Final answer:
Limestone (CaCO3) is a pure substance classified as a compound because it is made of calcium, carbon, and oxygen in a fixed ratio of 1:1:3, and cannot be physically separated into its elemental components. The correct classification for limestone is Option A.
Step-by-step explanation:
Limestone, or calcium carbonate (CaCO3), is a substance that can be classified based on its composition and properties. According to its chemical formula, it is a pure substance and more specifically, a compound, as it is made up of more than one element chemically combined in a fixed ratio. In CaCO3, the ratio of calcium (Ca) atoms, carbon (C) atoms, and oxygen (O) atoms is 1:1:3 respectively.
Regarding the option provided, calcium carbonate should be classified as "A- A pure substance, classified as compound" because it is composed of these elements in a specific chemical arrangement that cannot be physically separated into its components. In contrast, a mixture is a physical blend of two or more substances that are not chemically combined. For example, salt water is a homogeneous mixture because the composition is uniform throughout, while soil represents a heterogeneous mixture due to its non-uniform composition.
Limestone can be part of a larger geological formation that might include other minerals and substances, thereby creating a composite of materials that could be considered heterogeneous at a macroscopic scale. However, the substance CaCO3 by itself is classified as a compound due to its constant chemical composition and intrinsic properties.