Why should magnesium ribbon be cleaned before burning in air?

Magnesium ribbon should be cleaned before burning in air to remove the layer of magnesium oxide that naturally forms on the surface when the metal is exposed to air. This oxide layer protects the underlying metal from further reaction with oxygen and other elements in the air. However, when you want to burn magnesium for a demonstration or experiment, the presence of this oxide layer can impede the reaction.

Cleaning the magnesium ribbon typically involves rubbing it with a piece of sandpaper or steel wool to physically remove the oxide layer. Once the protective layer is removed, the underlying magnesium metal can react more readily with oxygen in the air to produce a bright white light and a significant amount of heat. The reaction produces magnesium oxide, as shown in the following equation:

[2Mg(s) + O₂(g) → 2MgO(s)

By cleaning the magnesium ribbon before burning it, you ensure a more vigorous and uniform reaction, which is important both for the consistency of experimental results and the visual effect in demonstrations.