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Fractional distillation is chosen in a situation where

1. the liquids have a wide gap between their boiling points
2. the liquids have only a slight difference in boiling points
3. the liquids boil at the same temperature
4. the liquids together form a constant-boiling mixture

Correct answer: the liquids have only a slight difference in boiling points

Solution

Fractional distillation is used when the liquids have only a small difference in boiling points; a wide boiling-point gap (the stored answer) is handled by simple distillation.

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