* Mixtures vs. Compounds: Mixtures are combinations of two or more substances that are not chemically bonded. They can be separated by physical means (like filtration, evaporation, etc.). Compounds, on the other hand, are formed when two or more elements chemically combine in a fixed ratio.

* Chemical Formulas Represent Compounds: Chemical formulas represent the specific types and numbers of atoms that make up a compound. For example, H₂O represents water, which is composed of two hydrogen atoms and one oxygen atom.

* Variable Composition of Mixtures: Mixtures can have varying ratios of their components. For example, a sugar-water mixture could have more or less sugar depending on how it's prepared. Because of this variability, a single formula couldn't represent all possible combinations.

Example:

* Salt water is a mixture. It contains salt (NaCl) dissolved in water (H₂O). While we know the formulas for salt and water, we don't write a formula for salt water because the amount of each component can vary.

In short, chemical formulas describe the fixed composition of compounds, while mixtures have variable compositions and therefore cannot be represented by a single formula.