Question

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

Answer 1

MAGNESIUM RIBBON MORE REACTIVE REACTS WITH OXYGEN TO FORM A LAYER OF MAGNESIUM OXIDE, THIS LAYER PREVENTS FURTHER REACTION AND HENCE, TO PERFORM OUR EXPERIMENT WE NEED TO CLEAN THE RIBBON TO GET  RID OF THE LAYER.

Answer 2

$<v> Magnesium is a very reactive metal. When stored, it reacts with oxygen to form a layer of magnesium oxide on its surface. This layer of magnesium oxide is quite stable and prevents further reaction of magnesium with oxygen. The magnesium ribbon is cleaned by sand paper for removing this layer so that the underlying metal can be exposed to air.$