The correct option is **D. Cooling water to obtain ice**. ### Explanation of the Correct Answer 1. **Definition of Physical Change**: A physical change is a change that affects one or more physical properties of a substance without altering its chemical composition. This means that the substance remains the same at the molecular level, even though its state or appearance may change. 2. **Cooling Water to Obtain Ice**: When water is cooled, it undergoes a physical change from a liquid state to a solid state (ice). The molecules of water slow down as the temperature decreases, and they arrange themselves into a solid crystalline structure. Importantly, the chemical composition of water (H₂O) remains unchanged throughout this process. When ice is heated, it will melt back into water, demonstrating that this is a reversible physical change. ### Why the Other Options Are Incorrect - **A. Adding iron filings to aerated water**: This is not a physical change because it can lead to a chemical reaction. Iron can react with oxygen in the water to form iron oxides (rust), which alters the chemical composition of the substances involved. Therefore, this process is more complex than a simple physical change. - **B. Adding sodium metal to water**: This is a chemical change. When sodium metal is added to water, it reacts vigorously, producing sodium hydroxide and hydrogen gas. This reaction is exothermic (releases heat) and results in the formation of new substances, which means the original chemical composition of both sodium and water is altered. - **C. Cooling a solution of Iron (II) sulphate to obtain the hydrated salt**: This process involves crystallization, which can be considered a physical change in some contexts. However, it is often associated with the formation of a new solid phase from a solution, which can involve changes in the chemical environment. The process can also lead to the formation of different hydrated forms of iron (II) sulfate, which complicates the classification as purely physical. ### Summary of Key Points - **Physical Change**: A change that does not alter the chemical composition of a substance. - **Correct Answer (D)**: Cooling water to form ice is a classic example of a physical change, as the water remains H₂O in both states. - **Incorrect Options**: - A involves potential chemical reactions (iron oxidation). - B is a clear chemical reaction (sodium and water). - C, while it may seem physical, can involve complex interactions and changes in chemical composition. ### Revision Summary - A physical change does not alter the chemical identity of a substance. - Cooling water to form ice is a reversible physical change. - Chemical changes involve the formation of new substances and changes in chemical composition. - Always consider the nature of the substances involved when determining if a change is physical or chemical.