Question

why should a magnesium ribbon be cleaned before burning in air

Answer 1

Magnesium ribbon be cleaned before burning in air because it form a layer of magnesium oxide outside it.

Answer 2

when magnesium ribbon remains exposed to moist air . a white layer of magnesium oxide is formed in its surface .this hinders the burning of magnesium . Hence this layer is first removed by rubbing with sand paper before burning