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Magnesium ribbon should be cleaned before burning because it contain impurities which also combines with oxygen and we'll not get pure magnesium oxide.
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Magnesium ribbon is a very reactive metal. When stored it reacts with oxygen to forma layer of magnesium oxide on its surface. This layer of Magnesium oxide being a stable compound prevents further reaction of mangnesium with oxygen. The magnesium is cleaned before burning in air to remove this layer so that the meta can be exposed to air properly.
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