Question

Why is it advised to rub magnesium ribbon with sand paper before burning it in air?

Answer 1

Thus, when sanded with sand paper, we are able to remove this extra layer that is impeding the burning, so that it can burn as if it weren't exposed to years of oxygen.

Answer 2

♤ When Magnesium ribbon is exposed in the atmosphere it form a layer of magnesium oxide on its surface.

♤ This layer of magnesium oxide is stable compound to prevent further reaction of magnesium with oxygen.

♤ Hence, it should be cleaned before burning in air to remove this layer so that the metal can be exposed to air properly.