Why should a magnesium ribbon be cleaned before burning in air?
Answer:
A magnesium ribbon should be cleaned before burning to remove the layer of magnesium oxide (MgO) that forms on its surface due to reaction with air.
Explanation:
Magnesium reacts with oxygen in the air to form a thin, white coating of magnesium oxide.
This oxide layer prevents the magnesium metal underneath from burning properly.
Cleaning the ribbon (by rubbing it with sandpaper) removes this layer, so that the pure magnesium can burn brightly and react efficiently with oxygen.