There are 92 naturally occurring elements. Compound - A substance formed by the combination of elements in fixed proportions. Compounds, unlike mixtures, cannot be separated by physical means. Mixture - A system of two or more distinct chemical substances. Homogeneous mixtures are those in which the atoms or molecules are interspersed, as in a mixture of gases or in a solution.
Heterogeneous mixtures have distinguishable phases, e.g. a mixture of iron filings and sulphur.
In a mixture there is no redistribution of valence electrons, and the components retain their individual chemical properties. However, in many cases the extent of this back reaction is negligibly small, and the reaction is regarded as irreversible. Biology - Any change in behavior of an organism in response to a stimulus. In a solution, the molecules of the solute are discrete and mixed with the molecules of solvent. There is usually some interaction between the solvent and solute molecules.