Question

why is it necessary to clean a magnesium ribbon before burning?

Answer 1

Otherwise magnesium already reacted with oxygen in air so while  burning its very diffcult if it is not cleaned by sand paper

Answer 2

Hello mate !!

Here's your answer

This is done to remove the layers of magnesium oxide and magnesium carbonate from the surface of magnesium ribbon. These layers are formed as a result of reaction with atmospheric oxygen and carbon dioxide. Magnesium ribbon is cleaned by rubbing with sand - paper after which magnesium ribbon burns smoothly.

Hope my answer will help you :))