Question

For a unary (one component) system, the maximum number of phases that can coexist in equilibrium at the triple point is

• A. 1
• B. 2
• C. 3
• D. 0

Hint:

To easily remember this concept, look at the name: Triple Point. The word "triple" directly indicates that three distinct phases (Solid, Liquid, and Gas) intersect and coexist in perfect equilibrium at that specific temperature and pressure.

Answer 1

The Correct Option is C

Concept: The number of phases that can coexist in a chemical system under stable thermodynamic equilibrium is regulated by Gibbs' Phase Rule. The rule is expressed as: \[ F = C - P + 2 \] where:
• \(F\) represents the degrees of freedom (the number of independent intensive variables, such as temperature and pressure, that can be varied without changing the number of phases in equilibrium).
• \(C\) represents the number of chemical components in the system.
• \(P\) represents the number of coexisting phases. Step-by-Step Calculation:
The problem statement specifies two physical conditions:
• The system is unary, meaning it contains exactly one chemical component: \[ C = 1 \]
• The system is at its triple point. A triple point represents an invariant state on a phase diagram where the degrees of freedom must equal exactly zero: \[ F = 0 \] Substituting these parameter values into Gibbs' Phase Rule equation: \[ 0 = 1 - P + 2 \] Simplifying the right-hand side: \[ 0 = 3 - P \quad \implies \quad P = 3 \] This proves that a maximum of 3 phases (typically solid, liquid, and gas) must coexist in equilibrium at the triple point of a single-component system. This matches Option (3).