Why should a magnesium ribbon be cleaned before burning in air ?

Magnesium ribbon should be cleaned before burning in air for the following reasons:

1. Oxidation: Magnesium metal forms a layer of magnesium oxide on its surface when exposed to air. This is due to the reaction of magnesium with oxygen present in the air. This layer of magnesium oxide is quite stable and prevents further reaction of magnesium with oxygen.

2. Effective Reaction: The presence of the oxide layer on the magnesium ribbon surface slows down the reaction of magnesium with oxygen when the ribbon is burnt. By cleaning the magnesium ribbon before burning, we ensure that the reaction between magnesium and oxygen is swift and complete.

3. Accurate Results: For experimental purposes, it's important to get accurate results. If the magnesium ribbon is not cleaned and the oxide layer is present, it will interfere with the reaction and the results obtained will not be accurate.

4. Safety: Magnesium burns with a dazzling white flame. If the ribbon is not cleaned and burns with an oxide layer, it can lead to spluttering, which might be dangerous.

So, the cleaning of the magnesium ribbon before burning is to ensure that the reaction is effective, the results are accurate, and the process is safe.