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An aqueous solution of CuSO4 is taken in a test tube. When which of the following gases is passed into it, a metal ion gets precipitated?

1. H2S(g)
2. HCl(g)
3. NH3(g) in excess
4. PH3(g)

Correct answer: H2S(g)

Solution

When H2S gas is bubbled into CuSO4 solution, the highly insoluble CuS (Ksp ~ 10⁻³⁶) precipitates as a black solid: CuSO4 + H2S -> CuS(s) + H2SO4. This is a standard Group II qualitative analysis reaction.

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