Salts are prepared according to their solubility. Soluble salts of sodium, potassium and ammonium are made by titration; other soluble salts by reacting an acid with excess metal, base or carbonate then crystallising; insoluble salts by precipitation, then filtering, washing and drying.
This page covers one Form 4 content standard: preparation of salts. A salt forms when the hydrogen of an acid is replaced by a metal or ammonium ion, and the whole standard is about choosing the right method to make a particular salt. The examiner gives you a target salt and expects you to select the method, list the steps and write the equation, so a clear decision routine is the skill to build.
Solubility first: the deciding rule
Every method choice starts by asking whether the salt is soluble or insoluble in water. The solubility rules you need are:
- All sodium, potassium and ammonium salts are soluble.
- All nitrates are soluble.
- Most chlorides are soluble, except lead(II) chloride and silver chloride.
- Most sulfates are soluble, except lead(II) sulfate, barium sulfate and calcium sulfate.
- Most carbonates are insoluble, except sodium, potassium and ammonium carbonate.
Learn these as a short table; the whole standard hinges on placing the target salt correctly. A single misclassification sends you to the wrong method.
Method 1, Soluble salts of sodium, potassium or ammonium (titration)
These salts cannot be made with excess solid, because the reactants (a soluble alkali or carbonate) are themselves soluble and nothing would be left to filter off. Instead you use titration to mix the exact volumes that just react, then evaporate to crystallise.
Example: to make sodium chloride, titrate sodium hydroxide against hydrochloric acid to find the exact neutralising volume, then repeat without indicator and evaporate: NaOH + HCl → NaCl + H₂O.
Method 2, Other soluble salts (excess reactant, then crystallise)
For a soluble salt whose metal is not sodium, potassium or ammonium, such as copper(II) sulfate or zinc chloride, react the acid with an excess of an insoluble metal, metal oxide (base) or metal carbonate. Because the solid is insoluble and in excess, the reaction goes to completion and the leftover solid is simply filtered off. The steps are:
- Add the solid in excess to the warm acid until no more dissolves (excess confirms the acid is used up).
- Filter to remove the unreacted excess solid.
- Evaporate the filtrate to the point of crystallisation, then cool.
- Crystals form; filter, then dry them between filter papers.
Example: CuO + H₂SO₄ → CuSO₄ + H₂O.
Method 3, Insoluble salts (precipitation)
An insoluble salt is made by precipitation (double decomposition): mix two solutions that each contain one of the ions needed, and the insoluble salt drops out as a precipitate. Then filter, wash the residue with distilled water, and dry it.
Example: to make lead(II) iodide, mix lead(II) nitrate solution with potassium iodide solution: Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃, where PbI₂ is the yellow precipitate.
Worked example
Question. You are asked to prepare a pure, dry sample of (a) barium sulfate and (b) zinc chloride. For each, state whether the salt is soluble or insoluble, name the method, and give a balanced equation.
Step 1, Classify barium sulfate. Sulfates are soluble except barium sulfate, so BaSO₄ is insoluble.
Step 2, Method for (a). Use precipitation: mix barium chloride solution with sodium sulfate solution. BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl. Filter, wash and dry the BaSO₄ precipitate.
Step 3, Classify zinc chloride. Chlorides are soluble except lead(II) and silver, so ZnCl₂ is soluble, and zinc is not Na/K/ammonium.
Step 4, Method for (b). Use excess reactant then crystallise: add excess zinc (or zinc oxide/carbonate) to dilute hydrochloric acid. Zn + 2HCl → ZnCl₂ + H₂. Filter off the excess, evaporate to crystallisation, cool and dry.
Answer. (a) BaSO₄ is insoluble → precipitation, BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl. (b) ZnCl₂ is soluble → excess reactant + crystallisation, Zn + 2HCl → ZnCl₂ + H₂.
Practice question
Question. Describe how you would prepare a pure, dry sample of potassium nitrate, KNO₃, and explain why crystallisation rather than evaporation to dryness is used at the final step.
Answer. Potassium nitrate is a soluble salt of potassium, so use titration: titrate potassium hydroxide against nitric acid with an indicator to find the neutralising volume; repeat without indicator using the same volumes (KOH + HNO₃ → KNO₃ + H₂O); then heat the solution to the point of crystallisation, cool, filter and dry. Crystallisation (evaporate part-way, then cool) is used because heating a nitrate to complete dryness can decompose it and would leave impurities; slow crystallisation gives pure crystals.
Exam tip
Always classify the salt first with the solubility rules, soluble or insoluble decides everything else. Then match: Na/K/ammonium soluble salt → titration; other soluble salt → excess metal/base/carbonate + crystallisation; insoluble salt → precipitation. Quote the finishing steps in order (filter, and for insoluble salts wash, then dry), and remember to crystallise rather than evaporate to dryness for salts that would decompose. Naming the method but omitting the ordered steps is a common way to lose the last marks.
Building the decision into a routine
Turn the standard into a two-question flowchart you can run in seconds. Question one: is the salt soluble? Use the rules above. Question two, if soluble: is the metal sodium, potassium or ammonium? If yes, titration; if no, excess reactant then crystallisation. If the salt is insoluble, it is precipitation. Practise until you can place any named salt, nitrates, chlorides, sulfates, carbonates, onto the correct branch without hesitating, because the method marks in Paper 2 depend entirely on that first classification being right.
The other reliable marks come from the finishing steps and their reasons. For an insoluble salt, you must wash the residue to remove soluble impurities before drying, a step easy to forget. For a soluble salt, you crystallise rather than evaporate fully so that hydrated or heat-sensitive salts are not damaged. Pairing each step with its reason, exactly as you did for safety rules in the first chapter, is what turns a described method into a full-mark answer.
Where this fits
This is content standard 6.6 of the Acids, Bases and Salts chapter. It applies neutralisation from standard 6.5 and leads into the qualitative analysis of the salts you make. Reinforce the methods with the chapter revision notes and rehearse equations in the worked examples. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers turn the solubility rules into a quick decision routine, because getting the classification right is what makes the rest of every salt-preparation question straightforward.
Quick recap
- Classify the salt with the solubility rules first.
- Na/K/ammonium soluble salt → titration; other soluble salt → excess reactant + crystallisation; insoluble salt → precipitation.
- Finishing: filter; for insoluble salts wash the residue; then dry.
- Crystallise (not evaporate to dryness) for heat-sensitive soluble salts.
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