A collection of molecules that are not all the same is called a mixture.

Here's why:

* Mixture: A combination of two or more substances where each substance retains its own chemical identity. The components of a mixture can be separated by physical means (like filtration, evaporation, etc.).

* Compound: A substance formed when two or more different elements are chemically bonded together in a fixed ratio. The components of a compound cannot be separated by physical means.

Examples of mixtures:

* Salt water: A mixture of salt (NaCl) and water (H2O).

* Air: A mixture of nitrogen (N2), oxygen (O2), carbon dioxide (CO2), and other gases.

* Sand and pebbles: A mixture of different types of rocks and minerals.