How do we prepare a pure, dry sample of a salt?
The solubility rules for salts, preparing a soluble salt from an acid and an insoluble base, and preparing an insoluble salt by precipitation.
A CCEA GCSE Chemistry answer on preparing salts, covering the general solubility rules, the method for making a soluble salt from an acid and an insoluble base by filtration and crystallisation, and the precipitation method for making an insoluble salt.
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What this dot point is asking
CCEA wants you to use the solubility rules to decide whether a salt is soluble, describe how to make a soluble salt from an acid and an insoluble base, and describe how to make an insoluble salt by precipitation.
Solubility rules
These rules decide your method: a soluble salt is crystallised from solution, while an insoluble salt is made by precipitation.
Making a soluble salt
The standard method uses an acid and an insoluble base (a metal, metal oxide, hydroxide or carbonate):
Using excess base makes sure all the acid is neutralised, and filtering removes the leftover base so the salt is pure. Crystallising rather than boiling dry gives proper crystals.
Making an insoluble salt
For example, to make insoluble lead iodide, mix lead nitrate solution with potassium iodide solution; the lead and iodide ions combine and a yellow precipitate forms. Then filter, wash and dry the precipitate to obtain the pure salt.
Worked example
Examples in context
- Example 1. Barium meals in medicine
- Insoluble barium sulfate is made by precipitation and swallowed as a "barium meal" so the gut shows up on X-rays. Because it is insoluble, the toxic barium ions are not absorbed, which is why the precipitation method that gives a pure insoluble solid matters.
- Example 2. Making fertiliser salts
- Soluble ammonium and potassium salts used as fertilisers are prepared by neutralising acids with the right base and crystallising the product. The soluble salt method scales straight from the lab bench to industrial production.
- Example 3. Filtering out hard-water scale
- When insoluble calcium carbonate forms as limescale, the same separation logic applies: it can be filtered from water because it is insoluble, just like a precipitated salt. The solubility rules that guide salt preparation also explain why some compounds form solids in everyday water systems.
Why the choice of method matters
The two methods exist because the product behaves differently in water. A soluble salt stays dissolved, so you cannot simply filter it out; you have to remove the only insoluble thing present (the excess base) and then crystallise the salt from its solution. An insoluble salt is the opposite: it will not stay in solution, so the moment the two ions meet it falls out as a precipitate that you can filter. Recognising the solubility of the target salt before you start is therefore the single most important decision, and CCEA often gives a marks for justifying the method as well as describing the steps.
Try this
Q1. State why excess insoluble base is added when preparing a soluble salt. [1 mark]
- Cue. To make sure all the acid is used up (neutralised).
Q2. Name the technique used to make an insoluble salt. [1 mark]
- Cue. Precipitation.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20195 marksDescribe how you would prepare pure, dry crystals of copper(II) sulfate from copper(II) oxide and dilute sulfuric acid.Show worked answer →
Markers want the steps in order with reasons.
Add excess copper(II) oxide to warm dilute sulfuric acid and stir. The copper oxide (an insoluble base) neutralises the acid to form copper(II) sulfate solution and water. Using excess ensures all the acid is used up.
Filter to remove the unreacted excess copper oxide, leaving a blue copper(II) sulfate solution (the filtrate).
Warm the filtrate to evaporate some water until it is saturated, then leave it to cool slowly so crystals form (crystallisation). Filter off the crystals and dry them between filter papers.
Markers reward using excess base, filtering off the excess, crystallising (not boiling to dryness) and drying the crystals.
CCEA 20214 marksBarium sulfate is insoluble. Describe how you would prepare a pure, dry sample of barium sulfate by precipitation.Show worked answer →
The marks are for mixing two soluble solutions, then filtering and washing.
Mix a solution of a soluble barium salt (such as barium chloride) with a solution of a soluble sulfate (such as sodium sulfate or dilute sulfuric acid). The barium ions and sulfate ions react to form insoluble barium sulfate, which appears as a white precipitate.
Filter the mixture to collect the barium sulfate as the residue, wash it with distilled water to remove soluble impurities, and dry it.
Markers reward mixing two soluble solutions to precipitate the insoluble salt, then filtering, washing and drying the precipitate.
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Sources & how we know this
- CCEA GCSE Chemistry specification (1110) — CCEA (2017)