Question

Temperature and pressure combination at which water co-exist is all forms is known as

• A. Magic point
• B. Universal point
• C. Absolute point
• D. Triple point

Hint:

Triple point of water: $T = 273.16\text{ K} = 0.01^\circ\text{C}$ and $P = 611.65\text{ Pa}$. Degree of freedom $F = 0$ (Invariant).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
According to Gibbs' Phase Rule (\(F = C - P + 2\)), a single-component system (\(C=1\)) with three phases in thermodynamic equilibrium (\(P=3\)) has zero degrees of freedom (\(F=0\)), defining a unique invariant thermodynamic point.
Key Formula or Approach:
\[ T_{\text{tp}} = 0.01^\circ\text{C} = 273.16\text{ K}, \quad P_{\text{tp}} = 611.65\text{ Pa} = 0.006\text{ atm} = 4.58\text{ mm Hg} \]

Step 2: Detailed Explanation:
The Triple Point of water is the unique, invariant state of thermodynamic equilibrium at which solid ice, liquid water, and water vapour coexist simultaneously in stable equilibrium.
It occurs at an exact temperature of \(273.16\text{ K}\) (\(0.01^\circ\text{C}\)) and a partial vapour pressure of \(611.65\text{ Pa}\) (\(4.58\text{ mm Hg}\)).

Step 3: Final Answer:
Hence, the state where all three phases coexist is the Triple point, matching option (D).