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. However, the chemical formula of water (H₂O) remains unchanged. This is a classic example of a physical change because: - The process is reversible (ice can melt back into water). - No new substances are formed. ### Explanation of the Other Options **A. Adding iron filings to aerated water**: - This is not a physical change because when iron filings are added to water, they can react with oxygen in the water to form iron oxides (rust) over time, especially if the water is aerated. This reaction alters the chemical composition of the iron, leading to the formation of new substances. Thus, it is a chemical change. **B. Adding sodium metal to water**: - This is a chemical change. When sodium metal is added to water, it reacts vigorously to produce sodium hydroxide and hydrogen gas. This reaction is exothermic (releases heat) and results in the formation of new substances, which means the original sodium and water are transformed into different chemical entities. Therefore, this is not a physical change. **C. Cooling a solution of Iron (II) sulphate to obtain the hydrated salt**: - This process involves cooling a solution to precipitate out the hydrated form of iron (II) sulfate. While it may seem like a physical change because it involves a change in state (from solution to solid), the formation of hydrated salts can involve changes in the chemical structure of the compound. The process can also lead to the formation of new crystalline structures, which may not be purely physical. Thus, it is more complex and can be considered a chemical change. ### Summary of Key Points - **Physical Change**: A change that does not alter the chemical composition of a substance (e.g., water to ice). - **Chemical Change**: A change that results in the formation of new substances with different chemical properties (e.g., sodium reacting with water). - **Reversibility**: Physical changes are often reversible, while chemical changes are not easily reversible. - **Examples**: Cooling water to ice is a straightforward physical change, while adding sodium to water is a clear chemical change. ### Revision Summary - **Correct Answer**: D (Cooling water to obtain ice is a physical change). - **Physical changes** do not alter the chemical identity of a substance. - **Chemical changes** involve the formation of new substances and often are not reversible. - Always consider the nature of the substances involved and whether new products are formed to determine if a change is physical or chemical.