Question

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

Answer 1

It is cleaned to remove the layer of oxides. As magnesium reacts with the gasses present in the air and form oxide (sulphide, etc,)....
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Answer 2

♤ When Magnesium ribbon is exposed in the atmosphere it form a layer of magnesium oxide on its surface.

♤ This layer of magnesium oxide is stable compound to prevent further reaction of magnesium with oxygen.

♤ Hence, it should be cleaned before burning in air to remove this layer so that the metal can be exposed to air properly.