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
To prepare a pure, dry sample of a soluble salt from an insoluble oxide or carbonate using a Bunsen burner and dilute acid.
A salt can be prepared and separated by acid-base neutralisation reaction.
- 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.
Diagram of the apparatus needed
The base is added in excess to use up all of the acid, which would become dangerously concentrated during the evaporation and crystallisation stages.
- Add 50cm3 dilute acid into a beaker and heat using a Bunsen burner flame.
- 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
- forms in the acid).
- Filter the mixture into an evaporating basin to remove the excess base.
- Gently heat the solution in a water bath or with an electric heater to evaporate water and to make the solution saturated.
- Check the solution is saturated by dipping a cold glass rod into the solution and seeing if crystals form on the end.
- Leave the filtrate in a warm place to dry and crystallise.
- Decant excess solution and allow the crystals to dry.
Hydrated copper(II) sulfate crystals should be bright blue and regularly shaped.
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’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.