Question

why should a magnesium ribbon be cleaned before burning in air ?​

Answer 1

Answer:

magnesium ribbon is cleaned before burning to remove the magnesium oxide which may slow down or interrupt the burning process...

Answer 2

$\huge\underline\mathbb\blue{Answer}$

Reason : Magnesium ribbon must be cleaned before burning in air so that the layer of magnesium oxide formed due to reaction of magnesium with air(oxygen) can be removed to get the desired chemical reaction.