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2.7.5Inorganic Chemistry

Prepare a Soluble Salt — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions

5 min read 5 parts Lesson 43 of 52

📚 Edexcel International GCSE Science Double Award 4SD0 - Chemistry Hub / Topical Revision
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Part 1 of 5Making a Soluble Salt

2.7.5 Prepare a Soluble Salt

An insoluble base such as copper(II) oxide reacts with an acid, and the excess solid is then removed.

1
Warm dilute sulfuric acid in a beaker, then add copper(II) oxide slowly, stirring until some solid remains undissolved.
2
Filter the mixture to remove the excess copper(II) oxide, leaving a solution of copper(II) sulfate.
3
Heat the filtrate gently to evaporate some water until the solution is saturated and crystals begin to form.
4
Leave the solution to crystallise, then decant the excess liquid and dry the crystals between filter paper.

Part 2 of 5Why the Base Is in Excess

Adding the base in excess is essential for producing a pure salt solution.

All acid used
  • An excess of solid base ensures that every mole of acid has reacted, so no acid is left in the solution.
Safe evaporation
  • Unreacted acid would become dangerously concentrated during evaporation and could spit or burn.
Filtration
  • Any unused solid base is removed by filtration, so only the salt solution and water remain.
Carbonates
  • If a carbonate is used, carbon dioxide escapes, so the base should be an oxide or hydroxide where possible.

Part 3 of 5Key Equation and Products

Key Equation and Products
Equation
  • Copper(II) oxide and sulfuric acidCuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l).
Salt
  • Copper(II) sulfate is the soluble salt that remains in the solution.
Crystal size
  • Slow evaporation of the filtered solution gives larger crystals than rapid heating.
Colour
  • Hydrated copper(II) sulfate crystals are bright blue.

Part 4 of 5Worked Example

Question Explain why excess copper(II) oxide is added to the sulfuric acid, and how the excess is removed.

  1. Adding the base in excess ensures all the sulfuric acid reacts.
  2. Without this, the leftover acid would become concentrated during evaporation.
  3. The solid base is removed by filtration, leaving only copper(II) sulfate solution and water.

Answer

  • The base is in excess so that all the acid is used up.
  • Any excess solid is removed by filtration before the solution is evaporated and crystallised.

Part 5 of 5Exam Checklist & Tips

2.7.5 – Key Points for the Exam

  • Soluble salt A salt that dissolves in water and can be obtained as crystals by evaporation.
  • Excess An amount of a reactant greater than needed, so the other reactant is fully used.
  • Filtration Separating an insoluble solid from a liquid by passing the mixture through filter paper.
  • Saturated solution A solution that cannot dissolve any more solute at that temperature.
  • Decant Pouring off the liquid above settled solid, leaving the solid behind.

Common Mistakes

  • Saying the base is in excess to speed up the reaction; its purpose is to use up all the acid.
  • Evaporating the solution to dryness, which can cause decomposition and loss of water of crystallisation.
  • Forgetting to filter out the excess solid base before evaporation.
  • Using a metal such as copper that does not react with dilute acid to make the salt.

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