What defines a mixture in scientific terms?

A mixture is defined scientifically as a combination of two or more substances that retain their individual properties. This is a fundamental characteristic of mixtures; the components do not chemically react with each other, meaning that each substance maintains its own physical and chemical properties. For example, if salt is mixed with sand, the salt and sand can be separated from each other, and each will retain its properties as salt and sand, respectively.

In contrast, the other options describe different concepts. A combination of two elements refers specifically to a binary compound, which is not applicable to mixtures where substances remain separate. A fixed composition of various elements describes a compound rather than a mixture, as compounds have set ratios and new properties. Lastly, a pure substance refers to a single type of material without any additives, which again does not align with the definition of a mixture that involves multiple substances.