ay634807
Answer: 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.
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Parul678
When magnesium ribbon remains exposed to moist air, a white layer of magnesium oxide and carbonate is formed on its surface. this hinders the burning of magnesium....hence this layer is first removed by rubbing with sand paper before burning
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