Equilibrium
Sab ruk gaya? Nahi. First feel it human-scale: run up a down-escalator and match the belt's speed — you stay put while sprinting flat-out (rate up = rate down, no net change). Then watch the same idea in a sealed container: liquid molecules evaporate while vapour condenses, until evaporation and condensation rates become equal and the vapour density freezes at the saturated vapour pressure. Tag a few liquid molecules and watch them reach the vapour while every count stays constant — proof equilibrium is dynamic, not dead.
match the escalator speed · then step the container to equilibrium · tag molecules to prove it's alive
5 minutes · +4 right, −1 wrong (real NEET marking) · one global leaderboard.
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.
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.
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.
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.
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.
Ek band container mein liquid apne vapour ke saath equilibrium par hai. Kaunsa statement SAHI hai?