2.
Step 2: Identify C and P for water at its triple point.
Pure water is a single species, so C = 1. At the triple point, solid ice, liquid water, and water vapour coexist, so P = 3.
Step 3: Substitute. \[ F = C - P + 2 = 1 - 3 + 2 = 0 \]
Step 4: Interpret physically.
F = 0 means the triple point is an invariant point: no freedom to independently vary temperature or pressure and still keep all three phases coexisting. This is why the triple point is used as a fixed reference for the Kelvin scale.
Step 5: Rule out the other options.
A negative degrees of freedom is not physically meaningful. Values of 1 or 3 would imply some variable remains free while all three phases persist, contradicting the triple point being a single fixed point. \[ \boxed{F = 0} \]
The Gibbs phase rule for a non-reacting system relates degrees of freedom F to the number of components C and phases P: F = C - P + 2.
Step 2: Identify C and P for water at its triple point.
Pure water is a single species, so C = 1. At the triple point, solid ice, liquid water, and water vapour coexist, so P = 3.
Step 3: Substitute. \[ F = C - P + 2 = 1 - 3 + 2 = 0 \]
Step 4: Interpret physically.
F = 0 means the triple point is an invariant point: no freedom to independently vary temperature or pressure and still keep all three phases coexisting. This is why the triple point is used as a fixed reference for the Kelvin scale.
Step 5: Rule out the other options.
A negative degrees of freedom is not physically meaningful. Values of 1 or 3 would imply some variable remains free while all three phases persist, contradicting the triple point being a single fixed point. \[ \boxed{F = 0} \]
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