A vapor is said to be saturated when?

Step 1: Analyze the question The question asks for the condition that defines a saturated vapour.

Step 2: The concept When a liquid is placed in a closed container, molecules escape from the surface into the space above (evaporation). Some of these vapour molecules also strike the surface and return to the liquid (condensation).

• At first, more molecules leave than return.
• As the vapour builds up, the return rate increases.
• Eventually, the rate of escape equals the rate of return. This is dynamic equilibrium.

At this point the space above the liquid cannot hold any more vapour at that temperature. The vapour is saturated, and its pressure is the saturated vapour pressure (SVP).

Step 3: Check each option

• A. A dynamic equilibrium exists between liquid molecules and vapour molecules. This is correct. The rates of evaporation and condensation are equal.
• B. More molecules return to the liquid than leave it. This is wrong. It would mean net condensation, and that is not equilibrium.
• C. The temperature of the vapour varies. This is wrong. SVP depends on temperature, but a varying temperature does not define saturation.
• D. The vapour pressure is atmospheric. This is wrong. That describes the condition for boiling, not saturation.

Step 4: Answer The correct option is A.

Exam tip: Remember "saturated = equal rates". Whenever you see "dynamic equilibrium" with the rate of leaving equal to the rate of returning, think saturated vapour. Also remember that "vapour pressure equals atmospheric pressure" is the definition of the boiling point, which is a common distractor in JAMB.