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

Magnesium ribbon should be cleaned before it is burnt in air for the following reasons:

1. Oxidation Layer Removal: Magnesium metal forms a layer of magnesium oxide on its surface when exposed to air. This layer acts as a protective barrier and prevents further reaction of magnesium with oxygen. Cleaning the ribbon removes this layer.

2. Accurate Results: For a fair test in a scientific experiment, it's important that the magnesium is in its pure form. Any impurities (like magnesium oxide) could affect the results of the experiment.

3. Faster Reaction: Once the oxide layer is removed, the magnesium can react directly with the oxygen in the air, making the reaction faster and more vigorous.

4. Brighter Flame: The removal of the oxide layer also results in a brighter flame when the magnesium ribbon is burnt.

The cleaning process usually involves using a piece of sandpaper to gently rub off the layer of magnesium oxide from the ribbon.