Chapter 4: Chemical changes
Required practical: Preparing a pure, dry sample of a soluble salt
from an insoluble oxide or carbonate: Practical sheet 4.8
Magnesium sulfate can be made by neutralising sulfuric acid using magnesium carbonate:
magnesium carbonate + sulfuric acid → magnesium sulfate + water + carbon dioxide
This salt can be recovered from the solution formed by evaporation, and can be purified by
crystallisation.
Apparatus
Wear eye protection and collect some 0.5 mol/dm3 sulfuric acid, two 100 cm3 beakers and a stirring
rod, a Bunsen burner, tripod, gauze, pipe-clay triangle, a heat-resistant mat and tongs, a conical flask,
two filter funnels and filter paper, an evaporating basin and two crystallising dishes. You will also need
some magnesium carbonate powder and distilled water.
Method
1. Pour about 20 cm3 of 0.5 mol/dm3 sulfuric acid into a 100 cm3 beaker.
2. Add the magnesium carbonate a little at a time, stirring after each addition, until no more reacts and
the mixture stays cloudy.
Filter to remove the excess carbonate.
3. Pour the magnesium sulfate solution into the evaporating basin and heat until about half the
solution has evaporated.
4. Test the solution by dipping a glass rod in it and then letting the rod cool. When small crystals form
on the glass rod, stop heating the solution.
5. Use tongs to pour the magnesium sulfate solution into a crystallising dish and leave it in a warm
place to finish crystallising.
6. Filter off the crystals and pour a little cold distilled water over them to wash away soluble impurities.
7. Dissolve the crystals in warm distilled water, and leave them to cool and recrystallise.
8. Filter off the crystals and dry them.
AQA GCSE: Chemistry: Teacher Pack
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