Students at a school in Trinidad were asked to prepare sodium chloride from sodium hydroxide solution. They added dilute hydrochloric acid to the alkali until the solution was neutral, then evaporated the solution to dryness. The sodium chloride crystals produced had a yellow colour. What was the most likely cause of this problem?

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Question

Students at a school in Trinidad were asked to prepare sodium chloride from sodium hydroxide solution. They added dilute hydrochloric acid to the alkali until the solution was neutral, then evaporated the solution to dryness. The sodium chloride crystals produced had a yellow colour. What was the most likely cause of this problem?

Accepted Answer

The solution was heated too strongly during evaporation — Award 1 mark for C. Heating too strongly causes thermal decomposition and discolouration of the salt. A is incorrect because slow evaporation produces better crystals, not discolouration. B is incorrect because excess acid would not cause yellow colour, though it would contaminate the product. D is incorrect because insufficient alkali would leave excess acid, not cause discolouration.

Answer

Excess hydrochloric acid was present in the solution

Answer

The solution was heated too strongly during evaporation

Answer

The solution was evaporated too slowly

Answer

Insufficient sodium hydroxide was used

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