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Reviewed CSCA Chemistry question · Standard

A mixture contains solid potassium nitrate (KNO3) and a small amount of solid barium sulfate (BaSO4). KNO3 is very soluble in water, while BaSO4 is insoluble. Which sequence of steps is most appropriate to obtain pure solid KNO3 from the mixture?

  1. Add water, stir, filter, then evaporate the filtrate to dryness.
  2. Add water, stir, then evaporate the entire mixture to dryness.
  3. Heat the mixture strongly to decompose BaSO4, then dissolve the residue in water and filter.
  4. Add water, stir, filter, then wash the residue with water and dry it.
Show the answer and explanation

Correct answer

A. Add water, stir, filter, then evaporate the filtrate to dryness.

Principle or equation

Filtration separates an insoluble solid from a soluble one. The soluble salt is recovered from the filtrate by evaporation.

Why this answer is correct

Adding water dissolves KNO3 but not BaSO4. Filtration removes the insoluble BaSO4. Evaporating the filtrate leaves pure KNO3. Option B would leave BaSO4 mixed with KNO3. Option C is unnecessary and may not decompose BaSO4 easily. Option D would recover BaSO4, not KNO3.

Example

If you have a sand-salt mixture, add water, filter to remove sand, then evaporate the filtrate to get salt.

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