A mixture contains solid potassium chloride (KCl) and a small amount of solid lead(II) chloride (PbCl2). KCl 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 potassium chloride from the mixture?
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Correct answer
A. Add cold water, filter, evaporate the filtrate to dryness.
Principle or equation
Separation based on solubility differences: a soluble substance can be dissolved and separated from an insoluble one by filtration; the soluble substance is recovered by evaporation.
Why this answer is correct
KCl is very soluble in cold water, while PbCl2 is sparingly soluble in cold water. Adding cold water dissolves most KCl, leaving PbCl2 as a residue. Filtration removes the solid PbCl2, and evaporating the filtrate yields pure KCl. Hot water would dissolve more PbCl2, contaminating the KCl.
Example
To separate NaCl from sand, add water, filter, and evaporate the filtrate to recover NaCl.
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