Question

A gas ‘A’ reacts with another gas ‘B’ in the presence of a catalyst to give a colourless gas ‘C’. The gas ‘C’ when comes in contact with air produces a brown gas ‘D’. The solution of ‘A’ in water turns red litmus blue. Explain the observations.

Answer 1

As the ‘A’ turns red litmus blue, it is a base. Now the gas ‘A’ combines with ‘B’ in the presence of a catalyst to give colourless gas nitrogen monoxide. It reacts with oxygen to give brown gas, which is nitrogen dioxide.

A = NH₃

B = O₂

C = NO

D = NO₂

Catalyst = Pt

Reactions:

\[\ce{4NH3 + 5O2 ->[Pt 800^\circ C] 4NO↑ + 6H2O + Heat}\]

\[\ce{2NO + O2 -> \underset{(Brown gas)}{2NO2}}\]

Litmus Test:

NH₃ dissolves in water to form NH₄OH (ammonium hydroxide), which is basic and hence turns red litmus blue.

\[\ce{NH3 + H2O -> NH4OH}\]