AQA GCSE Chemistry

Revision Notes

4.2.4 Soluble Salts

Preparing Soluble Salts

  • A soluble salt can be made from the reaction of an acid with an insoluble base
  • During the preparation of soluble salts, the insoluble reactant is added in excess to ensure that all of the acid has reacted
  • If this step is not completed, any unreacted acid would become dangerously concentrated during evaporation and crystallisation
  • The excess reactant is then removed by filtration to ensure that only the salt and water remain
  • Since all of the acid has reacted and the excess solid base has been removed then the solution left can only be salt and water
  • If a carbonate was used as the solid base instead of an oxide or hydroxide, then any carbon dioxide gas produced would have been released into the atmosphere
  • A common example is the preparation of copper(II) sulfate which can be made with copper(II) oxide and dilute sulfuric acid:

CuO (s) + H2SO4 (aq) CuSO4 (s) + H2O (l)

  • The acid could also be reacted with a metal to produce the salt, as long as the metal is above hydrogen in the reactivity series and not too reactive so that a dangerous reaction does not take place

Exam Tip

Exam questions often ask why the solid oxide is added in excess. This is done to avoid leaving any unreacted acid which would become dangerously concentrated during evaporation and crystallisation.

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.

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