Question

Why aluminum foils are used to cover cooked food though aluminum is a highly reactive metal?

Answer 1

1. As we know Aluminum (Al) is a highly reactive metal, but it forms a protective layer called Aluminum oxide $\left({\mathrm{Al}}_2{\mathrm{O}}_3\right)$ when reacts with moisture, light, and oxygen present in the air preventing the food from getting rancid.
2. This layer helps to slow down oxidation due to its passive nature.

The reaction of Aluminium with oxygen present in the air:

Thus, being so highly reactive, aluminum foils are used to cover food because of the formation of a layer called aluminum oxide.