A mixture contains solid potassium nitrate (KNO3) and a small amount of manganese dioxide (MnO2). Which method 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
Potassium nitrate is soluble in water; manganese dioxide is insoluble. Filtration separates the insoluble solid, and evaporation of the filtrate recovers the dissolved salt.
Why this answer is correct
KNO3 dissolves in water while MnO2 does not. The mixture is stirred with water, then filtered to remove MnO2. The filtrate contains dissolved KNO3; evaporating the water yields solid KNO3. Distillation would separate a liquid from a dissolved solid, but here we want the solid salt, not the water. Sublimation is not suitable because KNO3 does not sublime; MnO2 also does not sublime. A separating funnel is used for immiscible liquids, not for a solid-liquid mixture.
Example
To separate sand from salt, add water, filter to remove sand, and evaporate the filtrate to recover salt.
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