Unit 1 Atomic Structure and Properties

AP Chemistry Pure Substances and Mixtures

How to classify matter, homogeneous and heterogeneous mixtures with examples, and how mass percent reveals a mixture’s makeup or a sample’s purity, in four worked examples.

Sorting matter into pure substances and mixtures sounds like an early-chemistry topic, but AP Chemistry adds a quantitative edge: a pure compound always has the same composition by mass, a mixture does not, and that difference lets you work out what a mixture contains or whether a sample is pure. This page covers the classification and then the calculations, which build on molar mass and percent composition.

What Makes a Substance Pure: Elements and Compounds

A pure substance contains only one kind of particle: one kind of atom, molecule, or formula unit. That gives it a fixed composition and fixed properties, such as a sharp melting point.

Element: made of one kind of atom, such as gold (Au), oxygen gas (O₂), or carbon (C). It cannot be broken down by a chemical change.
Compound: two or more elements bonded in a fixed ratio, such as H₂O, NaCl, or CO₂. It can be broken into its elements only by a chemical change.

The fixed ratio is the law of definite proportions: every pure sample of a compound has the same mass ratio of its elements, whatever its size or source. That ratio is what an empirical formula is built from.

Homogeneous and Heterogeneous Mixtures

A mixture contains two or more kinds of particles that are not chemically bonded to one another, in proportions that can vary from sample to sample.

Homogeneous mixture: the same composition throughout, so you cannot see separate parts. A homogeneous mixture is also called a solution, whether it is a liquid (salt water), a gas (air), or a solid (brass).
Heterogeneous mixture: visibly different regions with different compositions, such as sand in water, granite, or oil floating on water.

Fig. 1 · How matter is classified

Classifying Common Examples of Matter

Material	Classification	Reason
Gold	Element	One kind of atom
Oxygen gas, O₂	Element	Molecules made of one kind of atom
Pure water, H₂O	Compound	Hydrogen and oxygen bonded in a fixed ratio
Table sugar, C₁₂H₂₂O₁₁	Compound	One kind of molecule
Carbon dioxide	Compound	Carbon and oxygen bonded in a fixed ratio
Salt water	Homogeneous mixture	Dissolved ions spread evenly; the salt content can vary
Air	Homogeneous mixture	Mostly N₂ and O₂, evenly mixed
Brass	Homogeneous mixture	An alloy of copper and zinc in varying proportions
Sand in water	Heterogeneous mixture	The sand settles into a separate layer
Granite	Heterogeneous mixture	Visible grains of different minerals
Oil and water	Heterogeneous mixture	Two separate liquid layers

Worked Example 1: Testing the Law of Definite Proportions With Water

Find the mass of hydrogen in 18.0 g and in 36.0 g of pure water, and show that both samples have the same composition.

Mass percent of H: molar mass of H₂O = 2(1.008) + 16.00 = 18.02 g/mol, so H = 2.016 / 18.02 × 100 = 11.19%.
18.0 g sample: 0.1119 × 18.0 g = 2.01 g H.
36.0 g sample: 0.1119 × 36.0 g = 4.03 g H.

Doubling the sample doubles the hydrogen, so the percentage, and the 7.94 : 1 mass ratio of oxygen to hydrogen, stays the same. A sugar-water mixture, by contrast, can be 5% sugar or 20% sugar and still be sugar water.

Worked Example 2: Finding the Composition of a Salt Mixture

A 2.000 g mixture of NaCl and KCl contains 1.100 g of chlorine. Find the mass percent of NaCl in the mixture.

Chlorine fractions: NaCl is 35.45 / 58.44 = 0.6066 Cl by mass; KCl is 35.45 / 74.55 = 0.4755 Cl by mass.
Set up: let x = grams of NaCl, so KCl = 2.000 − x. Then 0.6066x + 0.4755(2.000 − x) = 1.100.
Solve: 0.1311x = 0.149, so x = 1.14 g NaCl.
Mass percent: 1.14 / 2.000 × 100 = 56.8% NaCl (and 43.2% KCl).

This is the elemental-analysis reasoning the course describes: measure how much of one element is present, then use each component’s fixed composition to work back to the mixture’s makeup.

