Question

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

Answer 1

Magnesium ribbon must be cleaned before burning in air so that the layer of magnesium oxide (which is formed due to reaction of magnesium with air) can be removed in order to get the desired chemical reaction.

Answer 2

magnesium ribbon is cleaned to remove the layer of magnesium oxide from its surface so that it may come in contact with oxygen......