When the pressure surrounding a pure substance increases, what typically happens to its freezing point?

• A. The substance cannot freeze at higher pressure.
• B. The freezing point remains unchanged.
• C. The freezing point decreases.
• D. The freezing point increases.

Answer: (C)

Verified step by step guidance

1. Understand that the freezing point of a substance is the temperature at which it changes from liquid to solid under a given pressure.
2. Recall that pressure can affect the phase equilibrium between solid and liquid phases, described by the Clausius-Clapeyron relation.
3. Consider the volume change upon freezing: if the solid phase is denser than the liquid, increasing pressure typically raises the freezing point; if the solid is less dense, increasing pressure lowers the freezing point.
4. Apply this concept to common substances like water, where the solid (ice) is less dense than the liquid, so increasing pressure lowers the freezing point.
5. Conclude that the effect of pressure on freezing point depends on the relative densities of solid and liquid, and for many substances, increasing pressure increases the freezing point, but for substances like water, it decreases.