Worked Example 3: Deciding Whether a Sample Is Pure

A white solid sold as pure sodium chloride is analyzed and found to be 58.0% chlorine by mass. Is it pure?

Expected: pure NaCl is 35.45 / 58.44 × 100 = 60.66% Cl.
Measured: 58.0% Cl.
Conclusion: the sample is not pure. A pure compound always has the same composition, so the lower chlorine content means something else is present.

The direction of the error is a clue. The impurity must contain a smaller fraction of chlorine than NaCl does, for example KCl (47.55% Cl) or a chlorine-free substance. A value higher than 60.66% would point to a chlorine-rich impurity instead.

Worked Example 4: Choosing a Method to Separate a Mixture

Suggest a separation method for each mixture: (a) sand and water, (b) salt dissolved in water, (c) the colored dyes in a marker ink.

(a) Filtration. Sand is a solid that does not dissolve, so it stays on the filter paper.
(b) Evaporation or distillation. The water boils off, leaving the salt; distillation also collects the water.
(c) Chromatography. The dyes travel at different speeds through the stationary phase, depending on how strongly they interact with it.

Every method uses a physical difference, such as particle size, boiling point, or intermolecular attractions, so the components come out unchanged. Dye separation with R_f values is worked through in chromatography.

Five Pure Substance and Mixture Practice Questions With Answers

Classify each as an element, a compound, a homogeneous mixture, or a heterogeneous mixture: nitrogen gas, sodium chloride, vinegar, a chocolate-chip cookie.
Show answer
Element, compound, homogeneous mixture (acetic acid dissolved in water), heterogeneous mixture.
Carbon dioxide is 27.29% carbon by mass. How many grams of carbon are in 50.0 g of CO₂?
Show answer
0.2729 × 50.0 g = 13.6 g of carbon, whatever the source of the CO₂.
A 4.00 g mixture of NaCl and sand contains 1.82 g of chlorine. What is the mass percent of NaCl?
Show answer
Mass of NaCl = 1.82 / 0.6066 = 3.00 g, so the mixture is 75.0% NaCl.
Two samples of a white compound contain 40.0% and 32.0% carbon by mass. Can they be the same pure compound?
Show answer
No. A pure compound always has the same composition, so at least one sample is a different substance or impure.
Is brass a compound? Explain.
Show answer
No. Brass is a homogeneous mixture (an alloy) of copper and zinc whose proportions can vary, so it has no fixed composition.

Common Pure Substance and Mixture Mistakes

Calling a compound a mixture. The elements in a compound are chemically bonded in a fixed ratio; in a mixture they are not.
Calling every clear liquid pure. Salt water looks clear but is a homogeneous mixture.
Calling O₂ a compound. A molecule made of one kind of atom is an element.
Forgetting that solutions can be solids or gases. Air and brass are homogeneous mixtures too.
Using a mixture’s percent as a fixed constant. Only a pure compound has a fixed mass percent.
Separating a compound by physical means. Filtration or distillation cannot split water into hydrogen and oxygen.

Composition and purity calculations are quick Unit 1 points that reuse molar mass skills. To see how stronger Unit 1 answers would move your overall estimate, try the AP Chem Score Calculator.

Frequently Asked Questions

What is a pure substance?

A pure substance contains only one kind of particle, so its composition is fixed. Elements such as gold and compounds such as water are pure substances. Every sample of a compound has the same mass ratio of its elements.

What is the difference between a homogeneous and a heterogeneous mixture?

A homogeneous mixture has the same composition throughout, so you cannot see separate parts; it is also called a solution (salt water, air, brass). A heterogeneous mixture has visibly different regions (sand in water, granite, oil and water).

Is water a pure substance?

Pure water is: it is the compound H₂O, with a fixed composition of 11.19% hydrogen and 88.81% oxygen by mass. Tap water and seawater are homogeneous mixtures, because they contain dissolved substances.

Is air a pure substance?

No. Air is a homogeneous mixture of gases, mostly nitrogen and oxygen with smaller amounts of argon, carbon dioxide, and water vapor. Its composition can vary, which a pure substance's cannot.

How can you tell if a sample is pure?

Compare its measured composition with the composition of the pure substance. Pure NaCl is 60.66% chlorine by mass, so a sample labeled NaCl that measures 58.0% chlorine must contain something else.

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