Ajar is filled with water, and the lid is closed tightly. the jar is kept at constant temperature. what happens at the surface of the water? a. water molecules evaporate and condense at the same rate. b. water molecules evaporate and condense, but evaporation occurs more quickly. c. water molecules evaporate only and do not condense. d. water molecules condense only and do not evaporate.

The correct answer is A: Water molecules evaporate and condense at the same rate.

Evaporation is defined as the physical change from liquid to gas, and Condensation is the physical change from gas to liquid.

At any given temperature, these two processes occur at once, in a dynamic equilibrium.

When the lid is closed, evaporation occurs faster than condensation, and pressure increases. Then, when pressure reaches a critical value, condensation starts to occur faster than evaporation, until an equilibrium is reached, and the pressure of the water molecules in the gas phase is maximum for that temperature. The pressure at that point is called Vapor Pressure.

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