A vapour whose molecules are in dynamic equilibrium with those of its own liquid is said to be

Analyze the question: The question describes a vapour whose molecules are in dynamic equilibrium with the liquid it came from. We need the term for this state.

Explain the concept: In a closed container, molecules continually escape from the liquid surface (evaporation) and return to it from the vapour (condensation).

• At first, evaporation is faster than condensation.
• As more molecules enter the vapour, the rate of return increases.
• Eventually, the rate of evaporation equals the rate of condensation. This is dynamic equilibrium. Molecules are still moving in both directions, but the net amount of vapour no longer changes.

A vapour in this state is called a saturated vapour. It exerts the saturated vapour pressure (SVP), which depends only on temperature.

Step-by-step solution:

1. Unsaturated (A): The vapour has not yet reached equilibrium, so evaporation is still faster than condensation. More liquid can evaporate. This does not match.
2. Gaseous (B): This is a general description of a state of matter. It says nothing about equilibrium with the liquid. This does not match.
3. Saturated (C): Evaporation and condensation rates are equal, and the vapour holds the maximum number of molecules it can at that temperature. This matches.
4. Diffused (D): Diffusion is the spreading of particles from high to low concentration. It is unrelated to this situation.

Answer: C. saturated

Tip: Link the keywords. Dynamic equilibrium with its own liquid means saturated vapour. Remember also that saturated vapour pressure rises with temperature and does not depend on the volume of the container (as long as some liquid remains).