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

• A. Rust forming on iron
• B. Water boiling into steam
• C. A tree growing in height
• D. Combustion of a fuel

Answer: (B)

Water boiling into steam is indeed an example of a physical change because it involves a change in the state of matter from liquid to gas without altering the chemical composition of the water itself. During this process, the water molecules gain energy and move apart, but they remain H₂O regardless of whether they are in liquid or vapor form.

In contrast, rust forming on iron entails a chemical reaction where iron reacts with oxygen, producing iron oxide, which fundamentally changes the chemical structure of the substance. A tree growing in height is a result of biological processes rather than a physical change, as it involves growth and development at a cellular level, affecting its chemical makeup. Lastly, combustion of a fuel is another chemical change because it involves a reaction with oxygen to produce new substances, such as carbon dioxide and water, along with releasing energy. These responses showcase the distinction between physical changes, which are reversible and do not affect chemical composition, and chemical changes, which create new substances.