Chapter 3 · Matter and Energy
3.4Classifying Matter According to Its Composition
12 min · two checks
Predict
Is salt water an element, a compound, or a mixture?
The idea
- Separate elements, compounds, and mixtures.
- Separate homogeneous from heterogeneous mixtures.
A pure substance has fixed composition and a single set of properties. If it cannot be broken into simpler substances by chemical means, it is an element: copper, oxygen, gold. If it can, it is a compound: water breaks into hydrogen and oxygen, and it is always about 11% hydrogen and 89% oxygen by mass. Compounds are not blends you can pick apart with tweezers. Their elements are chemically combined.
A mixture’s composition can vary, and its parts keep their identities. If you can see different regions — granite, oil and vinegar, salad — it is heterogeneous. If it looks uniform at the scale of the sample — salt water, air, brass — it is homogeneous, also called a solution. Filtering separates some heterogeneous mixtures. Distillation and evaporation often separate homogeneous ones because the components still have their own boiling points. Classification is about composition, so do not confuse it with solid/liquid/gas.
Keep these
- Elements: one kind of atom. Compounds: elements chemically joined in a fixed ratio.
- Mixtures have variable composition. Solutions are homogeneous mixtures.
- Physical separation works on mixtures because the substances are not chemically bonded to each other.