Making soluble salts (except sodium, potassium and ammonium salts)
3 marks · NormalA student tries to make a pure sample of hydrated magnesium sulfate crystals using magnesium and dilute sulfuric acid.
Explain, in steps, what the student must do to obtain pure crystals and why each step is necessary.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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Also accept: Allow either bulleted or continuous prose if all steps and reasons are clear.
Common mistakes: Omitting filtration; Evaporating to dryness
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Marking points
M1 · 1 mark · Add excess magnesium to sulfuric acid until fizzing stops (so all acid reacts)
M2 · 1 mark · Filter off excess magnesium (to get pure solution)
M3 · 1 mark · Evaporate gently to crystallisation point, cool to form crystals (avoiding dryness to prevent anhydrous salt)
Worked solution
Add magnesium to dilute sulfuric acid until the reaction (fizzing) stops, showing all acid reacted.
Filter to remove excess magnesium. This ensures only the required ions remain.
Evaporate the solution by heating gently until crystals begin to form. Let it cool so hydrated magnesium sulfate crystals develop. Do not evaporate to dryness.