State one relevant observation — Ammonia gas is burnt in an atmosphere of excess oxygen.

State your observation : Calcium hydroxide is heated with ammonium chloride crystals.

Name the other ion formed when ammonia dissolves in water. Give one test that can be used to detect the presence of the ion produced.

From the list of the gases — Ammonia, ethane, hydrogen chloride, hydrogen sulphide, ethyne — Select the gas which is used as a reducing agent in reducing copper oxide to copper.