* Composition: Limestone is primarily composed of calcium carbonate (CaCO3), but it also contains other minerals and impurities.

* Variable Composition: The exact composition of limestone can vary depending on its origin and formation. This variation is a characteristic of mixtures.

* Physical Separation: The components of limestone can be separated by physical means, like crushing and grinding, further indicating it's a mixture.

In contrast, a compound is a pure substance with a fixed composition, like water (H2O) or table salt (NaCl).