Formation of rust on an iron nail is a ________ change.

Explanation

**Step-by-Step Solution:**

1. **Understanding Rust Formation**: Rust is formed when iron reacts with oxygen and moisture (water) in the environment. This process is known as rusting.

2. **Identifying the Process**: Rusting involves the degradation of iron, which means that the iron is being transformed into a different substance due to a chemical reaction.

3. **Chemical Composition of Rust**: The chemical formula for rust is Fe2O3·nH2O, which indicates that rust is a hydrated form of iron(III) oxide. This means that during rusting, new chemical compounds are formed.

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