What type of a change is rusting of iron?

Explanation

### Step-by-Step Solution:

1. **Understanding Rusting**: Rusting of iron occurs when iron comes into contact with moisture and oxygen in the air. This reaction leads to the formation of a new substance.

2. **Formation of Rust**: The chemical reaction that takes place can be summarized as:
- Iron (Fe) reacts with oxygen (O2) and water (H2O) to form hydrated ferric oxide (rust), which has the chemical formula Fe2O3·xH2O.

3. **New Substance Formation**: Since rust (hydrated ferric oxide) is a different substance from iron, we can conclude that a new substance is formed during this process.
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