Why should magnesium ribbon be cleaned before burning it?

Question:

Why should magnesium ribbon be cleaned before burning it?

Explanation:

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

1. 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. 

2. To remove the Magnesium oxide layer from the ribbon which may prevent or slow down the burning of magnesium ribbon. 

3. Unwanted impurities deposited on the magnesium ribbon can be removed and only pure magnesium can be used for the reaction. Hence, magnesium ribbon be cleaned before burning in air.