Question

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

Answer 1

Magnesium ribbon must be cleaned before burning in the air because the layer of magnesium oxide is formed on the ribbon due to the reaction of magnesium with air slowing down or interfering with the burning process. So it is cleaned in order to get the desired chemical reaction i.e 2Mg+O2⟶2MgO.

Answer 2

Magnesium gets covered with a layer of magnesium oxide when kept in air for a long time. This layer hinders the burning of magnesium. Hence, it is to be cleaned before burning.