Question

Which statement describes how vapor pressure occurs?

Some molecules of a substance have high enough kinetic energy to melt. These molecules exert pressure on the substance.
Some molecules of a substance have high enough kinetic energy to vaporize. These molecules exert pressure on the substance.
Some molecules of a substance have low enough kinetic energy to vaporize. These molecules exert pressure on the substance.

Answer 1

The second statement is correct:

Some molecules of a substance have high enough kinetic energy to vaporize. These vapor molecules exert pressure (the vapor pressure).

Notes:
- The first is wrong because "melt" refers to solid → liquid, not vaporization.
- The third is wrong because molecules need sufficiently high, not low, kinetic energy to escape into the gas phase.
- Vapor pressure rises with temperature because more molecules reach the required kinetic energy; at equilibrium the rate of evaporation equals the rate of condensation.