Practical Techniques - Salt Preparation (SQA National 5 Chemistry): Revision Note

Preparation of soluble salts

What is a salt?

• A salt is a compound that is formed when the hydrogen atom in an acid is replaced by a metal

• For example, if we replace the H in HCl with a potassium atom, then the salt potassium chloride is formed, KCl

How to make a soluble salt

• There are two options for making soluble salts

• The method you choose depends on whether your starting base is soluble or insoluble

Option 1: Using an insoluble base

• This method is used to make a soluble salt from an insoluble base, such as:

  • An insoluble metal oxide

  • An insoluble metal hydroxide

  • An insoluble metal carbonate

Method:

1. React

   • Gently warm the dilute acid and add the insoluble base in excess (add it until no more reacts and you can see some unreacted solid).

2. Filter

   • Filter the mixture to remove the unreacted excess solid base.

   • The liquid that passes through (the filtrate) is your pure salt solution.

3. Evaporate and crystallise

   • Gently heat the filtrate in an evaporating basin to evaporate some of the water.

   • Once crystals start to form, leave the solution to cool and crystallise slowly.

4. Dry

   • Remove the crystals and pat them dry.

• For full details, see the Producing Soluble Salts from Carbonates or Oxides revision note

Example: preparation of pure, hydrated copper(II) sulfate crystals

• Acid = dilute sulfuric acid 

• Insoluble base = copper(II) oxide

• Method

  1. Add dilute sulfuric acid into a beaker and heat using a Bunsen burner flame

  2. Add copper(II) oxide (insoluble base), a little at a time to the warm dilute sulfuric acid and stir until the copper(II) oxide is in excess (stops disappearing)

  3. To check the acid has been neutralised touch the glass rod onto indicator paper

  4. Filter the mixture into an evaporating basin to remove the excess copper(II) oxide

  5. Heat the solution to evaporate half of the 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 crystals to dry or blot to dry with filter paper

• Equation of reaction:

copper(II) oxide + sulfuric acid → copper(II) sulphate + water

CuO (s) + H₂SO₄ (aq) → CuSO₄ (aq) + H₂O (l)

Practical tip

• Allowing the filtered solution to evaporate slowly over about a week will result in the formation of larger crystals

• Heating the filtered solution will result in the formation of smaller crystals

Option 2: Using a soluble base / alkali

• This method is used to make a soluble salt from a soluble base

  • For example, the alkali sodium hydroxide

Method:

1. Titrate

   • Perform a titration using an indicator to find the exact volume of acid needed to neutralise a fixed volume of the alkali.

2. Repeat without indicator

   • Repeat the experiment using the exact same volumes of acid and alkali, but without the indicator

   • This creates a pure, uncontaminated salt solution.

3. Evaporate and crystallise

   1. Gently evaporate the water from the pure salt solution and allow crystals to form, as in Option 1

• For full details, see the Producing Soluble Salts by Titration revision note

Preparation of insoluble salts

• Insoluble salts can be prepared using a precipitation reaction

• The solid salt obtained is the precipitate, thus in order to successfully use this method the solid salt being formed must be insoluble in water, and the reactants must be soluble

Using two soluble reactants to prepare an insoluble salt

Method

1. Dissolve soluble salts in water and mix together using a stirring rod in a beaker

2. Filter to remove precipitate from mixture

3. Wash residue with distilled water to remove traces of other solutions

4. Leave in an oven to dry

Example: Preparation of pure, dry lead(II) sulfate crystals using a precipitation reaction

• Soluble salt 1 = lead(II) nitrate        

• Soluble salt 2 = potassium sulfate

• Method

  1. Dissolve lead(II) nitrate and potassium sulfate in water and mix together using a stirring rod in a beaker

  2. Filter to remove precipitate from mixture

  3. Wash precipitate with distilled water to remove traces of potassium nitrate solution

  4. Leave in an oven to dry

• Equation of reaction

lead(II) nitrate + potassium sulfate → lead(II) sulfate + potassium nitrate

Pb(NO₃)₂ (aq) + K₂SO₄ (aq) → PbSO₄ (s) + 2KNO₃ (aq)