Heat of vaporisation versus pressure:\nHow does the latent heat of vaporisation change with increasing pressure, and what happens at the critical pressure?

Introduction / Context:
Understanding how latent heat of vaporisation varies with pressure is central to phase-equilibrium calculations, boiler and condenser design, and the thermodynamics of power cycles. As a liquid approaches its critical point, the distinction between liquid and vapour fades, and this is reflected directly in the behaviour of the heat of vaporisation.

Given Data / Assumptions:

• Latent heat of vaporisation refers to the energy required to convert saturated liquid to saturated vapour at the same temperature and pressure.
• System is a single-component pure fluid under equilibrium phase change conditions.
• Critical point is defined where the liquid and vapour phases become indistinguishable.

Concept / Approach:
From the Clapeyron/Clausius–Clapeyron relation, latent heat λ is tied to the slope of the saturation curve and the specific volume change between vapour and liquid. With rising pressure (and temperature along the saturation line), the difference in specific volumes shrinks. Consequently, λ decreases and must vanish at the critical point where the two phases coalesce.

Step-by-Step Solution:

Verification / Alternative check:
Steam tables corroborate this trend: compare λ at 1 bar to values near the critical point of water (around 22.06 MPa). The latent heat diminishes steadily and becomes zero at the critical point.

Why Other Options Are Wrong:

• Increases: Opposite to observed data and theory.
• Becomes zero at critical pressure (alone): Incomplete; it also decreases continuously before reaching zero.
• Remains constant: Contradicted by property tables for all common fluids.

Common Pitfalls:
Confusing heat of vaporisation with sensible heat; assuming constant latent heat across pressures; forgetting that phase boundaries vanish at the critical point.

Final Answer:
both decreases and becomes zero at critical pressure