Correct statement on purification methods

Steps:

• Recall that pure substances have fixed boiling points, while mixtures boil over ranges.
• Note crystallization dissolves and recrystallizes soluble substances, not insolubles.
• Understand chromatography separates based on adsorption differences, not color.
• Confirm fractional distillation exploits boiling point variations in liquids like air.

Why D is correct:

• Fractional distillation separates liquid air's components by boiling points; oxygen boils at -183°C, distilling as a distinct fraction.

Why the others are wrong:

• A: Pure liquids boil at a single temperature; a range indicates impurities.
• B: Crystallization separates soluble substances; insolubles require filtration.
• C: Chromatography separates any substances with differing affinities, including colorless ones via UV or other detection.

Final answer: D