Question 1

Why is equilibrium called 'dynamic'?

Accepted Answer

Because the two opposing processes never stop — they simply run at equal rates. In a liquid–vapour system at equilibrium, molecules keep evaporating and keep condensing at the same rate, so the amount of vapour stays constant even though molecules are continuously crossing the surface both ways. 'Dynamic' captures this hidden microscopic activity behind a macroscopically unchanging state. The escalator analogy is exact: a runner matching a down-escalator's speed stays in place while still sprinting.

Question 2

What is saturated vapour pressure?

Accepted Answer

When a liquid is kept in a closed container, it evaporates until the rate of evaporation equals the rate of condensation. At that point the space above the liquid holds a fixed amount of vapour, and the pressure exerted by this vapour is the saturated vapour pressure. It depends only on the nature of the liquid and the temperature — not on the amount of liquid or the container's volume.

Question 3

How do we know molecules are still moving at equilibrium?

Accepted Answer

Tag a few molecules with a radioactive label. Even after the liquid–vapour system looks perfectly still, the tagged molecules soon appear in the vapour phase, while the total liquid and vapour amounts never change. This shows evaporation and condensation are both still happening — the equilibrium is dynamic, not frozen.

Question 4

What does the rate-vs-time graph look like as liquid–vapour equilibrium is reached?

Accepted Answer

The evaporation rate stays roughly constant (it depends on temperature and surface, not on how much vapour is present). The condensation rate starts at zero and rises as vapour builds up. The condensation curve climbs until it meets the flat evaporation line; from that moment the two rates are equal and the vapour density is constant. This meeting point is equilibrium — a commonly tested graph.

Question 5

Why must the container be closed for liquid–vapour equilibrium?

Accepted Answer

Equilibrium needs vapour to be able to build up until condensation can match evaporation. In an open container the vapour escapes, so condensation can never catch up with evaporation and the liquid simply keeps evaporating until it is gone. Only in a closed vessel do the two rates become equal and a true dynamic equilibrium (with a definite saturated vapour pressure) form.

Dynamic Equilibrium — Why a Flat Graph Isn't a Dead Reaction

Practice

Learned it? Now prove it under the clock.