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Since Magnesium is an reactive metal in the reactivity series, when it is exposed to oxygen it will form metal oxides. For example, when oxygen is mixed with one of these metals for an extended period of time, you will see rust form and a brownish color coat over it as a tint. With Magnesium, what will happen if exposed to oxygen for too long is create a white coating on the outside of it which will impede and effect the burning process.
Thus, when sanded with sand paper, we are able to remove this extra layer that is impeding the burning, so that it can burn as if it weren't exposed to years of oxygen.
When exposed to air many metals will slowly react with oxygen in the atmosphere to form metal oxides and Magnesium is one of those metals. You are probably familiar with rust which is simply the oxide that forms when iron is exposed to oxygen in the air. With magnesium, the oxide layer forms a white coating on the outside of the metal that will slow down or interfere with the burning process. The ribbons should be sanded with a fine sandpaper, therefore, to remove the layer. Burning magnesium results in magnesium oxide and also some magnesium nitride (reaction with nitrogen in the air). Burning speeds up the conversion of magnesium to magnesium oxide in air.
magnesium,being a reactive metal in the activity series, gets easily reacted wen comes in contact with oxygen. thus magnesium oxide is formed. in order to get appropriate results, it has to be cleaned, so that magnesium oxide is removed and the experiment is successfully performed.
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