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

Explanation

### Step-by-Step Solution:

1. **Understanding Magnesium Ribbon**: Magnesium is a highly reactive metal. When exposed to air, it can react with oxygen.

2. **Formation of Magnesium Oxide**: When magnesium burns in air, it reacts with oxygen to form magnesium oxide (MgO). The chemical reaction can be represented as:
\[ 2 \text{Mg} + \text{O}_2 \rightarrow 2 \text{MgO} \]
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