Question

1. why should a magnesium ribbon be cleared before burning in air ?
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Answer 1

it should be cleaned before burning in air to remove the layer of oxide formed at its obove layer

if thats not clered it will not burn

Answer 2

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.