Question

why is a magnesium ribbon. rubbed with sand paper before burning it in air

Answer 1

magnesium ribbon is rubbed with Sandpaper before burning in air because to remove the oxidized layer is a magnesium ribbon..
due to the contact of moisture and air magnesium tends to form an layer... therefore be rubbed with Sandpaper to avoid this formation

Answer 2

hey...!!
here's ur answer ,

the magnesium ribbon is rubbed with sand paper before burning it in air, to cleared the protective layer of calcium carbonate ( CaCO3).

so it may be readly react with O2 .

hope it might be helpful :) ##