Question

Why should a magnesium ribbon be cleaned before burning in air?

Answer 1

because if it kept for a long time it develop a layer of magnesium oxide which is quite satble reduces the reaction of magnesium with oxygen hence it should be cleaned.

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.