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 potassium nitrate from the mixture?
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Correct answer
A. Add water, filter, then evaporate the filtrate to dryness
Principle or equation
Filtration separates an insoluble solid from a soluble solid. The soluble potassium nitrate dissolves in water, leaving insoluble barium sulfate as the residue. The filtrate contains dissolved KNO3, which can be recovered by evaporation of the water.
Why this answer is correct
Adding water dissolves KNO3 but not BaSO4. Filtration removes BaSO4. The filtrate is a solution of KNO3; evaporating the water yields pure solid KNO3. Washing the residue would discard the KNO3. Heating would not separate them. Distillation would collect water, not the salt.
Example
Mixture + water → stir → filter → residue BaSO4; filtrate KNO3(aq) → evaporate → KNO3(s).
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