**Correct Option: A. A substance consisting of two or more elements chemically bonded together** ### Detailed Explanation: 1. **Definition of a Compound**: - A compound is defined as a substance formed when two or more different elements are chemically bonded together. This means that the atoms of these elements are held together by chemical bonds, which can be ionic, covalent, or metallic. The resulting compound has unique properties that are different from the individual elements that make it up. 2. **Chemical Bonding**: - In a compound, the elements are combined in fixed ratios. For example, water (H₂O) is a compound made of two hydrogen atoms and one oxygen atom. The chemical bond between these atoms is what gives water its unique properties, such as being a liquid at room temperature and having a high specific heat capacity. 3. **Properties of Compounds**: - Compounds have distinct physical and chemical properties that differ from those of the individual elements. For instance, sodium (Na) is a highly reactive metal, and chlorine (Cl) is a toxic gas, but when they combine to form sodium chloride (NaCl), commonly known as table salt, the resulting compound is safe to consume and has entirely different properties. ### Why Other Options Are Incorrect: - **Option B: A mixture of two or more substances that can be separated by physical means**: - This option describes a mixture, not a compound. Mixtures consist of two or more substances that are physically combined, and they retain their individual properties. For example, a salad is a mixture of various vegetables, and you can separate them easily without any chemical reaction. In contrast, compounds cannot be separated by physical means; they require chemical reactions to break down into their constituent elements. - **Option C: A pure substance that cannot be broken down into simpler substances**: - This option is misleading because it describes an element rather than a compound. An element is a pure substance that cannot be broken down into simpler substances by chemical means. Compounds, on the other hand, are made up of two or more elements and can be broken down into those elements through chemical reactions. - **Option D: A solution that contains a solute dissolved in a solvent**: - This option describes a solution, which is a homogeneous mixture of two or more substances. In a solution, the solute (the substance being dissolved) is uniformly distributed within the solvent (the substance doing the dissolving). While solutions can contain compounds, they are not compounds themselves. For example, saltwater is a solution of sodium chloride (a compound) dissolved in water (the solvent). ### Summary of Key Points: - A compound is formed from two or more elements that are chemically bonded together. - Compounds have unique properties that differ from the individual elements. - Mixtures can be separated by physical means, while compounds require chemical reactions for separation. - Elements are pure substances that cannot be broken down, while compounds can be decomposed into their constituent elements. This understanding of compounds is fundamental in chemistry, as it lays the groundwork for studying chemical reactions, stoichiometry, and the properties of materials.