Question

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

Answer 1

magnesium is a reactive element and when it comes in contact of air it forms a layer of oxide on its surface which prevent it from burning so mg ribbon is cleaned before burning
hope it helps

Answer 2

it should be cleaned as mg reacts with CO2 present in 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