* Composition: Limestone is primarily composed of calcium carbonate (CaCO3). However, it also contains varying amounts of other minerals, impurities, and fossilized remains. These variations in composition make it a mixture.

* Heterogeneity: Even within a single piece of limestone, you can often see different colors, textures, and mineral components, indicating a lack of uniform composition.

* Separation: The components of limestone can be separated by physical means. For example, you can dissolve some of the minerals in acids, leaving behind the calcium carbonate.

While limestone is mainly made up of calcium carbonate, the presence of other substances makes it a mixture rather than a pure substance.