Soluble Salts (AQA GCSE Chemistry: Combined Science)

• 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
• The water is evaporated by heating until small crystals begin to appear
  • This is typically once half of the water is left 
  • Allowing the filtered solution to evaporate slowly over a period of days results in the formation of larger crystals 
• 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 (aq) + H₂O (l)

Copper sulfate crystals

After filtering and heating copper sulfate crystals will form. Larger crystals will form if the water in the solution is left to evaporate slowly

Photo by Wikimedia commons

• Acids can react with metal to produce the salt if:
  • The metal is above hydrogen in the reactivity series
  • The metal is not too reactive, which could result in a dangerous reaction