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

Dear student !

Magnesium is a reactive metal and amongst the most common elements found on the earth, so it reacts with atmospheric oxygen gas and makes the magnesium oxide which covers the surface of Mg-metal.

So, when we perform the oxidation of magnesium we clean the Mg-ribbon by a sand paper so that the magnesium oxide is removed and only the metal could be burnt.