Potassium trioxonitrate (V) can be obtained from its solution by----------

Analyze the question: Potassium trioxonitrate(V) is KNO₃ (potassium nitrate), a soluble salt. We want to recover it as a solid from its solution.

Concept: Different separation techniques are used for different mixtures:

• Distillation separates a liquid (solvent) from a solution, so you collect the liquid, not the solid. It is used when you want the water or the solvent.
• Evaporation removes the solvent completely by heating to dryness. It leaves a solid residue, but it can decompose heat-sensitive salts and gives only a powder with impurities.
• Crystallization involves heating the solution to concentrate it (to the saturation point), then cooling it so that pure crystals form. It is used for salts that are much more soluble in hot water than in cold water.
• Filtration separates an insoluble solid from a liquid. Because KNO₃ is dissolved, it would pass through the filter paper.

Solution:

1. KNO₃ is soluble in water, so filtration cannot separate it (D is out).
2. Distillation would give us water, not the salt (A is out).
3. KNO₃ has a solubility that rises sharply with temperature. This is the classic case for crystallization: concentrate the solution, cool it, and pure crystals of KNO₃ form, which can then be filtered and dried.
4. Evaporation to dryness could work in a crude sense, but the standard method for obtaining a pure, well-formed salt like KNO₃ from its solution is crystallization.

Answer: C. crystallization

Tip: For salts from solution, remember the sequence "Heat, cool, crystals form." Use evaporation to dryness only for salts that are not much affected by temperature, such as NaCl. Salts whose solubility changes strongly with temperature, such as KNO₃, are obtained by crystallization.