Question

why should a magnesium ribbon be cleaned before burning in air​

Answer 1

Magnesium ribbon must be cleaned before burning in air so that the layer of magnesium oxide (which is formed due to reaction of magnesium with air) can be removed in order to get the desired chemical reaction.

Answer 2

Magnesium is a highly reactive element. It forms magnesium oxide when it combines with oxygen. This Magnesium oxide is stable and it prevents further reaction with oxygen. Therefore to remove this coating it is cleaned with sand paper.