Vapour is said to be saturated on top of an enclosed liquid if _________

Analysis of the Question

The question asks for the condition that defines a saturated vapour in an enclosed space above a liquid. This is a fundamental concept in the study of Heat Energy and Kinetic Theory of Matter.

Explanation of the Concept

In an enclosed container, molecules at the surface of a liquid constantly escape into the space above (a process called vaporization or evaporation). Simultaneously, some of the vapour molecules collide with the liquid surface and return to the liquid state (a process called condensation).

• Unsaturated Vapour: Initially, the rate of vaporization is higher than the rate of condensation.
• Saturated Vapour: As more molecules enter the vapour phase, the pressure increases. Eventually, the number of molecules escaping the liquid per second becomes exactly equal to the number of molecules returning to the liquid per second. At this point, the vapour is said to be saturated, and the space above the liquid can hold no more vapour at that specific temperature.

Step-by-Step Solution

1. Identify the state of equilibrium: When a vapour is saturated, it is in a state of dynamic equilibrium.
2. Analyze the rates: Dynamic equilibrium means that while both processes (vaporization and condensation) are still occurring, they are happening at the same rate.
3. Evaluate the options: • If the rate of condensation were greater (Option A), the vapour would be decreasing. • If there were no vaporization or condensation (Option C), the system would be static, not dynamic. • If the rate of condensation were less (Option D), the vapour would still be unsaturated. • Since the rates must be equal for saturation to exist, Option B is the correct description.

Conclusion

Exam Tip: Always remember that "saturated" in physics often implies a state of dynamic equilibrium. Whether it is a saturated solution in Chemistry or a saturated vapour in Physics, the key is that the forward and backward processes are occurring at the same rate!