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CIE iGCSE Co-Ordinated Science C7.3 Preparation of salts Exam Style Questions Paper 2

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Question

Copper(II) sulfate is produced by reacting copper(II) oxide with dilute sulfuric acid. The stages in the process to produce pure dry crystals are listed.

  1. Leave to crystallise in a cool place.
  2. Filter the reaction mixture.
  3. Press the crystals between dry filter papers.
  4. Add copper(II) oxide until it is in excess.
  5. Heat the filtrate to concentrate it.
  6. Heat the dilute sulfuric acid.

What is the correct order for these stages?

A. 6 → 4 → 2 → 5 → 3 → 1
B. 6 → 4 → 1 → 2 → 5 → 3
C. 6 → 4 → 2 → 5 → 1 → 3
D. 4 → 6 → 2 → 1 → 5 → 3

▶️ Answer/Explanation
The acid is heated first, then excess copper(II) oxide is added to ensure all acid reacts.
The mixture is then filtered to remove unreacted oxide, the filtrate is heated to concentrate it, left to crystallise, and finally the crystals are pressed dry.
✅ Answer: (C)

Question

Crystals of copper(II) sulfate, a soluble salt, are made by reacting excess copper(II) oxide with dilute sulfuric acid.

After filtering off the unreacted copper(II) oxide, the solution is heated until it is saturated. It is then left to cool.

Which statements about this preparation are correct?

  1. Excess copper(II) oxide is used in order to ensure a high yield is obtained.
  2. After filtering, the solution is heated to evaporate some of the water.
  3. When the saturated solution cools, crystals of copper(II) sulfate begin to appear.
  4. After cooling, the water is rapidly removed by evaporation.

A. 1 and 3
B. 1 and 4
C. 2 and 3
D. 2 and 4

▶️ Answer/Explanation
Excess copper(II) oxide is used to ensure all the acid reacts, not specifically for “a high yield” phrasing — the true purpose is to guarantee no leftover acid contaminates the salt, so statement 1 is not the intended reason and is considered incorrect here.
After filtering, heating the solution does evaporate some water until saturation point is reached, making statement 2 correct.
Cooling a saturated solution lowers its solubility limit, so crystals of copper(II) sulfate begin forming as it cools, making statement 3 correct; statement 4 is wrong since crystallisation relies on slow cooling, not rapid evaporation after cooling.
✅ Answer: (C)

Question

Information about the solubility in water of some calcium compounds is listed.

  • Calcium hydroxide is soluble.
  • Calcium carbonate is insoluble.
  • Calcium chloride is soluble.

Which method is used to prepare pure calcium chloride?

A. Add excess calcium hydroxide to dilute hydrochloric acid, filter, then crystallise.
B. Add excess calcium carbonate to dilute hydrochloric acid, filter, then crystallise.
C. Add excess dilute hydrochloric acid to calcium hydroxide, filter, then crystallise.
D. Add excess dilute hydrochloric acid to calcium carbonate, filter, then crystallise.

▶️ Answer/Explanation
To ensure all the acid reacts and none remains in the final product, an excess of the insoluble solid reactant is used.
Calcium carbonate is insoluble, so excess unreacted solid can simply be filtered off, leaving pure calcium chloride solution.
Crystallisation of the filtrate then yields pure solid calcium chloride: \(CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2\).
✅ Answer: (B)

Question

Copper carbonate is insoluble in water.

Which method can be used to make copper carbonate?

A. adding aqueous sodium carbonate to aqueous copper sulfate and filtering off the product
B. adding solid calcium carbonate to aqueous copper sulfate and filtering off the product
C. adding solid copper oxide to solid calcium carbonate and heating the mixture
D. heating copper metal in carbon dioxide

▶️ Answer/Explanation
Copper carbonate, an insoluble salt, is prepared by precipitation: mixing two soluble solutions whose ions combine to form the insoluble product.
Mixing aqueous sodium carbonate with aqueous copper sulfate produces insoluble copper carbonate, which is separated by filtration.
✅ Answer: (A)

Question

Zinc sulfate is a soluble salt. Zinc sulfate is prepared by reacting zinc with dilute sulfuric acid.

Which statement about this preparation is correct?

A. Excess dilute sulfuric acid is used and after the reaction has finished the unreacted acid is removed by evaporation.
B. Excess dilute sulfuric acid is used and after the reaction has finished the unreacted acid is removed by adding an aqueous base.
C. Excess zinc is used and after the reaction has finished the unreacted zinc is removed by filtration.
D. Excess zinc is used and after the reaction has finished the zinc sulfate is purified by distillation.

▶️ Answer/Explanation
Excess zinc is used to ensure all the acid reacts. The unreacted zinc (solid) is removed by filtration, leaving a solution of zinc sulfate. Evaporation/crystallisation then gives pure zinc sulfate crystals.
✅ Answer: (C)
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