Equilibrium in physical processes — NEET & JEE Questions

Question 1 · difficulty L1 · recall

At equilibrium between a liquid and its vapour in a closed container, the rate of evaporation is

• A. equal to the rate of condensation
• B. greater than the rate of condensation
• C. zero
• D. less than the rate of condensation at all times

Question 2 · difficulty L2 · understanding

A saturated solution in contact with undissolved solute is at equilibrium because

• A. dissolution and crystallisation occur at equal rates
• B. all molecular motion stops
• C. the solute can no longer dissolve at all
• D. the solvent evaporates completely

Question 3 · difficulty L2 · understanding

For a pure substance at its melting point, solid and liquid can coexist in equilibrium when

• A. melting and freezing rates are equal
• B. only melting occurs
• C. only freezing occurs
• D. temperature must continuously rise

Question 4 · difficulty L2 · understanding

Which condition is essential for establishing a stable liquid-vapour equilibrium?

• A. A closed system at fixed temperature
• B. An open beaker with vapour continuously escaping
• C. Continuous removal of liquid
• D. A temperature that changes rapidly

Question 5 · difficulty L3 · application

Water is sealed in a container with some empty space above it. After sufficient time at constant temperature, the vapour pressure becomes constant. What does this indicate?

• A. Dynamic liquid-vapour equilibrium
• B. All water molecules have stopped moving
• C. Evaporation has permanently ceased
• D. Only condensation continues

Question 6 · difficulty L3 · application

A saturated sugar solution contains excess solid sugar. If a small amount of labelled sugar crystal is added without changing temperature, what is expected at equilibrium?

• A. Labelled and unlabelled sugar can exchange between solid and solution even though concentration stays constant
• B. The labelled crystal can never dissolve
• C. All solid dissolves completely
• D. The solution concentration must steadily increase

Question 7 · difficulty L3 · application

Ice and liquid water coexist at equilibrium at the melting point. If a little heat is supplied while temperature remains at the melting point, the immediate effect is mainly

• A. conversion of some ice to water
• B. a large rise in temperature
• C. complete cessation of melting
• D. conversion of water to ice

Question 8 · difficulty L3 · application

A volatile liquid is at liquid-vapour equilibrium in a sealed vessel. Some vapour is suddenly removed while temperature is kept constant. What happens next?

• A. Net evaporation occurs until equilibrium is re-established
• B. Net condensation occurs immediately
• C. Nothing changes
• D. The liquid freezes

Question 9 · difficulty L4 · application

A saturated solution with excess solid solute is diluted by adding solvent at the same temperature. What happens immediately after mixing?

• A. Some solid dissolves until saturation is restored
• B. More solute crystallises immediately
• C. No change occurs because solid is present
• D. The solubility constant becomes zero

Question 10 · difficulty L4 · application

At equilibrium between a solid and its vapour, the macroscopic amount of each phase is constant. Which microscopic statement is correct?

• A. Sublimation and deposition continue at equal rates
• B. Neither process occurs
• C. Only sublimation occurs
• D. Only deposition occurs