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

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

• Magnesium very reactive element which rapidly reacts with oxygen in the air to form a white layer of magnesium oxide and this layer will not burn.
• To remove the Magnesium oxide layer from the ribbon 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.