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Composition of Mixtures

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A mixture is two or more pure substances physically combined, and unlike a compound its proportions are not fixed. One sample can be richer in a component than another and both are still the same mixture. Analyzing a mixture means separating what varies (the blend) from what does not (the fixed composition of each component within it).

UNIT 1 TOPIC 1.4 • COMPOSITION OF MIXTURES MIXTURE ANALYZER AP Chemistry · Unit 1 · Atomic Structure & Properties Pure substances have a fixed composition; mixtures contain two or more components in proportions that can vary. PURE SUBSTANCE one type of particle all identical → fixed ratio • Element: one kind of atom • Compound: one kind of molecule / formula unit • Composition is fixed — definite proportions (Topic 1.3) MIXTURE two or more types Sample A (more blue) Sample B (more orange) Same two components, different ratio → composition VARIES Separable by physical means (filtering, distillation, chromatography). ELEMENTAL ANALYSIS composition by mass → relative numbers of atoms → identity & purity mass % of each element → ÷ molar mass → mole ratio of atoms Purity check Pure CaCO₃ is 40.04% Ca by mass. A sample measures 32.0% Ca. Purity ≈ 32.0 / 40.04 ≈ 80% CED ANCHOR 1.4.A Pure substances contain particles of a single type; mixtures contain two or more types in variable proportions. Elemental analysis gives the relative numbers of atoms and can determine a sample's purity.
A pure substance is one kind of particle in a fixed ratio; a mixture holds two or more kinds whose proportions can change from sample to sample. Two samples of the same mixture (more blue here, more orange there) are still the same mixture at different compositions.
Mixture Decomposer · Open the sandbox →

The mistakes here come from importing rules that only hold for pure compounds: expecting a mixture to obey definite proportions, reading a blend as if it were a single pure compound, and confusing a component's percent of the mixture with an element's percent inside that component. A mixture's makeup is a choice per sample, not a law.

The work

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Lesson
Composition of Mixtures ›

Mixtures blend pure substances in proportions that vary, which breaks the fixed-ratio intuition from compounds. The lesson draws the line between blend and compound, then closes with a ten-scenario applet that decomposes samples into their components.

Skill check · 10 scenarios
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Diagnostic
10-item topic check ›

Ten items spanning the three Topic 1.4 misconceptions: applying definite proportions to a mixture, reading a blend as a single pure compound, and confusing a component's percent of the mixture with an element's percent within it.

Not started · 10 items · ~15 min
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Targeted Practice
Drill a single misconception ›

Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.

Take the diagnostic to identify your misconceptions
Diagnostic first