- Iron in contact with water and oxygen will rust
- Rust forming is a kind of chemical change
- Hence, iron in contact with water and oxygen is a chemical change

- Iron in contact with water and oxygen is a chemical change
- Chemical change is a kind of chemical reaction
- Therefore, iron in contact with water and oxygen is a chemical reaction

- Iron in contact with water and oxygen is a chemical reaction
- Chemical reactions cause new substances to form
- Hence, iron in contact with water and oxygen will form new substance

Therefore, iron in contact with water and oxygen will form new substance
Question: Which of these involves the formation of a new chemical substance?
Context: - Chemical reactions cause new substances to form 
- Iron in contact with water and oxygen will rust 
- Chemical change is a kind of chemical reaction 
- Rust forming is a kind of chemical change