Question

Why should magnesium ribbon be rubbed with sandpaper before burning it ?

Answer 1

With magnesium the oxide layer form a white coating on the outside of the metal that will slow down or interfere with the burning process. This ribbon should be sandpapers with a fine sandpapers therefore, to remove the layer ,------ thus magnesium oxide is formed

Answer 2

When any metal is left in contact with air (oxygen), it forms an oxide coating on the surface of the metal..Similarly in case of magnesium it reacts with oxygen present in air and formed a layer of magnesium oxide which will slow down the burning process. Thus we shoul rub it with a fine sandpaper for fastening the reaction.