Why magnesium ribbon is rubbed with sandpaper before burning it?
Remove oxide layer from the surface
When exposed to air many metals will slowly react with oxygen in the atmosphere to form metal oxides. Magnesium is one of those metals. You are probably familiar with rust which is simply the oxide that forms when the iron is exposed to oxygen in the air. With magnesium, the oxide layer forms a white coating on the outer metal surface that will slow down or interfere with the burning process. The ribbons should be rubbed with a fine sandpaper to remove the layer.