Answer :
In qualitative analysis, when H2S gas is passed through an aqueous solution of salt acidified with HCl, a black ppt of CuS is formed.
CuSO4 + H2S + dil HCl → CuS + H2SO4
(Black ppt)
When ppt of CuS is boiled with dilute HNO3,it forms a blue coloured solution due to the following reactions given below:
3CuS + 8HNO3→ 3Cu(NO3)2 + 2NO + 3S + 4H2O
S + 2HNO3→ H2SO4 + NO
2Cu2+ + SO42- + 2NH3 + 2H2O → Cu(OH)2 + CuSO4 + 2NH4OH
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