Question

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

Answer 1

to remove the oxide layer formed by the ribbon when coming in contact with atmosphere.

Answer 2

 Magnesium ribbon is a very reactive metal. When stored it reacts with oxygen to form a layer of magnesium oxide on its surface. This layer of magnesium oxide being a stable compound prevents further reaction of magnesium with oxygen. The magnesium is cleaned before burning in air to remove this layer so that the metal can be exposed to air properly.

Hope this helps!!☺