Q 1. Why should a magnesium ribbon be cleaned before burning in air?
Magnesium is a very reactive metal so it reacts with oxygen to form a layer of magnesium oxide on its surface.
This layer is stable oxide so prevent further reaction of magnesium with oxygen. To remove this layer a magnesium ribbon is cleaned using sandpaper before burning in air.
Magnesium ribbon must be clean clean for burning in air because the layer of magnesium oxide is format on the rubbing due to reaction of magnesium with air and this slow down or interferes with burning process so it is cleaned in order to get the decide chemical reaction