Question

why should a magnesium ribbon be cieaned before in air?

Answer 1

Hey there !

When magnesium ribbon is exposed to the air it forms a layer of magnesium oxide on its surface. This layer of magnesium oxide being a stable compound prevents further reaction of magnesium with oxygen. Therefore it has to be cleaned.

Hope it helps you !

Answer 2

HERE IS YOUR ANSWER

A magnesium ribbon when kept in open for a long time acquires a layer of oxide and carbonate. When magnesium ribbon is burnt without removing the layer of oxide and carbonate then it may not burn easily. This is because the compounds formed are stable and thus they further don't react with oxygen while burning.
Thus it is suggested to clean the magnesium ribbon with a sand paper before burning to remove the layer of oxide and carbonate so that it may burn easily.

HOPE IT IS USEFUL