Question

why do we need to clean the sandpaper before burning

Answer 1

Because on the metals sometimes there are layer of dust , gangue and also some times there are layer of oxides which can change the products of the reaction thus we use sandpaper to clean before combustion or burning...

Answer 2

Hey!

Ur answer is:-
Magnesium ribbon must be cleaned before burning in air so that the layer of magnesium oxide (which is formed due to reaction of magnesium with air) can be removed in order to get the desired chemical reaction.

#riShu