A mixture contains solid potassium nitrate (KNO3) and a small amount of solid lead(II) chloride (PbCl2). KNO3 is very soluble in water, while PbCl2 is sparingly soluble in cold water but more soluble in hot water. Which sequence of steps is most appropriate to obtain pure solid KNO3 from the mixture?
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Correct answer
A. Add cold water, filter, then evaporate the filtrate to dryness.
Principle or equation
To separate a soluble salt from a sparingly soluble one, use a solvent in which the desired salt dissolves readily and the impurity does not. Cold water dissolves KNO3 but not PbCl2, so filtration removes PbCl2, and evaporation recovers KNO3.
Why this answer is correct
KNO3 is very soluble in cold water, while PbCl2 is sparingly soluble in cold water. Adding cold water will dissolve KNO3, leaving PbCl2 as a solid. Filtration removes PbCl2. Evaporating the filtrate yields pure KNO3. Hot water would dissolve some PbCl2, contaminating the KNO3.
Example
To separate NaCl from CaCO3, add water (NaCl dissolves, CaCO3 does not), filter, then evaporate the filtrate.
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