Question

why should magnesium ribbon be cleaned before burning in air
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Answer 1

Magnesium ribbon must be cleaned before burning in air so that the layer of magnesium oxide (which is formed due to reaction of magnesium with air) can be removed in order to get the desired chemical reaction.

Answer 2

Magnesium is highly reactive metal and some dust get settle on it if not cleaned then observation which we want doesn't happens and a new product is formed and also to remove oxide layer it must be cleaned