Question

Why should be magnesium ribbon be cleaned before burning in air? ​

Answer 1

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When magnesium ribbon is exposed to air, a white protective and unreactive layer of magnesium oxide is formed on its surface . This layer hinders the burning of magnesium. Hence it is cleaned by rubbing with sand paper before burning...

Answer 2

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$\pink \bigstar$Magnesium is very reactive metals and store it react with oxygen to form a layer magnesium oxide on its surface layer of magnesium oxide is quite stable and prevent future for the reaction of magnesium with oxide. Magnesium ribbon is cleared by sandpaper to remove this layer so that underline metal can be exposed to into air.
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