Past Paper Exam

7.3.12 — Salt Synthesis Masterclass

Cambridge IGCSE Chemistry 0620 · Papers 4 & 6 · 15 Questions · 40 Marks

Section A

Method Selection

Section B

Full Method Descriptions

Section C

Reagent Choices

Section AMethod Selection

Which method should be used to prepare pure, dry crystals of Sodium Nitrate (NaNO₃)? Justify your answer.

[2 marks]💡 Think: Is sodium nitrate soluble? Can you filter out excess reactant?

Which method should be used to prepare a pure, dry sample of Barium Sulfate (BaSO₄)? Justify your answer.

[2 marks]💡 Check solubility: Is BaSO₄ soluble or insoluble in water?

Which method should be used to prepare pure, dry crystals of Magnesium Chloride (MgCl₂)? Justify your answer.

[2 marks]💡 Magnesium chloride is soluble. Can you use a base that won't contaminate the product?

A student wants to prepare Lead(II) Sulfate (PbSO₄). Which method is most appropriate and why?

[2 marks]💡 Check solubility rules: Is PbSO₄ soluble?

Which method is used to prepare Potassium Sulfate (K₂SO₄)? Explain why the excess solid method cannot be used here.

[2 marks]💡 Potassium sulfate is soluble. What kind of base would you need?

Section BFull Method Descriptions

Briefly outline the steps to make pure, dry crystals of Magnesium Chloride starting from Magnesium Carbonate and dilute Hydrochloric acid.

[6 marks]💡 6 marks = 6 key steps. Think: react → filter → evaporate → cool → filter → dry

Describe fully how you would prepare pure, dry crystals of Sodium Chloride starting from Sodium Hydroxide solution and dilute Hydrochloric acid.

[6 marks]💡 NaCl is soluble — you cannot filter off excess. Which method must you use?

Describe how to prepare a pure, dry sample of Copper(II) Sulfate crystals from Copper(II) Oxide and dilute Sulfuric acid.

[6 marks]💡 CuSO₄ is soluble. CuO is an insoluble base. Which method suits this perfectly?

Describe how to prepare a pure, dry sample of Silver Chloride (AgCl) precipitate from aqueous silver nitrate and dilute hydrochloric acid.

[5 marks]💡 Is AgCl soluble? Which method is used for insoluble salts?

Explain why the crystallisation step requires heating only to the 'crystallisation point' and NOT to complete dryness. What would happen if the solution were boiled to dryness?

[4 marks]💡 Think about what might happen to the salt crystals at very high temperatures.

Section CReagent Choices

Suggest two aqueous reagents that could be used to prepare Lead(II) Chloride (PbCl₂) by the precipitation method.

[2 marks]💡 You need a source of Pb²⁺ ions and a source of Cl⁻ ions, both soluble.

Suggest two reagents to prepare Zinc Sulfate (ZnSO₄) using the excess solid method.

[2 marks]💡 You need an insoluble zinc compound (metal, oxide, hydroxide, or carbonate) and a suitable acid.

Suggest two aqueous reagents to prepare Barium Sulfate (BaSO₄) by precipitation.

[2 marks]💡 Both reagents must be soluble, and when mixed, must produce an insoluble BaSO₄ precipitate.

A student wants to make Iron(II) Sulfate (FeSO₄). Suggest suitable starting reagents and the method. Explain why iron(II) sulfate cannot be made by titration.

[3 marks]💡 Iron is a metal — what does it react with? Why can't titration work here?

State the reagents and describe the method to prepare a pure dry sample of Calcium Carbonate (CaCO₃). What type of reaction is this?

[3 marks]💡 Is CaCO₃ soluble or insoluble? Which two aqueous reagents provide Ca²⁺ and CO₃²⁻ ions?

📋 The Three Golden Rules of Salt Synthesis

Soluble salt from soluble base → TITRATION (e.g., NaCl, NaNO₃, K₂SO₄)

Soluble salt from insoluble base/metal/carbonate → EXCESS SOLID (e.g., MgCl₂, CuSO₄, ZnSO₄)

Insoluble salt → PRECIPITATION (e.g., BaSO₄, PbCl₂, AgCl)

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