What constitutes a substance in physical science?

In physical science, a substance is defined as a form of matter that has a definite and consistent composition. This means that a substance always has the same chemical and physical properties regardless of the amount or the state it is in. For example, pure water (H₂O) is always made up of two hydrogen atoms and one oxygen atom, which is a specific and unchanging composition.

Contrasting this with other options, a form of matter with variable composition would not qualify as a pure substance; mixtures, for instance, can consist of different proportions of components and do not have a fixed composition. Similarly, a substance that can be physically separated indicates a mixture, as pure substances cannot be separated into their components by physical means. Lastly, while a combination of elements can describe some substances (like compounds), it does not inherently define the concept of a substance, since it lacks the emphasis on a definite composition. Thus, focusing on the definition of a substance leads to the conclusion that it must have a fixed and uniform composition.