A pure substance is matter made of only one type of particle, either a single element or a single compound, so its composition by mass is fixed. In AP Chem (Topic 1.3), this fixed composition is what makes empirical formulas and the law of definite proportions possible.
A pure substance is matter that contains only one kind of particle throughout the entire sample. That particle can be an element (like a chunk of copper, which is all Cu atoms) or a compound (like water, which is all H₂O molecules). Either way, every sample of that substance has the same composition and the same physical and chemical properties, no matter where it came from or how big the sample is.
The CED splits compounds into two flavors here. Some pure substances exist as individual molecules (covalent compounds like CO₂), while others are atoms or ions locked together in fixed proportions described by a formula unit (ionic compounds like NaCl). The fixed-proportion part is the whole point. Because a pure compound always has the same ratio of elements by mass (that's the law of definite proportions), you can take mass-percent data from any sample and work out its empirical formula, the lowest whole-number ratio of atoms. None of that math is valid for a mixture, where the composition can be anything.
Pure substances live in Topic 1.3 (Elemental Composition of Pure Substances) in Unit 1, supporting learning objective 1.3.A. That LO asks you to explain the quantitative relationship between a substance's elemental composition by mass and its empirical formula. The hidden assumption behind every empirical formula problem is purity. The law of definite proportions (EK 1.3.A.2) only holds for pure substances, so before you trust any mass-percent calculation, you have to know the sample is pure. This idea also quietly underpins Topic 1.4 (mixtures and mass spectrometry), where varying composition is the giveaway that you're not dealing with a pure substance. Get this concept down early, because stoichiometry, formulas, and lab analysis all build on it.
Keep studying AP Chemistry Unit 1
Law of Definite Proportions (Unit 1)
This law is basically the definition of a pure compound stated in math. If a substance is pure, the mass ratio of its elements is identical in every sample. Water is always about 11% hydrogen and 89% oxygen by mass, whether it came from a glacier or your faucet.
Empirical Formula (Unit 1)
Empirical formula calculations only work on pure substances. Convert mass percents to moles, divide by the smallest, and you get the lowest whole-number atom ratio. Try that on a mixture and you get a meaningless answer, because the ratio changes from sample to sample.
Mixtures (Unit 1)
Mixtures are the opposite case, two or more substances combined in variable proportions. The classic exam test is composition data. If the element ratio varies across samples, it's a mixture; if it's constant, it's a pure substance.
Compounds (Unit 1)
Compounds are one of the two types of pure substances (elements are the other). A compound is still pure even though it contains multiple elements, because those elements are chemically bonded in a fixed ratio rather than just blended together.
This term shows up mostly in Unit 1 multiple-choice questions, usually as a reasoning check rather than a recall question. A classic stem gives you a sample of mixed Cu and Zn powders and asks why you can't calculate a single empirical formula from the element masses. The answer hinges on purity. Cu and Zn aren't chemically combined, so there's no fixed ratio and no formula. Another common setup is data interpretation, like a mineral whose silicon-to-oxygen ratio varies across a sample. Varying composition tells you the sample is a mixture, not a pure substance. No released FRQ uses the phrase 'pure substance' verbatim, but the concept is baked into every empirical formula and percent composition calculation, so you're using it constantly even when the question doesn't say the words.
A homogeneous mixture (like salt water) looks uniform, so it gets mistaken for a pure substance. The difference is composition. A pure substance has one type of particle in a fixed ratio that never changes. A homogeneous mixture is evenly blended but contains multiple substances in proportions you can vary. You can make salt water saltier; you can't make water 'more hydrogen-y.' Uniform appearance is not the same as fixed composition.
A pure substance contains only one type of particle, either an element or a compound, and has the same composition throughout.
The law of definite proportions says every pure sample of a compound has the same mass ratio of elements, which is what separates compounds from mixtures.
Empirical formula calculations are only valid for pure substances, because mixtures have no fixed atom ratio to find.
Some pure substances exist as individual molecules (like CO₂), while ionic substances are described by formula units (like NaCl).
If composition data varies between samples of the same material, the material is a mixture, not a pure substance.
A pure substance is matter made of only one type of particle, either a single element or a single compound, with a fixed composition by mass. Topic 1.3 of the AP Chem CED ties this to the law of definite proportions and empirical formulas.
Yes. A compound counts as a pure substance even though it contains multiple elements, because those elements are chemically bonded in a fixed ratio. Water is always 2 hydrogens to 1 oxygen, so every H₂O sample is identical.
No. Salt water is a homogeneous mixture. It looks uniform, but you can dissolve more or less salt in it, so its composition isn't fixed. Pure substances always have the exact same composition by mass.
Both look uniform, but a pure substance has one type of particle in an unchangeable ratio, while a homogeneous mixture blends multiple substances in proportions you can adjust. Fixed composition is the test, not appearance.
Because a mixture has no fixed mass ratio between its components. A Cu and Zn powder mix could be 30/70 or 60/40, so calculating a 'formula' from the masses gives a number that only describes that one sample, not a real chemical formula.