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4.2.3Soluble salts

AQA GCSE Chemistry (8462), Higher tier · Chemical changes › Reactions of acids

Practise Soluble salts. 19 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.

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Required practical: Making salts (method, variables and exam tips)

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A student made copper sulfate crystals from copper oxide and dilute sulfuric acid.
The steps of the method are shown in the wrong order:
  • A filter the mixture
  • B add copper oxide to warm acid until no more dissolves
  • C leave the solution to cool so that crystals form
  • D heat the filtrate to evaporate some of the water
 Give the correct order of the steps.
B, A, D, C
A student made crystals of magnesium sulfate by reacting magnesium carbonate with dilute sulfuric acid. Write a word equation for the reaction.
magnesium carbonate + sulfuric acid → magnesium sulfate + water + carbon dioxide
Write a balanced symbol equation, including state symbols, for the reaction.
CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l)
A student makes copper(II) sulfate solution by adding copper(II) oxide to warm dilute sulfuric acid until some of the black solid is left undissolved. The student filters the mixture, then heats the filtrate to evaporate some of the water.
The student then investigates how the place where the hot solution is left to cool affects the size of the crystals that form. The student pours 20 cm3 of the hot solution into each of three identical evaporating dishes. One dish is left in a refrigerator, one on the laboratory bench and one in a warm cupboard. After two days, the student measures the length of the largest crystal in each dish. What is the dependent variable in this investigation?
The length of the largest crystal.

Sample questions

Written for this site in the style of AQA exam questions. They are not taken from real past papers.

Question 1Easy4 marks
A student made copper sulfate crystals from copper oxide and dilute sulfuric acid.
The steps of the method are shown in the wrong order:
  • A filter the mixture
  • B add copper oxide to warm acid until no more dissolves
  • C leave the solution to cool so that crystals form
  • D heat the filtrate to evaporate some of the water
(a) Give the correct order of the steps.[2]
(b) Explain why the student added copper oxide until no more dissolved.[1]
(c) Name the apparatus used to remove the excess copper oxide.[1]
Show the answer and mark scheme
(a) Answer: B, A, D, C
  • B first and C last
  • A before D: B, A, D, C
(b)
  • to make sure all of the acid had reacted / was used up
(c)
  • filter funnel and filter paper
Question 2Medium6 marks
A student made crystals of magnesium sulfate by reacting magnesium carbonate with dilute sulfuric acid.
(a) Write a word equation for the reaction.[1]
(b) The student warmed the acid before adding the magnesium carbonate.
Give a reason why.[1]
(c) How would the student know when enough magnesium carbonate had been added?[2]
(d) The student evaporated the solution using a water bath instead of heating it strongly with a Bunsen burner.
Suggest why.[1]
(e) Suggest how the student could dry the crystals.[1]
Show the answer and mark scheme
(a) Answer: magnesium carbonate + sulfuric acid → magnesium sulfate + water + carbon dioxide
  • magnesium carbonate + sulfuric acid → magnesium sulfate + water + carbon dioxide
(b)
  • to increase the rate of reaction / so the solid reacts faster
(c)
  • no more fizzing / bubbles
  • some solid is left (undissolved) in the beaker
(d)
  • gentle heating stops the solution spitting / stops the salt decomposing / allows crystals to form (instead of a powder)
(e)
  • pat dry between pieces of filter paper / leave in a warm place / leave on a windowsill
Question 3Hard8 marks
Copper(II) sulfate is a soluble salt. It can be made by reacting insoluble copper(II) oxide with dilute sulfuric acid.
(a) Write a balanced symbol equation, including state symbols, for the reaction.[2]
(b) Describe a method to make pure, dry crystals of copper(II) sulfate from copper(II) oxide and dilute sulfuric acid.[6]
Show the answer and mark scheme
(a) Answer: CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l)
  • CuO + H2SO4 → CuSO4 + H2O
  • state symbols (s), (aq), (aq), (l)
(b)
  • measure a known volume (e.g. 40 cm3) of dilute sulfuric acid into a beaker using a measuring cylinder
  • warm the acid gently using a Bunsen burner (do not boil)
  • add copper(II) oxide a spatula at a time, stirring
  • until copper(II) oxide is in excess / no more dissolves / black solid remains
  • filter to remove the excess copper(II) oxide and collect the blue filtrate
  • heat the filtrate in an evaporating basin using a water bath / electric heater until about half the water has evaporated / crystals start to form
  • leave the solution to cool so that crystals form
  • remove the crystals (by filtering) and pat them dry with filter paper / leave in a warm place

Marked with levels of response: the full level descriptors are in the app.

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