Question

why should a magnesium ribbon be cleaned before burning in air

Answer 1

magnesium ribbon should be cleaned before burning in air so that the layer of magnesium oxide can be removed the two get desirable chemical reaction ...

Answer 2

a magnesium which we use generally has a coating of magnesium oxide which act as a protective layer. so the magnesium ribbon cleaned  with the help of sandpaper so that it readily and easily combined with oxygen in air and burn easily