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JAMB Chemistry 1990 Question 40

Question 40

Qualitative Analysis

Which of the following compounds gives a yellow residue when heated and also reacts with aqueous sodium hydroxide to give a white gelatinous precipitate soluble in excess sodium hydroxide solution?

  1. A.(NH4)2CO3
  2. B.ZnCO3
  3. C.Al2(SO4)3
  4. D.PbCO3.
Show the answer and explanation

Answer: B (ZnCO3)

Explanation

ZnCO3 decomposes on heating to ZnO, which is yellow when hot. Zn2+ gives a white gelatinous Zn(OH)2 precipitate that dissolves in excess NaOH.