Substance

A substance is a material whose property values are uniform throughout. It has a phase_of_matter value of one, meaning it is a single component material; substances are the building blocks of mixtures.

There is no entity whose name is "substance." No physical thing carries that word as its name. Substance is the name of a class. Its members are individual things (water in this glass, salt in this dish, the iron of this nail) whose feature in common is that what is true of one part of them is true of every part of them. Pick any region of a substance, sample its features, and you find the same values you would find anywhere else in it. That uniformity is what membership in the class requires.

A substance has all the features of the material class plus those that distinguish it. It exists, it can be perceived, it is composed of matter, and it serves as a building block. Beyond that, its property values are uniform throughout, and it has only one component. The phase_of_matter property has the value one. These are the marks by which we recognize a substance and separate it, in our descriptions, from a mixture.

Substances sit between two roles in the hierarchy of materials. From above, a substance is a kind of material. From below, substances are the components out of which mixtures are built. A learner who can pick up a piece of the world and say with confidence, "this is a substance, that is a mixture," has done the first work of macroscopic chemistry. The criterion is the same in either direction: how many components, and are the values uniform?