Question

Magnesium metal burns in air to give a white ash. When this ash is treated with water, the odour of ammonia can be detected ? Explain.

Answer 1

Magnesium burns in air to form Magnesium oxide. So, the ash is magnesium oxide.
When this ash means MgO is treated with water it form:
MgO + H2O to form Mg(OH)2

Answer 2

Magnesium metal burns in air to form white ash that contains magnesium oxide and also traces of magnesium nitride. It is because magnesium reacts with the oxygen in the air to form magnesium oxide and with nitrogen to form magnesium nitride.

This when reacts with water form magnesium hydroxide along with ammonia and thereby the smell of ammonia can be detected.