Question

Why magnesium ribbon be cleaned before burning in air?

Answer 1

Magnesium ribbon should be cleaned before burning in air in order to remove the layer of magnesium oxide and thereby get the desired chemical reaction.

Answer 2

When exposed to air magnesium ribbon acquires a coating of magnesium oxide by slow reaction with oxygen present in air.
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In order to perform the burning of magnesium ribbon, we have to clean it with sand paper to remove the coating of magnesium oxide so that the experiment can take place smoothly.

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