At the melting point, how does the vapor pressure of a substance in the solid state compare to that in the liquid state?

Introduction / Context:Understanding vapor pressures of solid and liquid phases is essential for sublimation, drying, and melting operations. Around the melting point, the liquid generally has a higher tendency to evaporate than the solid.

Given Data / Assumptions:

• Substance at its melting point (equilibrium between solid and liquid).
• Standard behavior (no unusual polymorphic anomalies considered).

Concept / Approach:Vapor pressure reflects the escaping tendency of molecules. Liquids typically have higher surface molecular mobility than solids; consequently, at the same temperature (near the melting point), the equilibrium vapor pressure over the liquid exceeds that over the solid (the sublimation pressure).

Step-by-Step Solution:Compare phase tendencies: liquid molecules are less constrained.Therefore, P_vap,liquid > P_vap,solid at the same T.Select “less” for the solid relative to the liquid.

Verification / Alternative check:Clausius–Clapeyron considerations show different slopes; empirical data (e.g., ice vs liquid water near 0°C) support lower sublimation pressure than liquid vapor pressure at comparable temperatures.

Why Other Options Are Wrong:“More” contradicts observed behavior; “same” would only occur under special rare conditions; “depends” is unnecessarily vague here.

Common Pitfalls:Confusing triple-point relationships with melting-point comparisons; mixing sublimation and vaporization concepts.

Final Answer:less