Trend of latent heat near the critical point:\nHow does the heat of vaporisation change with pressure, and what happens at the critical point?

Introduction / Context:
Phase-change energetics are vital for boilers, condensers, and power cycles. The latent heat of vaporisation reflects the energy required to separate liquid and vapor phases; its behavior with pressure reveals what happens as a fluid approaches the critical state.

Given Data / Assumptions:

• Pure fluid undergoing equilibrium vaporisation along the saturation curve.
• Critical point: liquid and vapor phases become indistinguishable.

Concept / Approach:
The Clausius–Clapeyron relation shows latent heat λ ∝ T * Δv * (dP_sat/dT). As pressure increases toward the critical pressure, the specific volume difference Δv = v_g − v_l shrinks. Consequently, λ decreases and must reach zero at the critical point where Δv → 0.

Step-by-Step Reasoning:

Verification / Alternative check:
Steam tables show the latent heat of water declining with pressure and vanishing at the critical point (~22.06 MPa), validating the trend.

Why Other Options Are Wrong:

• Increase or remain constant: Contradicted by data and theory.
• Only “becomes zero” without noting the continuous decrease omits the trend.

Common Pitfalls:
Confusing latent heat with sensible heat; assuming a constant latent heat across pressures.

Final Answer:
Both decreases with pressure and becomes zero at the critical point