The correct option for the question "Which of the following are chemical changes?" is **A. I and II**. Let's break down each option to understand why this is the case. ### Step-by-Step Explanation 1. **Understanding Chemical Changes**: - A **chemical change** involves a transformation that alters the chemical composition of a substance. This means that the original substances are converted into different substances with new chemical properties. - In contrast, a **physical change** does not change the chemical composition of a substance. The substance may change its state (like solid to liquid) or appearance, but its chemical identity remains the same. 2. **Analyzing Each Option**: - **I. Cooking of food**: - This is a chemical change. When food is cooked, the heat causes chemical reactions that change the structure and composition of the food. For example, proteins denature, carbohydrates caramelize, and new flavors and aromas are developed. The original ingredients cannot be recovered in their initial form after cooking. - **II. Digestion of food**: - This is also a chemical change. Digestion involves breaking down food into smaller molecules through enzymatic reactions. The complex molecules in food (like proteins, fats, and carbohydrates) are chemically altered into simpler molecules (like amino acids, fatty acids, and sugars) that the body can absorb. Again, the original substances are transformed into new substances. - **III. Freezing of water**: - This is a physical change. When water freezes, it changes from a liquid to a solid state (ice), but its chemical composition (H₂O) remains unchanged. The molecules are still water molecules; they are just arranged differently in a solid structure. - **IV. Water is heated up**: - This is also a physical change. Heating water increases the kinetic energy of the water molecules, which may lead to a change in state (like boiling), but the chemical composition of the water (H₂O) does not change. It remains water regardless of its temperature. ### Why Other Options Are Incorrect: - **B. I, II, and III**: This option incorrectly includes the freezing of water as a chemical change. While I and II are correct, III is a physical change. - **C. III and IV**: This option includes two physical changes (freezing and heating of water) and excludes the correct chemical changes (I and II). - **D. All of the above**: This option is incorrect because it includes physical changes (III and IV) along with the correct chemical changes (I and II). ### Summary of Key Points: - **Chemical Changes**: Involve a change in chemical composition (e.g., cooking and digestion). - **Physical Changes**: Do not alter the chemical identity (e.g., freezing and heating of water). - **Correct Answer**: A. I and II are chemical changes; III and IV are physical changes. ### Revision Summary: - **Chemical changes** result in new substances; examples include cooking and digestion. - **Physical changes** do not change the chemical identity; examples include freezing and heating water. - Always analyze the nature of the change to determine if it is chemical or physical. - Remember that in chemical changes, the original substances cannot be recovered in their initial form.