Assertion: Magnesium ribbon burns with a dazzling flame in air.Reason: A white substance magnesium nitride is formed.

The assertion and the reason provided are partially correct.

Assertion: Magnesium ribbon does indeed burn with a dazzling white flame when exposed to air. This is due to the high reactivity of magnesium with the oxygen present in the air, which results in the production of a large amount of heat and light.

Reason: However, the reason provided is incorrect. When magnesium burns in air, it primarily forms magnesium oxide (MgO), not magnesium nitride. The reaction is as follows:

2Mg + O2 → 2MgO

The white substance that is formed is magnesium oxide, which is a white powdery substance. Magnesium nitride (Mg3N2) can be formed when magnesium burns in an atmosphere of nitrogen, but not in air.