When silver nitrate solution is added to potassium iodide solution then the sol produced is :

Cu²⁺ salt reacts with potassium iodide to give

An inorganic Compound 'X' on treatment with concentrated H₂SO₄ produces brown fumes and gives dark brown ring with FeSO₄ in presence of concentrated H₂SO₄. Also Compound 'X' gives precipitate 'Y', when its solution in dilute HCl is treated with H₂S gas. The precipitate 'Y' on treatment with concentrated HNO₃ followed by excess of NH₄OH further gives deep blue coloured solution, Compound 'X' is :

Reagent, 1-napthylamine and sulphanilic acid in acetic acid is used for the detection of