Why should a magnesium ribbon be cleaned before it is burnt in air?
Magnesium ribbon should be cleaned before burning in air to remove the basic magnesium carbonate coating on it. This layer should be removed so that the ribbon may readily combine with oxygen while burning in air. Basic magnesium carbonate layer consists of magnesium carbonate and magnesium hydroxide.
Magnesium 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 is quite stable and prevents further reaction of magnesium with oxygen. The magnesium ribbon is cleaned by sand paper for removing this layer so that the underlying metal can be exposed to air.