IGCSE · Practice questions · Preparing soluble salts

Plan the preparation.

Six original Cambridge-style questions on preparing soluble salts: method, reasons for each step and choosing reactants.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Excess, filter, crystallise.

Insoluble reactant: excess solid method.
Na, K, NH₄ salts: titration method.
Point of crystallisation, then cool; never to dryness.
Filter, wash, dry the crystals at the end.

Recall

[5 marks]

Describe how to prepare pure, dry crystals of copper(II) sulfate from copper(II) oxide and dilute sulfuric acid.

Warm dilute sulfuric acid in a beaker. ✓
Add copper(II) oxide a little at a time, stirring, until some remains unreacted (excess). ✓
Filter to remove the excess copper(II) oxide. ✓
Heat the filtrate until it reaches the point of crystallisation, then leave it to cool. ✓
Filter off the crystals, wash with a little cold water and dry between filter papers. ✓

Explain

[2 marks]

Explain why an excess of copper(II) oxide is used, and how you would know it was in excess.

To make sure all the sulfuric acid reacts, so the salt is not contaminated with acid. ✓
Black solid remains undissolved at the bottom of the beaker. ✓

Recall

[4 marks]

Describe how to prepare a pure sample of potassium nitrate crystals from potassium hydroxide solution and dilute nitric acid.

Titrate: pipette potassium hydroxide into a flask, add indicator, add nitric acid from a burette until the colour changes; record the volume. ✓
Repeat with the same volumes without indicator. ✓
Heat the solution to the point of crystallisation and leave to cool. ✓
Filter off, wash and dry the crystals. ✓

Analysis

[3 marks]

Name the acid and a suitable insoluble solid to prepare (a) zinc chloride and (b) magnesium sulfate, and name the method.

(a) hydrochloric acid with zinc (or zinc oxide or zinc carbonate). ✓
(b) sulfuric acid with magnesium (or magnesium oxide or magnesium carbonate). ✓
Both by the excess solid method (react, filter, crystallise). ✓

Explain

[2 marks]

Why is the salt solution heated only to the point of crystallisation and not evaporated to dryness?

Evaporating to dryness can decompose the salt or drive off its water of crystallisation. ✓
Cooling a saturated solution slowly forms pure, well-shaped crystals. ✓

Recall

[2 marks]

Write a balanced equation for the reaction of zinc carbonate with dilute sulfuric acid, and state one observation.

ZnCO₃ + H₂SO₄ → ZnSO₄ + H₂O + CO₂ ✓
Fizzing (a gas that turns limewater milky); the solid disappears. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.