Assertion: Coal gas is a mixture of methane, hydrogen and carbon monoxide.
Reason: It is obtained when coal is burnt in excess of air.
Coal gas is obtained by the destructive distillation of coal. The destructive distillation process involves heating of powdered coal in the absence of air. The composition of coal gas varies but the main components are hydrogen, methane and carbon monoxide. Thus, the assertion is correct but the reason is an incorrect statement.