A saturated solution at 30$^0$C will normally produce crystals at a temperature of

Analyze the question: A solution that is saturated at 30°C holds the maximum amount of solute it can dissolve at that temperature. The question asks at which temperature crystals will normally form.

Concept: For most solids (such as KNO₃ and NaNO₃), solubility increases as temperature rises. So:

• Heating the solution makes it unsaturated, because more solute can dissolve.
• Cooling the solution lowers solubility, so the solution holds more solute than it can keep dissolved. The excess comes out as crystals.

Solution:

• At 50°C, 40°C and 35°C, which are all above 30°C, solubility is higher. The solution becomes unsaturated and no crystals form.
• At 20°C, which is below 30°C, solubility is lower. The excess solute crystallizes out.

Answer: D (20°C)

Tip: "Cool down, crystals crown." For most solids, cooling a saturated solution produces crystals, and heating it dissolves more solute. Watch for the exception of gases, whose solubility falls as temperature rises.