The correct option is **B. Mercury**. ### Detailed Explanation: To understand why mercury is the correct answer, we first need to define what a mixture is. A mixture is a combination of two or more substances that retain their individual properties and can be separated by physical means. In contrast, a pure substance has a uniform and definite composition. 1. **Analyzing the Options:** - **A. Air**: Air is a mixture of gases, primarily nitrogen, oxygen, carbon dioxide, and other trace gases. Each component retains its properties and can be separated (e.g., through fractional distillation). Therefore, air is a mixture. - **B. Mercury**: Mercury is a chemical element with the symbol Hg. It is a pure substance, meaning it consists of only one type of atom and has a uniform composition. It does not retain the properties of different substances because it is not a combination of different materials. Thus, mercury is not a mixture. - **C. Milk**: Milk is an emulsion, which is a type of mixture where fat droplets are dispersed in water. Although it appears homogeneous, it can be separated into its components (like cream and skim milk) through processes like centrifugation. Therefore, milk is a mixture. - **D. Cement**: Cement is a mixture of various compounds, including calcium silicates, aluminates, and other materials. When mixed with water, it forms a paste that can harden, but the individual components can still be identified and separated. Hence, cement is also a mixture. 2. **Why Other Options Are Incorrect:** - **Air (A)**: As mentioned, air is a mixture of gases, and it can be separated into its components. This characteristic makes it a mixture. - **Milk (C)**: Despite its creamy appearance, milk is a mixture of water, fats, proteins, and other substances. It can be separated into its components, confirming its status as a mixture. - **Cement (D)**: Cement is composed of various materials that can be mixed and separated. This property of retaining individual characteristics makes it a mixture. ### Summary of Key Points: - **Pure Substance vs. Mixture**: A pure substance has a uniform composition, while a mixture consists of two or more substances that retain their individual properties. - **Mercury**: As a pure element, mercury does not qualify as a mixture. - **Air, Milk, and Cement**: All these options are mixtures because they consist of multiple components that can be separated. ### Revision Summary: - A mixture retains the properties of its components and can be separated physically. - Mercury is a pure substance, not a mixture. - Air, milk, and cement are all mixtures due to their composite nature. - Understanding the definitions of pure substances and mixtures is crucial for identifying them correctly.