Question

Identify the salt R from the following observation.

Salt R is black in colour. On reacting with concentrated HCl, it liberates a pungent greenish yellow gas which turns moist starch iodide paper blue-black.

Answer 1

Observations:

A black compound that reacts with concentrated HCl to liberate a pungent greenish-yellow gas (Chlorine, Cl₂) which turns moist starch-iodide paper blue-black is a classic test for manganese dioxide acting as an oxidizing agent.

Chemical Equation:

\[\ce{\underset{Black colour}{MnO2} + 4HCl (conc.) -> MnCl2 + 2H2O + \underset{Greenish-yellow gas}{Cl2 ^}}\]

Salt R is Manganese dioxide (MnO₂).