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A sodium salt of an unidentified anion gives a white precipitate with MgCl2 solution ONLY on boiling, not in the cold. Which anion is most likely present?

1. SO4²-
2. HCO3-
3. CO3²-
4. NO3-

Correct answer: HCO3-

Solution

Na2SO4 gives MgSO4 (soluble, no ppt). Na2CO3 gives MgCO3 (white ppt even in cold). NaNO3 gives Mg(NO3)2 (soluble). NaHCO3 gives Mg(HCO3)2 which is soluble in cold, but on boiling it decomposes to give MgCO3 (white precipitate), CO2, and H2O. This matches 'precipitate only on boiling'.

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