Which of the following methods can be used to obtain pure water from a mixture of sand, water and methanoic acid?

Analyze the question

The mixture has three components:

• Sand: an insoluble solid
• Water: the liquid we want
• Methanoic acid (HCOOH): a miscible liquid with a boiling point of about 101 °C, very close to water's 100 °C

Concept

• Filtration removes insoluble solids, but not dissolved substances.
• Simple distillation separates a liquid from non-volatile substances.
• Methanoic acid is volatile, so if you distil the mixture directly it will come over with the water. Its boiling point is so close to water's that even fractional distillation would not separate them cleanly.
• To get around this, convert the acid into a non-volatile salt. Neutralising with NaOH gives sodium methanoate, an ionic salt that does not vaporise:

HCOOH + NaOH → HCOONa + H₂O

Solution

1. Neutralise the methanoic acid with NaOH to form sodium methanoate, which is non-volatile.
2. Distil the mixture. Pure water vaporises and is condensed and collected. The sodium methanoate and the sand stay behind in the flask.

Why the other options fail:

• A: Filtration only removes sand. The sodium methanoate stays dissolved in the water, so the water is not pure.
• C: Fractional distillation struggles because the boiling points of water and methanoic acid are too close. Sand is also still present.
• D: Filtration removes the sand, but distilling afterwards would carry the volatile methanoic acid over with the water.

Answer: B, neutralisation with NaOH followed by distillation.

Exam tip

When an unwanted volatile acid or base is mixed with the liquid you want, neutralise it first to make a non-volatile salt, then distil. Also remember that filtration only removes insoluble solids.