A mixture is a material composed of more than one component, in which some property values are not uniform throughout. Its phase_of_matter value is greater than one, and its components are substances that can be separated by methods based on differences in physical features.
There is no entity whose name is "mixture." Like material and substance, the word names a class. Its members are particular many component things (the sand and water in a jar, the air in this room, a handful of pebbles and salt) whose feature in common is that they are built from more than one substance, and the substances inside them keep enough of their own character to be told apart.
A mixture has all the features of the material class. It exists, it can be perceived, it is composed of matter, and it serves as a building block. Beyond that, its property values are not all uniform throughout. There are two or more components, and those components are substances. The components are mutually independent of one another; each keeps its own features inside the mixture. And because the components differ in physical features, those differences are the lever by which we separate them again. Filtering, settling, boiling, sifting; the methods are physical because the differences they exploit are physical.
This is where the hierarchy of materials becomes useful. Materials are composed of substances and mixtures. Substances, in turn, are the components of mixtures. The chain runs both ways. We meet a piece of the world; we ask how many components it has and whether its values are uniform throughout. The answer places it in a class. The class tells us what to expect of it.
Question
Sand stirred into water sits at the bottom and refuses to dissolve. Salt stirred into water disappears into the liquid. Are both mixtures?
Reveal Merrill's answer →Hide answer
Both have more than one component, and in both the components can be separated by methods that exploit differences in physical features (let the sand settle and pour off the water; boil the salt water and recover the salt). Both belong to the broader idea of a many component material. The distinction between them, the appearance of uniformity in one and not the other, is what carves the solution class out of the mixture class.