If we reduce the temperature of a product the vapor pressure goes ______.

• A. Unknown
• B. Up
• C. No change
• D. Down

Answer: (D)

Vapor pressure decreases when you lower the temperature. Vapor pressure is the pressure exerted by the vapor in equilibrium with its liquid or solid, and it depends on temperature because evaporation requires molecules to have enough energy to escape the surface. When temperature drops, fewer molecules have that energy, so fewer enter the vapor phase and the vapor pressure falls. This is why cooling a product reduces its volatility. The other options don’t fit: lowering temperature does not increase vapor pressure, and vapor pressure does change with temperature, so “no change” or “unknown” aren’t correct.