Answer :
Magnesium ribbon is rubbed before burning to remove a coating of basic magnesium carbonate, which allows the magnesium to burn with a bright white flame, producing magnesium oxide (MgO). Therefore, the correct option is A
Magnesium ribbon is typically rubbed before burning to remove the coating of basic magnesium carbonate that forms on the surface when it is exposed to the air. This coating is formed as magnesium reacts with carbon dioxide and water vapor present in the atmosphere, resulting in a layer of magnesium oxycarbonate that protects the metal and decreases its reactivity. By removing this passivation layer, the ribbon can burn more readily when ignited, producing a bright white flame. The product of this combustion is a fine white powder, known as magnesium oxide (MgO), which demonstrates the chemical change from reactive metal to oxide compound.
