CAIE IGCSE Chemistry Preparation and Crystallisation of Soluble Salts

Practise preparing pure dry soluble-salt crystals by choosing titration or excess solid, removing impurities and controlling evaporation and crystallisation.

Exam points

for an alkali, use titration to find exact volumes and repeat without indicator for purity
for an insoluble solid, add excess to consume all acid and filter off the residue
evaporate to near saturation, cool to crystallise, then filter, wash and dry the crystals

[Maximum number: 4]

Sulfur is an important element.

(a)

Dilute sulfuric acid is used to make salts known as sulfates.

A method consisting of three steps is used to make zinc sulfate from zinc carbonate.
step 1 Add an excess of zinc carbonate to $20 \mathrm{~cm}^{3}$ of $0.4 \mathrm{~mol} / \mathrm{dm}^{3}$ dilute sulfuric acid until the reaction is complete.
step 2 Filter the mixture.
step 3 Heat the filtrate until a saturated solution forms and then allow it to crystallise.

[ 4 ]

(i)

State two observations which would show that the reaction is complete in step 1.

1

2

[ 2 ]

M1 No more fizzing;

M2 $\left(\mathrm{ZnCO}_{3}\right)$ stops dissolving or a (white) solid remains / is visible

(ii)

Why is it important to add an excess of zinc carbonate in step 1?

[ 1 ]

To use up all the acid / $\mathrm{H}^{+}$ ions

(iii)

Name another zinc compound which could be used to make zinc sulfate from dilute sulfuric acid using this method.

[ 1 ]

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7.3.1—Preparation, separation and question 1

Question (a)

Question (i)

Question (ii)

Question (iii)