Question

why should a magnesium ribbon should be cleaned before burning​

Answer 1

Magnesium ribbon must be cleaned before burning in air so that the 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.

Answer 2

Magnesium Ribbon should be rubbed or cleaned befor burning so that it burns easily.. When we clean it or rub it we will remove the oxide layer which is formed on the magnesium ribbon...when we bring the ribbon with the oxide layer it will take some time to catch fire, if cleaned it can catch fire easily and we can get the white powder...

Hope it helps...