Preparation of salts: four questions to try now
Real past-paper questions, the answer key from the mark scheme, and the explanation that goes with it. No account needed to answer them.
Question 1
Which row describes the solubility of lead(II) chloride and lead(II) sulfate in water? Each answer gives, in order: lead(II) chloride; lead(II) sulfate.
Answer: D.
The solubility rules make chlorides soluble apart from silver and lead, and sulfates soluble apart from barium, calcium and lead, so lead appears on both exception lists and both compounds are insoluble. Lead is the awkward metal precisely because it breaks the pattern twice over, and the white precipitate of lead(II) chloride is why silver nitrate tests are acidified with nitric acid rather than hydrochloric acid. Answering soluble for the chloride applies the general rule while forgetting that lead is one of its two exceptions. Answering soluble for the sulfate repeats that mistake on the other rule, where lead stands alongside barium and calcium.
Question 2
Four stages used to prepare an insoluble salt are listed. 1 drying 2 filtration 3 precipitation 4 washing In which order are the stages done?
Answer: B.
An insoluble salt is made by precipitation, so the sequence has to begin by forming the solid, which happens when two solutions are mixed and the salt appears as a suspension. The solid is then separated from the leftover solution by filtration, rinsed on the paper with distilled water to remove soluble impurities, and finally dried, giving the order 3, 2, 4, 1. Washing before filtering would simply dilute the mixture in the beaker and remove nothing, since the impurities would still be in the liquid surrounding the solid. Drying before washing is worse, because the residue would then have to be wetted all over again, undoing the step just carried out. No sequence can open with filtration either, since until precipitation has occurred there is no solid for the paper to collect.
Question 3
Which methods of salt preparation are suitable for copper(II) chloride?
1 Add copper(II) carbonate to dilute hydrochloric acid.
2 Add copper to dilute hydrochloric acid.
3 Warm copper(II) oxide with dilute hydrochloric acid.
Answer: C.
A salt forms when an acid meets a metal, a metal oxide, a metal hydroxide or a metal carbonate, with the anion coming from the acid and the metal from the other reactant. Hydrochloric acid supplies the chloride in all three suggestions, and both copper(II) carbonate and copper(II) oxide react with it to give copper(II) chloride, releasing carbon dioxide and water in the first case and water alone in the second. Method 2 is the failure, because copper sits below hydrogen in the reactivity series and cannot displace hydrogen from dilute hydrochloric acid, so nothing happens however long the mixture is left. Choosing all three assumes every metal dissolves in acid, which is precisely the assumption copper is famous for breaking, and that same unreactivity is why copper is used for water pipes and roofing.
Question 4
The solubility of some salts is shown.
chloride nitrate sulfate carbonate
barium soluble soluble insoluble insoluble
lead(II) insoluble soluble insoluble insoluble
potassium soluble soluble soluble soluble
zinc soluble soluble soluble insoluble
Which two aqueous solutions produce an insoluble salt when mixed together?
Answer: C.
A precipitate appears only if the ions swap partners to give a compound the table lists as insoluble, so each pair has to be re-paired before checking. Lead(II) nitrate with potassium carbonate gives lead(II) carbonate, marked insoluble, alongside potassium nitrate, which stays dissolved, so a solid does form. Barium chloride with zinc nitrate produces barium nitrate and zinc chloride, and the table calls both soluble, so nothing appears. Potassium nitrate with zinc sulfate gives potassium sulfate and zinc nitrate, again both soluble. Barium nitrate with lead(II) nitrate is the emptiest choice of the four, because both compounds are nitrates and there is no new pairing available to make.
What this practice covers
These questions are drawn from past CIE 0620 Chemistry papers and filtered to preparation of salts. You answer, you find out immediately whether you were right, and you get the reasoning for the correct option and for each distractor. Wrong answers go to a mistakes locker so you can come back to exactly those.
Practice is free. You need an account only so your progress and your mistakes are still there next time.
Keep going Paper 1 and Paper 2 MCQs →
What examiners see students get wrong here
These are the errors that cost marks on preparation of salts, taken from our own topic notes. Read them before you practise and you will recognise the traps in the questions.
- Using a metal to make a sodium or potassium salt, when sodium and potassium react violently with acid.
- Trying to filter off excess alkali, which is soluble.
- Leaving the indicator in during the second titration, so the crystals are coloured.
- Evaporating the solution to dryness instead of to the point of crystallisation, which spoils the crystals.
- Forgetting to wash the precipitate, leaving the soluble salt behind.
- Saying the excess solid is added so the reaction goes faster, rather than to use up all the acid.
- Choosing precipitation for a soluble salt, which would leave both salts in the same solution.
Revise it first
If any of the above is unfamiliar, work through the notes before practising: Preparation of salts revision notes.