Which technique is used to separate substances with large differences in boiling point, such as water and a soluble salt?

• A. Simple Distillation
• B. Fractional Distillation
• C. Filtration
• D. Chromatography

Answer: (A)

The technique being tested is distillation, which relies on differences in boiling points to separate components. When one substance boils at a much lower temperature than the other, heating the mixture causes the volatile component to turn into vapor and be captured as a liquid again after condensation, while the non-volatile component stays behind. Here, water boils at 100°C while the salt does not vaporize at that temperature, so you can distill off the water and leave the salt as a residue. Because the difference in boiling points is large, simple distillation is sufficient. Fractional distillation is used when the boiling points are closer together, filtration is for separating an insoluble solid from a liquid, and chromatography is for separating components that move at different speeds through a medium.