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

Magnesium ribbon should be cleaned with sandpaper before burning in air due to the following reasons: 

• Magnesium is very reactive element which rapidly reacts with oxygen in the air to form a white layer of magnesium oxide, which may prevent or slow down the burning of magnesium ribbon. 

• Unwanted impurities deposited on the magnesium ribbon can be removed and only pure magnesium can be used for the reaction.

Magnesium ribbon should be cleaned with sandpaper before burning in air because Magnesium is very reactive element which rapidly reacts with oxygen in the air to form a white layer of magnesium oxide, which may prevent or slow down the burning of magnesium ribbon.