An unknown salt on treatment with dilute HCl evolves a colourless gas having a rotten egg smell. When an aqueous solution of this salt is treated with sodium nitroprusside solution, a characteristic colour is observed. Identify the colour of the resulting complex.

Step-by-step solution

The evolution of a colourless gas with a rotten egg smell on treatment with dilute HCl indicates the presence of the sulphide anion ( S ²⁻ ). The gas evolved is hydrogen sulphide ( H ₂ S ). As a confirmatory test, when an aqueous solution of a sulphide salt is treated with sodium nitroprusside ( Na ₂[ Fe ( CN )₅ NO ] ), a purple or violet colouration is obtained due to the formation of the complex [ Fe ( CN )₅ NOS ]⁴⁻ . S ²⁻ + [ Fe ( CN )₅ NO ]²⁻ [ Fe ( CN )₅ NOS ]⁴⁻ (Purple/Violet) A black precipitate is formed in