Making soluble salts (except sodium, potassium and ammonium salts)
2 marks · HardWhen making hydrated zinc sulfate crystals from zinc oxide and sulfuric acid, the final solution is left to cool so that crystals form. Explain, in two points, why crystallisation is used instead of evaporation to dryness to obtain hydrated salt crystals.
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Also accept: Crystallisation allows water of crystallisation to be included in the salt, forming hydrated crystals. Evaporation to dryness drives off the water, forming an anhydrous white powder.
Common mistakes: Only one reason given (either about water of crystallisation or about appearance).; Vague statements about 'better crystals' without stating water is lost in evaporation.
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Marking points
B1 · 1 mark · Crystallisation is used because it allows formation of hydrated salt containing water of crystallisation.
B2 · 1 mark · Evaporation to dryness removes water, forming anhydrous salt (white powder) instead of crystals.
Worked solution
Crystallisation is used so that water of crystallisation remains with the salt, forming hydrated crystals. Evaporation to dryness would remove this water from the salt, resulting in an anhydrous (without water) powder instead.