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Salts and precipitation reactions explained

Syllabus VMBO: Basic chemistry · structure of matter

Choose ions that form a poorly soluble salt and track the remaining ions.

Track the ions

Mixing salt solutions brings their ions together. Some combinations form a poorly soluble salt. Always use the supplied table and conditions. Our model predicts solid calcium carbonate at sufficient concentrations.

Not everything precipitates

With calcium chloride and sodium carbonate, Ca²⁺ and CO₃²⁻ form the precipitate. Na⁺ and Cl⁻ remain in solution in this model. A filter can remove the solid product; that does not automatically make the remaining water pure.

Worked example

A supplied table lists CaCO₃ as poorly soluble; the sodium salts used are soluble. What happens when CaCl₂ and Na₂CO₃ solutions are mixed?

  1. Choose the combination Ca²⁺ with CO₃²⁻.
  2. Check charge: +2 and −2 form a neutral formula.

Ca²⁺(aq) + CO₃²⁻(aq) → CaCO₃(s).