A cotton plug is kept at the mouth of test tube during the preparation of oxygen by KMnO4. Why?

Explanation

### Step-by-Step Solution:

1. **Understanding the Reaction**: The question involves the preparation of oxygen gas using potassium permanganate (KMnO4). When KMnO4 is heated, it decomposes to produce potassium manganate (K2MnO4), manganese dioxide (MnO2), and oxygen gas (O2).

2. **Chemical Equation**: The decomposition reaction can be represented as:
\[ 2 \text{KMnO}_4 \xrightarrow{\text{heat}} \text{K}_2\text{MnO}_4 + \text{MnO}_2 + \text{O}_2 \]
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