Salt preparations
2 marks · EasyExplain why it is not suitable to prepare pure sodium chloride by mixing sodium carbonate with dilute hydrochloric acid and evaporating.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
The mixture will contain impurities because not all the substances used and produced are soluble; insoluble impurities will contaminate the sodium chloride. Also, evaporation may leave behind other salts besides sodium chloride.
Also accept: Other ions or salts may be left after evaporation, so the sodium chloride is not pure.; Impurities may remain because not all products are soluble.
Common mistakes: It is suitable because both products are soluble.; You can get pure sodium chloride by evaporating any mixture.
Show mark scheme and worked solution
Marking points
M1 · 1 mark · Identifies the reason as possible contamination by insoluble materials or impurities.
M1 · 1 mark · States that the method does not produce a pure sample (reference to method or process flaw).
Worked solution
Mixing sodium carbonate with hydrochloric acid produces sodium chloride and carbon dioxide, but may also leave insoluble materials or other unwanted ions (such as leftover carbonate). Evaporating the solution may not remove all impurities, so the sodium chloride obtained is not pure.