Pressure dependence of freezing point: The freezing point of a liquid decreases when pressure is increased if, during solidification, the liquid __________.

Introduction:
Engineers must predict how phase-change temperatures shift with pressure, especially for materials with anomalous solid–liquid behavior. Water is the classic example where increasing pressure lowers the freezing point, allowing ice skating and pressure melting at the blade contact.

Given Data / Assumptions:

• Liquid undergoing freezing (liquid → solid).
• External pressure is increased slightly.
• We consider the sign of volume change on freezing.

Concept / Approach:
From the Clapeyron relation for solid–liquid equilibrium: dT/dP = T * ΔV / ΔH_fus, where ΔV = V_solid − V_liquid. If the liquid expands on freezing, the solid has a larger specific volume (ΔV > 0). Then dT/dP is positive; however, note that for water the conventional observation is that increasing pressure lowers the freezing point. Interpreting carefully: when the solid is less dense (volume larger) than the liquid, applying pressure favors the denser phase (liquid), so to maintain equilibrium the freezing temperature must decrease—thus the freezing point drops as pressure rises.

Step-by-Step Solution:
Identify volume change: liquid expands on freezing ⇒ solid less dense.Pressure favors the phase with lower volume (the liquid here).To freeze under higher pressure, the system needs a lower temperature.Therefore, freezing point decreases when pressure is increased.

Verification / Alternative check:
Water expands upon freezing; experimentally, higher pressure lowers its melting/freezing point—consistent with the reasoning above.

Why Other Options Are Wrong:

• Contracts or no volume change: Then pressure would not lower the freezing point; it would raise it or leave it almost unchanged.
• “Either”: The direction is specific; expansion on freezing is required for the decrease.

Common Pitfalls:
Confusing sign conventions in ΔV; always ask which phase is denser to infer the pressure effect.

Final Answer:
expands