Question

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

Answer 1

Magnesium ribbon is cleaned before burning in air because there is a layer of magnesium oxide layer on it..

Answer 2

magnesium ribbon is a very reactive metal.when stored it reacts with oxygen to form magnesium oxide. this layer of magnesium oxide being stable compound prevents further reaction of oxygen wiht mg.  thus this is done for the removal of this layer so as to expose the metal to air properly