Question

Why we should clean a magnesium before it reaction with air

Answer 1

we should clean a magnesium ribbon because the magnesium is a reactive metal so its surface reacts with oxygen in the atmosphere and form a layer of magnesium oxide on its surface. so to get a better result during an experiment magnesium ribbon should be cleaned  

Answer 2

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

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.Hope this will help you