Question

Why do we rub magnesium ribbon with sab paper before ignating'

Answer 1

When exposed to air many metals  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 the burning process.Thus , rubbing with sand paper helps removes this oxide layer due to which burning speeds up