Question

why should a magnesium ribbon be cleaned before the beginning in air ​

Answer 1

Explanation:

When magnesium ribbon burns in air, it combines with the oxygen to form magnesium oxide

2Mg(s)+O2===2MgO(s)

Answer 2

$<v> Magnesium is a very reactive metal. When stored, it reacts with oxygen to form a layer of magnesium oxide on its surface. This layer of magnesium oxide is quite stable and prevents further reaction of magnesium with oxygen. The magnesium ribbon is cleaned by sand paper for removing this layer so that the underlying metal can be exposed to air.$