Preparing Crystals of a Soluble Salt (WJEC GCSE Chemistry: Combined Science)

Introduction

• 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
• Salts are an important branch of chemistry due to the varied and important uses of this class of compounds
• These uses include fertilisers, batteries, cleaning products, healthcare products and fungicides
• Some salts can be extracted by mining but others need to be prepared in the laboratory
  

Apparatus

• Stirring rod
• 100 cm³ beaker
• Filter funnel and paper
• Evaporating dish
• 50 cm³ measuring cylinder
• 0.5 mol dm^–3 sulfuric acid, H₂SO₄
• Copper(II) oxide
• Spatula
• Indicator paper

Diagram

Adding acid to a solid metal, insoluble base or insoluble carbonate

Diagram showing the preparation of soluble salts

 

Method

1. Add dilute acid into a beaker and heat using a Bunsen burner flame
2. Add the insoluble metal, base or carbonate, a little at a time, to the warm 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)
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 base
5. Heat the solution to evaporate half of the 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
6. Leave the filtrate in a warm place to dry and crystallise
7. Decant excess solution and allow crystals to dry or blot to dry with filter paper

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. Check the solution is saturated by dipping a cold, glass rod into the solution and seeing if crystals form on the end
6. Leave the filtrate in a warm place to dry and crystallise
7. 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.

Analysis of results

• This method can be used in the formation of other soluble salts such as potassium chloride and potassium sulfate