Which of the following statements about the solubility of a salt is correct?

Analyze the question: We are asked which statement about solubility and crystallization is correct.

Concept: Crystallization by cooling works because a hot saturated solution holds more solute than a cold one. When the solution cools, the solution can no longer hold all the dissolved salt, so the excess comes out as crystals. This works best for salts whose solubility changes a lot with temperature (for example, KNO₃ or CuSO₄). It works poorly for salts whose solubility barely changes (for example, NaCl), which are usually recovered by evaporating the water instead.

Evaluate each option:

• A. Incorrect. A salt whose solubility increases with temperature crystallizes easily on cooling, because less can stay dissolved at the lower temperature.
• B. Incorrect. If solubility does not depend on temperature, cooling does not force any salt out of solution, so it would not normally crystallize on cooling.
• C. Correct. If solubility rises considerably with temperature, cooling a hot saturated solution leaves much of the salt unable to stay dissolved, so it crystallizes out. This makes crystallization an efficient way to separate and purify such a salt.
• D. Incorrect. Solubility, and how it changes with temperature, directly determines whether and how well crystallization works.

Exam tip: Remember "big change with temperature = crystallize by cooling; small change = evaporate." NaCl (small change) is obtained by evaporation, while KNO₃ (large change) is obtained by cooling.