Question

why should a magnesium ribbon be cleaned before burning in air?​

Answer 1

Magnesium ribbon is cleaned with a sand paper before burning to remove the layer of magnesium oxide. It combines with the oxygen present in the atmosphere to form a layer of magnesium oxide (MgO). This layer interferes in the further reaction of metal.

Answer 2

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MAGNESIUM RIBBON IS CLEANED BEFORE BURNING IN AIR BECAUSE THE CLEANING WILL REMOVE THE MAGNESIUM OXIDE LAYER TO GIVE DESIRED REACTION