Question

why magnesium ribbon is rubbed with Sandpaper before burning

Answer 1

It is rubbed with sandpaper because magnesium form a layer called magnesium oxide when exposed to air. Rubbing is done to remove the protective layer of MgO from the surface of the magnesium ribbon.

Answer 2

Answer: It's done to make burning process faster.

Explanation: When Magnesium ribbon is exposed to air, it reacts with the oxygen to form a layer of Magnesium Oxide( MgO ) which slows down the burning process. Thus we rub it with sandpaper to remove this Magnesium Oxide layer so that th burning process becomes faster.