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

Answer: 

Sandpaper

Magnesium ribbon should be cleaned with sandpaper before burning in the air because of the following reasons:

1) Magnesium is more reactive element  therefore it reacts rapidly with atmospheric oxygen to form a white layer of magnesium oxide on the surface of magnesium ribbon. And this magnesium oxide will not burn.

 

2) Removing the Magnesium oxide layer from the ribbon can prevent or slow down the burning of the magnesium ribbon

3) Unwanted impurities embedded in the magnesium ribbon can be removed and only pure magnesium can be used to react.