Question

A red solid is insoluble in water. However it becomes soluble if some KI is added to water. Heating the red solid in a test tube results in liberation of some violet coloured fumes and droplets of a metal appear on the cooler parts of the test tube. The red solid is

(a) (NH₄)₂Cr₂O₇ 

(b) HgI₂

(c) HgO 

(d) Pb₃O₄.

Answer 1

(b) : The precipitate of mercuric iodide dissolves in excess of potassium iodide forming a complex, K₂HgI₄.