Question

Why do we rub magnesium ribbon with a cloth before heating?

Answer 1

Magnesium is a reactive metal. When exposed to air many metals will slowly react with oxygen to form metal oxides. With magnesium, the oxide layer forms a white coating on the outside of the metal that will slow down or interfere with the burning process. So, one should rub a strip of magnesium with sandpaper to remove such layers.