Limestone, which is primarily composed of calcium carbonate (CaCO3), is classified as a compound. This classification arises because it consists of two elements, calcium (Ca) and carbon (C), combined with three oxygen (O) atoms in a specific ratio and structure. Unlike elements, which contain only one type of atom, compounds like limestone are formed when two or more different atoms bond together chemically.
A homogeneous mixture, on the other hand, would have a uniform composition throughout, like saltwater or air, where individual components are not distinguishable. Conversely, a heterogeneous mixture, such as salad or concrete, has visibly different substances or phases within it. Limestone as a pure compound does not exhibit these characteristics; its structure remains consistent regardless of where you sample it from, thus reinforcing its identity as a compound rather than a mixture.