Question

Why should magnesium ribbon be cleaned before
burning it in air?​

Answer 1

As magnesium when exposed to air reacts with oxygen to form magnesium oxide, thus it has to be cleaned to remove the layer of the protective magnesium oxide

Answer 2

Magnesium ribbon generally reacts with atmospheric Oxygen to form magnesium oxide or with other compounds to form undesirable products. Thus, we clean the ribbon before it is burnt in the air so that there is no change in the reaction.

Chemical Equation:

$\rm 2Mg + O_2 ---->2MgO$