Question

why should magnesium ribbon be cleaned before burning in air​

Answer 1

Here is your answer ⤵⤵⤵⤵

Magnesium ribbon should be cleaned before burning in air because in the presence of air, the first layer of the ribbon get oxidised which will not allow the ribbon to burn.

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Answer 2

A magnesium ribbon should be cleaned before burning in air to remove the protective layer of magnesium oxide from its surface so that it may combine with the oxygen of air on heating.