Preparation of salts
Preparing soluble salts by titration or excess solid, and insoluble salts by precipitation.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 7.3.1Describe the preparation, separation and purification of soluble salts.
- 7.3.2Describe the preparation of insoluble salts by precipitation.
- 7.3.3Recall the general solubility rules for common salts.Supplement
7 minute read
Preparing salts
Which method is used to prepare a salt depends on whether the salt is soluble or insoluble in water.
Solubility rules
- All sodium, potassium and ammonium salts are soluble.
- All nitrates are soluble.
- Most chlorides are soluble, except silver chloride and lead(II) chloride.
- Most sulfates are soluble, except barium sulfate, calcium sulfate and lead(II) sulfate.
- Most carbonates are insoluble, except sodium, potassium and ammonium carbonates.
Preparing a soluble salt (excess solid method)
Used when the salt comes from an acid and an insoluble base (a metal, metal oxide or metal carbonate):
- Warm the dilute acid and add the insoluble solid a little at a time, until it is in excess (no more dissolves).
- Filter to remove the unreacted excess solid, keeping the filtrate.
- Gently heat the filtrate to evaporate some water until the solution is saturated (a hot, saturated solution).
- Allow it to cool and crystallise, then filter off and dry the crystals.
Excess solid is used so that all the acid reacts — ensuring the salt is not contaminated with leftover acid.
Preparing a soluble salt (titration method)
Used when both reactants are soluble (an acid and an alkali): add a measured volume of alkali to a measured volume of acid, using an indicator to find the exact volume needed for neutralisation, then repeat without indicator using those measured volumes, and evaporate and crystallise as before.
Preparing an insoluble salt (precipitation)
Mix two soluble solutions, each containing one of the ions needed, so they react to form an insoluble product immediately: filter, wash with distilled water to remove other soluble impurities, then dry.
Think of it like this
Choosing a method is like choosing how to invite two guests to the same party: if one guest (a solid) cannot dissolve on their own, you add plenty of them and remove the leftovers afterwards (excess solid method); if both guests are already "in solution", you need to measure exact amounts carefully so they arrive in the right proportion (titration).
Common misconceptions
- Using excess acid instead of excess solid. Using excess solid is correct because the unreacted solid is easy to filter off, whereas excess acid would remain dissolved in the salt solution and contaminate the product.
- Forgetting to filter before evaporating and crystallising in the excess solid method — without filtering first, the final crystals would be contaminated with the unreacted solid.
- Trying to prepare an insoluble salt by evaporation. Precipitation happens immediately on mixing; evaporating a solution containing an insoluble salt does nothing useful, since the precipitate has already formed and settled.
In the exam
- For excess-solid method questions, the four steps (react with excess, filter, evaporate/crystallise, dry) are usually each worth a mark — write them as separate, clearly labelled steps.
- When explaining precipitation, name both ions coming together explicitly, e.g. "Ba²⁺ ions and SO₄²⁻ ions combine to form insoluble barium sulfate".