Making Soluble Salts

To make a soluble salt, you can react an acid with an insoluble base. The following steps can be followed to create soluble salts:

1. Start by adding an excess of the insoluble base to an acid, such as dilute sulfuric acid (H2SO4).

  • Make sure to allow all of the insoluble base to react with the acid before proceeding to the next step. Continue adding the base until you are confident that the reaction has finished.
  • In reactions between metal carbonates and acids, there is effervescence (fizzing) due to the release of carbon dioxide.

2. After the reaction, remove any excess reactant using filtration.

  • This ensures that the only compounds remaining in the beaker or container will be the salt and water.

3. To separate the salt and water, gently heat the solution to evaporate the water.

  • This will leave salt crystals behind in the beaker.

For example, you can prepare copper (II) sulphate by reacting copper (II) oxide with dilute sulphuric acid:

CuO (s) + H2SO(aq) ⟶ CuSO(s) + H2O (l)

The labelled steps of making a soluble salt including the reaction, filtration, evaporation and collection of crystals.

Why Do We Add the Insoluble Reactant in Excess?

When making soluble salts from insoluble bases, we add the insoluble reactant in excess. This will leave a small amount of insoluble reactant, which we can remove using filtration. In this case, the acid is the limiting reactant (the reactant that is fully used up first).

This might seem wasteful, and some students may ask why it’s not better to calculate the exact required quantities before making the soluble salt.

The answer is quite simple. Acids are dangerous substances and can cause serious harm. During evaporation and crystallisation, the unreacted acid can become dangerously concentrated, so any leftover acid can be a severe hazard.

Therefore, we add the base in excess to ensure that all the acid has reacted to form a salt. This prevents any acid from being left over, which decreases the risk of damage, injury, or harm associated with making soluble salts.

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