Why do we rub the magnesium ribbon with a sandpaper before burning it?
When exposed to air many metals will slowly react with oxygen in the atmosphere to form metal oxides and magnesium is one of those metals. Like rusting in case of iron, 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. Rubbing with sand paper helps to remove this oxide layer due to which burning speeds up.