Question

why should a magnesium ribbon be cleaned before burning in airu

Answer 1

Answer:

Magnesium ribbon must be cleaned before burning in air so that the the layer of magnesium oxide (which is formed due to reaction of magnesium with air) can be removed in order to get the desired chemical reaction

Answer 2

magnesium ribbon reacts with carbon dioxide present in the air to form a protective and inert layer of magnesium carbonate. This layer is unreactive and hence, needs to be cleaned with sand paper before burning in air.