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2.7.7Atoms, Elements and Compounds

Prepare an Insoluble Salt — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 15 of 26 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Part 1 of 5The Precipitation Reaction

2.7.7 Prepare an Insoluble Salt

Two soluble salts react, and the insoluble salt separates out as a solid.

1
Measure equal volumes of the two soluble salt solutions and mix them together in a beaker, stirring to ensure reaction.
2
A solid precipitate forms immediately because one of the products is insoluble in water.
3
Filter the mixture so the precipitate is retained on the filter paper and the solution passes through.
4
Wash the precipitate with distilled water to remove traces of the other solutions, then leave it to dry.

Part 2 of 5Choosing the Reactants

The two reactants must both be soluble, and their products must include one insoluble salt.

Soluble reactants
  • Both starting salts must dissolve in water so that their ions can mix freely.
Insoluble product
  • The salt to be made must be insoluble, so it separates as the precipitate.
Nitrates as sources
  • All nitrates are soluble, so silver nitrate or lead(II) nitrate are often used as the source of silver or lead ions.
Double decomposition
  • The reaction type AB + CD → AD + CB is also called double decomposition, since ions swap partners.
Insoluble saltTypical reactants
Silver chlorideSilver nitrate + sodium chloride
Lead(II) iodideLead(II) nitrate + potassium iodide
Barium sulfateBarium chloride + sodium sulfate

Part 3 of 5Ensuring a Good Yield

Ensuring a Good Yield
Slight excess
  • One solution is added in slight excess so that the other ion is fully precipitated.
Measured volumes
  • Using fixed volumes of each solution allows the amount of precipitate to be compared between experiments.
Washing
  • Washing removes soluble contaminants, which would otherwise remain in the dried precipitate.
Drying
  • The precipitate is dried at a low temperature to avoid changing the solid before it is weighed or used.

Part 4 of 5Worked Example

Question Write the balanced symbol equation, state symbols and ionic equation for the precipitation of silver chloride from silver nitrate and sodium chloride solutions.

  1. Symbol equationAgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq).
  2. The sodium and nitrate ions are spectators, so they are removed from the ionic equation.
  3. Ionic equationAg⁺(aq) + Cl⁻(aq) → AgCl(s).

Answer

  • AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq);
    • ionic equation Ag⁺(aq) + Cl⁻(aq) → AgCl(s).

Part 5 of 5Exam Checklist & Tips

2.7.7 – Key Points for the Exam

  • Precipitate An insoluble solid formed when two solutions react together.
  • Precipitation A reaction in which an insoluble solid is formed from solutions.
  • Spectator ion An ion that is present in the same form on both sides of an equation and takes no part in the reaction.
  • Filtration Separating a solid from a liquid by passing the mixture through filter paper.
  • Double decomposition A reaction in which two ionic compounds swap ions to form two new compounds.

Common Mistakes

  • Trying to make an insoluble salt by reacting an insoluble base with an acid;
    • precipitation needs two soluble reactants.
  • Forgetting to wash the precipitate, which leaves soluble contaminants in the solid.
  • Filtering off the solution and keeping it as the product, when the precipitate is the required salt.
  • Writing the state symbol (aq) for the precipitate instead of (s).

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