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.

Required Practical 1: Preparation of a Soluble Salt using an Insoluble Base

Objective:
To prepare a pure, dry sample of a soluble salt from an insoluble oxide or carbonate using a Bunsen burner and dilute acid.

Hypothesis:
A salt can be prepared and separated by acid-base neutralisation reaction.

Materials:

  • 1.0 mol/dm3 dilute sulfuric acid
  • Copper (II) oxide powder
  • Spatula & glass rod
  • Measuring cylinder & 100cm3 beaker
  • Bunsen burner
  • Tripod, gauze & heatproof mat
  • Filter funnel & paper, conical flask
  • Evaporating basin and dish.

IGCSE & GCSE Chemistry revision notes

Diagram of the apparatus needed

Practical Tip:
The base is added in excess to use up all of the acid, which would become dangerously concentrated during the evaporation and crystallisation stages.

Method:

  1. Add 50cm3 dilute acid into a beaker and heat using a Bunsen burner flame.
  2. Add the insoluble oxide slowly to the hot dilute acid and stir until the base is in excess (i.e. until the base stops disappearing and a suspension of the base
  3. forms in the acid).
  4. Filter the mixture into an evaporating basin to remove the excess base.
  5. Gently heat the solution in a water bath or with an electric heater to evaporate water and to make the solution saturated.
  6. Check the solution is saturated by dipping a cold glass rod into the solution and seeing if crystals form on the end.
  7. Leave the filtrate in a warm place to dry and crystallise.
  8. Decant excess solution and allow the crystals to dry.

Results:
Hydrated copper(II) sulfate crystals should be bright blue and regularly shaped.

Evaluation:
Describe how your crystals compare to the description in the results section. If different suggest an explanation.

Conclusion: Acid-base reactions produce salt and water with the regular shape of the salt reflecting the ionic lattice structure in its bonding.

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.