Question 1

Why should a magnesium ribbon be cleaned before it is burnt in air?

With time, the magensium ribbon gets covered with a protective coating of basic magnesium carbonate.

This coating is formed due to the exposure of magensium ribbon to moist air for a long period of time. The coating is called protective because it is quite stable and once formed, prevents further reaction of magnesium with air. Hence, sand paper is used to scrape off the coating so that the underlying metal can be exposed for further reaction.