Identify the gas from the following observations:
A reddish brown gas having a pungent smell. It turns moist potassium iodide paper brown.
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.
Name a gas which turns Nessler's reagent brown.
Name a gas which turns:
Alkaline pyrogallol solution brown.