A saturated solution of sugar in water is kept in contact with some undissolved sugar at a fixed temperature and pressure. Which of the following is true for this system?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Sugar keeps dissolving and separating out at the same rate, so the concentration of the solution remains constant.
Two opposite processes go on when a solid is in contact with its solution: dissolution (solute goes into the solution) and crystallisation (solute particles separate out). In a saturated solution the two take place at the same rate, so a dynamic equilibrium is set up. Both processes continue, but the concentration of the solute does not change.
Why the other options are incorrect
Option (1) Dissolution of sugar has stopped, so no sugar particles enter or leave the solution. — Incorrect. The equilibrium is dynamic, not static. Sugar particles still go into the solution and separate out; only their rates are equal.
Option (2) Crystallisation is faster than dissolution, so the amount of undissolved sugar keeps increasing. — Incorrect. At equilibrium the number of particles going into the solution equals the number separating out. So the amount of undissolved sugar stays the same.
Option (4) More sugar can still be dissolved in the solution at the same temperature. — Incorrect. This describes an unsaturated solution. A saturated solution already holds the maximum amount of solute at that temperature and pressure.
NCERT (2026-27 edition):
1.3.1 Solubility of a Solid in a Liquid – “Some solute particles in solution collide with the solid solute particles and get separated out of solution. This process is known as crystallisation. A stage is reached when the two processes occur at the same rate. Under such conditions, number of solute particles going into solution will be equal to the solute particles separating out and a state of dynamic equilibrium is reached.
\( \displaystyle \text{Solute} + \text{Solvent} \rightleftharpoons \text{Solution} \qquad (1.10) \)
At this stage the concentration of solute in solution will remain constant under the given conditions, i.e., temperature and pressure. … Such a solution in which no more solute can be dissolved at the same temperature and pressure is called a saturated solution. An unsaturated solution is one in which more solute can be dissolved at the same temperature.”