Fractional distillation separates and purifies both gaseous components

Steps:

• Identify mixtures: A and C are soluble solid-liquid; B is insoluble solid-liquid; D is miscible gases.
• Assess methods: Crystallisation/evaporation recover pure solute but impure/contaminated solvent; filtration recovers pure liquid but wet solid; fractional distillation collects both by boiling points.
• Check purity of both: Only fractional distillation yields separate pure fractions for both components.
• Confirm D: Nitrogen (b.p. -196°C) and oxygen (b.p. -183°C) distill at different temperatures.

Why D is correct:

• Fractional distillation exploits boiling point differences (definition: repeated vaporization-condensation in a column) to collect pure nitrogen and oxygen streams from liquefied air.

Why the others are wrong:

• A: Crystallisation yields pure copper sulfate crystals, but mother liquor retains dissolved solute, so water remains impure.
• B: Filtration yields pure ethanol filtrate, but sand residue retains ethanol, requiring extra drying for purity.
• C: Evaporation yields pure salt residue, but water vaporizes and is not collected as a pure liquid sample.

Final answer: D