Which of the following is an example of a chemical change?

Analysis

The question asks us to identify a chemical change from the given options. To do this, we must distinguish between physical changes and chemical changes.

Concept: Physical vs. Chemical Change

• Physical Change: This is a change in which no new substance is formed. The chemical composition of the material remains the same. Examples include changes in state (melting, freezing, boiling), dissolving (in most cases), and physical separation techniques (filtration, distillation).
• Chemical Change: This is a change that results in the formation of one or more new substances with entirely different chemical properties. These changes are usually irreversible and often involve the absorption or release of energy (heat, light, or gas production).

Step-by-Step Evaluation

1. A. Dissolution of salt in water: When salt dissolves in water, it forms a solution. You can recover the salt simply by evaporating the water. No new chemical substance is created; it is a physical process.
2. B. Rusting of iron: Rusting occurs when iron reacts with oxygen and moisture in the air to form a new substance called hydrated iron(III) oxide (rust). This is a chemical reaction that changes the composition of the iron, and it cannot be reversed by simple physical means.
3. C. Melting of ice: This is a change of state from solid to liquid. The chemical formula remains $H_2O$. It is a classic example of a physical change.
4. D. Separating a mixture by distillation: Distillation uses the difference in boiling points to separate components of a mixture. The substances involved do not change their chemical identity during this process.

Conclusion

Since rusting of iron involves the creation of a new substance through a chemical reaction, it is the only chemical change listed.

Exam Tip

Whenever you see a question about physical vs. chemical changes, ask yourself: "Can I easily get the original substance back using a physical method like heating or filtering?" If the answer is yes, it is likely a physical change. If the substance has permanently transformed into something else (like burning wood or rusting metal), it is a chemical change.