magnesium ribbon essential to be cleaned before burning in air because to withdraw basic magnesium carbonate laminate  on it and this is very bright so it harms our eyes responsible in decrease the rate of burning .

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Oxygen reacts with metals and forms oxides .
so , Magnesium reacts with oxygen and forms magnesium oxide. if oxide forms reaction does not take place properly . to avoid that one we have to clean magnesium ribbon before burning.

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