Pure, dry crystals of a soluble salt can be made in two ways. Use an excess of an insoluble solid when you can filter off what is left over; use a titration when both reactants are solutions.
A salt forms when the hydrogen of an acid is replaced by a metal or ammonium ion. To make a soluble salt from an insoluble solid: add excess solid (metal, base or carbonate) to warm acid, filter off the excess, heat the filtrate to the point of crystallisation, leave to cool, then filter, wash and dry the crystals. For sodium, potassium and ammonium salts, react an acid with an alkali using a titration, then repeat without the indicator and crystallise.
Fig. 1: making copper(II) sulfate crystals by the excess solid method
Section 01
The excess solid method.
Use this method when the other reactant is an insoluble solid, so any unreacted solid can be filtered off.
Example: copper(II) sulfate from copper(II) oxide and dilute sulfuric acid, as in Fig. 1.
Why excess? Adding solid until no more dissolves makes sure all the acid has reacted, so the salt is not contaminated with acid.
Why warm? Warming speeds up the reaction.
Point of crystallisation: heat until crystals appear on a cold glass rod dipped into the solution. Do not evaporate to dryness: that can decompose the salt and gives no crystals of water of crystallisation.
Other examples: zinc sulfate from zinc and sulfuric acid; magnesium chloride from magnesium carbonate and hydrochloric acid.
Examiner trap
Filter before crystallising, to remove the excess solid; then filter again after crystallising, to separate the crystals from the remaining solution. And the salt is in the filtrate in the first filtration, not in the residue.
Stage 1 · Learn
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Which method is used to make copper(II) sulfate from copper(II) oxide and sulfuric acid?
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Why is excess copper(II) oxide added?
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How is the excess solid removed?
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Which salt should be made by titration rather than by the excess solid method?
Section 02
The titration method.
Sodium, potassium and ammonium compounds are soluble, so an excess cannot be filtered off. Instead, find the exact volumes that react.
1. Pipette 25.0 cm3 of alkali (for example sodium hydroxide) into a conical flask and add a few drops of indicator.
2. Add acid from a burette until the indicator just changes colour. Record the volume of acid used.
3. Repeat with the same volumes but without the indicator, so the salt is not contaminated.
4. Heat the solution to the point of crystallisation, cool, filter, wash and dry the crystals.
Example: NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l).
Stage 2 · Exam
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A student makes zinc sulfate crystals. After filtering off the excess zinc, what should be done next?
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In the titration method, why is the titration repeated without indicator?
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Which pair of reactants would be suitable for making magnesium chloride by the excess solid method?
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Why is the acid often warmed when making a salt by the excess solid method?
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