Question

Why do the surface of the magnesium ribbon must be cleaned before burning

Answer 1

Because it have a layer of magnesium oxide on its surface as

Being in direct contact with the surrounding.....

Hope it helps you out!!!!

Answer 2

Answer:

magnesium ribbon reacts with carbon dioxide present in the air to form a protective and inert layer of magnesium carbonate. This layer is unreactive and hence, needs to be cleaned with sand paper before burning in air.