In a qualitative analysis when H₂S is passed through the solution of a salt acidified with HCl, a black precipitate is obtained. On boiling the precipitate with dilute HNO₃, it forms a solution of blue colour. Addition of an excess of an aqueous solution of ammonia to this solution will give ________.

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⁺ + SO₄^2- + 2NH₃ + 2H₂O → Cu(OH)₂ + CuSO₄ + 2NH₄OH