Required Practical: Preparation of a Soluble Salt (AQA GCSE Chemistry: Combined Science)

Objective:

To prepare a pure, dry sample of a soluble salt from an insoluble oxide or carbonate using a Bunsen burner and dilute acid

Hypothesis:

A salt can be prepared and separated by an acid-base neutralisation reaction

Materials:

• 1.0 mol/dm³ dilute sulfuric acid
• Copper(II) oxide
• Spatula & glass rod
• Measuring cylinder & 100 cm³ beaker
• Bunsen burner
• Tripod, gauze & heatproof mat
• Filter funnel & paper, conical flask
• Evaporating basin and dish.

The steps in the preparation of a soluble salt

Practical Tip:

The base is added in excess to use up all of the acid, which would become dangerously concentrated during the evaporation and crystallisation stages

Method:

1. Add 50 cm³ dilute acid into a beaker and warm gently using a Bunsen burner
2. Add the insoluble oxide slowly to the hot dilute acid and stir until the base is in excess (i.e. until the base stops dissolving and a suspension of the base forms in the acid)
3. Filter the mixture into an evaporating basin to remove the excess base
4. Gently heat the solution in a water bath or with an electric heater to evaporate the water and to make the solution saturated
5. 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 the crystals to dry

Results:

Hydrated copper(II) sulfate crystals should be bright blue and regularly shaped

Evaluation:

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