Question

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

Answer 1

Answer: 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 has oxide layer on the surface area so, it reacts slowly in air. because of that we clean the magnesium ribbon before the burning in air.