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Reviewed CSCA Chemistry question · Easy

A mixture contains solid calcium carbonate (CaCO3) and solid sodium chloride (NaCl). CaCO3 is insoluble in water, while NaCl is very soluble. Which sequence of steps is most appropriate to obtain pure solid NaCl from the mixture?

  1. Add water, filter, evaporate the filtrate to dryness
  2. Add water, filter, wash the residue and dry it
  3. Heat the mixture strongly, then collect the sublimed solid
  4. Add water, then distill the mixture to collect the water
Show the answer and explanation

Correct answer

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

Principle or equation

Separation based on solubility: add water to dissolve soluble NaCl, filter to remove insoluble CaCO3, then evaporate the filtrate to obtain solid NaCl.

Why this answer is correct

Adding water dissolves NaCl but not CaCO3. Filtration separates the insoluble CaCO3 from the NaCl solution. Evaporating the filtrate removes water, leaving pure NaCl solid. Washing the residue would give CaCO3, not NaCl. Heating would not separate them because CaCO3 decomposes only at high temperatures and NaCl does not sublime. Distillation would collect water, not NaCl.

Example

Mix the sample with water, stir, filter. The filtrate contains NaCl in water. Evaporate the water to get NaCl crystals.

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