Making soluble salts from insoluble substances

A soluble salt can be prepared by reacting an acid with a suitable insoluble reactant including:

  • a metal
  • a metal oxide
  • a metal hydroxide
  • a carbonate

The insoluble reactant chosen depends upon the particular salt required.

For example, copper does not react with dilute acids, so this metal cannot be used. On the other hand, sodium is too reactive to be used safely.

As the reaction between metals and acids produces flammable hydrogen, chemists usually make salts by reacting a metal oxide or a metal carbonate with an acid.

Choosing reactants

The table shows some examples of the salts produced by different combinations of insoluble reactants and acids.

Hydrochloric acidSulfuric acidNitric acid
Copper oxideCopper chlorideCopper sulfateCopper nitrate
Aluminium hydroxideAluminium chlorideAluminium sulfateAluminium nitrate
Zinc carbonateZinc chlorideZinc sulfateZinc nitrate

Making a salt

To make a soluble salt from an acid and an insoluble reactant:

  1. Add powdered insoluble reactant to acid in a beaker, one spatula at a time, stirring to mix. Continue adding powder until it is in excess (some unreacted powder is left over). All the acid has now reacted.
  2. Filter the mixture in the beaker to remove the excess solid. The filtrate now contains only the salt and water.
  3. Heat the solution in an evaporating dish over a water bath. Stop heating when small crystals start to appear around the edge of the evaporating basin. The solution is now saturated.
  4. Leave the saturated solution at room temperature for a day or two. This gives time for large crystals to form.
  5. If necessary, dry the crystals by dabbing gently with filter paper.
Question

Give the names of two substances that react to make zinc sulfate.

Sulfuric acid and zinc or zinc oxide or zinc carbonate or zinc hydroxide.