Question

why should magnesium ribbon be cleaned before burning in air​

Answer 1

Answer:

magnesium ribbon be cleaned before burning in air in order to remove the oxide layer formed due to its reaction with oxygen.

Answer 2

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magnesium is reactive metal which is stored react with oxygen to form a layer magnesium oxide on its surface this layer of magnesium oxide is a quite stable and prevent for the reaction of magnesium with oxygen. the magnesium ribbon is cleared by sandpaper to remove this layer so that the underlying metal can be exposed into air.
$\huge \boxed{ \fcolorbox{white}{pink}{hope it help u }}$