Concept:Cooling a saturated solution lowers the solute's solubility, causing excess solute to crystallise out.
Explanation:A solution saturated at
30∘C holds the maximum amount of solute at that temperature.
When the temperature is lowered below
30∘C, the solubility of the solute decreases.
The solution then has more solute than it can hold at the lower temperature, so crystals form.
Among the given options, only
20∘C is below
30∘C.
Temperatures of
35∘C,
40∘C, and
50∘C are higher and would increase solubility, not produce crystals.
Answer:D.
20∘C