The chemical reaction that takes place when a magnesium wire is burnt in air is a combustion reaction.

Here's the breakdown:

* Reactants: Magnesium (Mg) and Oxygen (O₂) from the air.

* Products: Magnesium oxide (MgO) and heat and light energy.

The balanced chemical equation for this reaction is:

2 Mg(s) + O₂(g) → 2 MgO(s) + heat + light

Explanation:

* Magnesium (Mg) is a highly reactive metal.

* When exposed to oxygen in the air, it rapidly reacts, producing magnesium oxide (MgO).

* This reaction releases a significant amount of heat and light, which is why you see a bright white flame when magnesium burns.

Additional Notes:

* This reaction is highly exothermic, meaning it releases a lot of energy.

* Magnesium oxide is a white powdery substance.

* The reaction is very fast and can be quite dangerous if not handled properly.