Which of these is the most preferred separation technique for the isolation of solutes where the purity of the constituent is of utmost importance?

Analyze the question: You need the technique best suited to isolating a solute when the purity of the product matters most.

Explain the concept:

• Sieving separates solid particles by size. It is used for mixtures of solids, not for solutes in solution.
• Distillation separates liquids by differences in boiling point, or separates a solvent from a dissolved solid. It mainly recovers the liquid, not the solute.
• Recrystallization purifies a solid solute. The impure solid is dissolved in a minimum amount of hot solvent and the solution is cooled slowly. The desired compound crystallizes out in a very pure form, because impurities stay dissolved in the solvent (the mother liquor) or are removed by filtering the hot solution. Crystals form slowly and in an orderly way, so impurities are excluded from the crystal lattice.
• Precipitation forms an insoluble solid quickly by a chemical reaction. The precipitate tends to trap (occlude) impurities, so it is usually less pure.

Solution:

1. The goal is to isolate a solute, which rules out sieving and distillation.
2. Purity is the priority, so we want a method that excludes impurities. Slow, controlled crystal growth does this better than rapid precipitation.
3. Recrystallization is the standard laboratory method for obtaining pure solid compounds.

Answer: C. recrystallization

Exam tip: Link purity + solid solute with recrystallization. Link liquids with different boiling points with distillation. Link solids of different particle sizes with sieving. Slow crystal growth means high purity, while fast precipitation means trapped impurities.