Question

why should a magnesium ribbon be cleaned before burning in air meritnation

Answer 1

magnesium ribbon must be cleaned with sand paper to remove the layer of hydrogen from it...

Answer 2

A magnesium ribbon be cleaned before burning in air to avoid the layer of magnesium oxide which form due to the presence of oxygen and chemical equation is :-

Mg + O2 ------> MgO2

So when we rub magnesium ribbon then the oxide layer get seprated and we got fresh magnesium