Separation of Solutions and Mixtures
Connect mixture structure to a separation method; the full measurement pathway follows in the next chapter.
AP Chemistry Topic 3.9 · CED printed p. 68 / PDF p. 75. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- IMF and molecular polarity
- Ratios and measured distances
Separation needs a physical difference to act on
Ordinary filtration retains a suspended insoluble solid, but dissolved molecules and ions generally pass through with the solvent. You cannot separate the molecular components of a homogeneous liquid solution by ordinary filter paper. Choose a method based on a property that differs: interaction with a stationary surface, volatility, or an insoluble solid phase. “Separate a mixture” is too broad to determine a method without knowing its components and phases.
Chromatography uses competition between a mobile phase and a stationary phase. A solute that interacts more strongly with the stationary material relative to the mobile phase is retained more and travels less or elutes later. In paper, thin-layer and column forms, the same competition matters even though the apparatus differs. On a polar stationary phase with a relatively nonpolar mobile phase, more strongly polar or hydrogen-bonding components often travel less. Always specify the phases before inferring polarity; a different stationary/mobile system can change the result.
Read a chromatogram with its conditions
Measure travel from the origin/baseline, not from the bottom of the sheet. A useful ratio is Rf = solute travel / solvent-front travel under the same run. It is dimensionless. Comparisons need matched stationary material, mobile solvent and conditions. Coincident Rf values support possible shared identity but do not prove identity by themselves; different substances can happen to migrate similarly. A larger solvent polarity may alter both solute solubility in the mobile phase and competition for stationary sites, so use the actual observed change rather than an unconditional rule.
Distillation uses different vapor tendencies
Distillation separates volatile components through differences in their vapor pressures and intermolecular interactions. The more volatile component contributes more strongly to the vapor than it does to the liquid under appropriate conditions, so condensed vapor becomes enriched in that component. A lower pure-component boiling point at the same external pressure often signals greater volatility, but the actual mixture behavior also matters.
A single distillation does not promise a perfectly pure fraction. Similar volatilities can make separation poor; repeated vaporization–condensation through a fractionating setup can improve separation when the mixture permits it. The useful AP claim explains enrichment in terms of relative attractions and vaporization, not “the lower boiling compound leaves and the other is completely absent.”
Let's work through it
Work it through
On the specified polar stationary phase and relatively nonpolar mobile phase, A travels 1.2 cm and B 4.8 cm while the solvent front travels 6.0 cm. A separate binary liquid has a vapor enriched in component Y.
- Rf,A = 1.2/6.0 = 0.20; Rf,B = 4.8/6.0 = 0.80.
- A is more strongly retained relative to this mobile phase, consistent with stronger interactions with the polar stationary phase. Under this specified comparison, A is the more strongly polar/retained candidate.
- If another run changes mobile solvent, do not compare Rf as an identity fingerprint without matched conditions.
- A distillate enriched in Y means Y has the greater vapor tendency under those mixture conditions. The condensed fraction contains a greater proportion of Y, not necessarily pure Y.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
Can ordinary filtration separate dissolved ethanol from water?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
Can ordinary filtration separate dissolved ethanol from water?
Worked answer and reason
No. Both molecular components pass with the liquid; another method must use their differing physical interactions or volatility.
Filtration acts on a suspended solid phase, not ordinary dissolved molecular identity.
Retrieval 2
On a polar stationary phase with relatively nonpolar mobile phase, what does a lower Rf suggest?
Worked answer and reason
Stronger retention by the stationary phase relative to mobile-phase transport, often consistent with greater polarity in that system.
Migration depends on competition between both phases.
Retrieval 3
Does the first condensed vapor from a two-liquid solution have to be a pure component?
Worked answer and reason
No. It is often enriched in the more volatile component while retaining some of the other.
Components contribute different partial vapor tendencies rather than an absolute all-or-none switch.
Catch the tempting mistake
- The farther spot is always the more polar substance.
- The result depends on the stationary and mobile phases.
- Different Rf values measure the number of molecules in a spot.
- They compare travel distances, not particle count.
- Distillation works because it breaks covalent molecules apart.
- It mainly separates intact volatile species using differences in vaporization behavior.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 3 problem 21 · ap application · data
- Unit 3 problem 22 · ap synthesis · macroscopic
- Unit 3 problem 23 · ap synthesis · data
Scientific calculator workspace
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Scientific calculator
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For the teacher
- Require the stationary/mobile phase names in polarity claims.
- Give students a mixture description and ask them to justify method selection before interpreting data.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 3.9.A
- Explain the results of a separation experiment based on intermolecular interactions.
- 3.9.A.1
- The components of a liquid solution cannot be separated by filtration. They can, however, be separated using processes that take advantage of differences in the intermolecular interactions of the components. i. Chromatography (paper, thin-layer, and column) separates chemical species by taking advantage of the differential strength of intermolecular interactions between and among the components of the solution (the mobile phase) and with the surface components of the stationary phase. The resulting chromatogram can be used to infer the relative polarities of components in a mixture. ii. Distillation separates chemical species by taking advantage of the differential strength of intermolecular interactions between and among the components and the effects these interactions have on the vapor pressures of the components in the mixture.
Science references for this teaching
- AP Chemistry Course and Exam Description, effective Fall 2024 (supplied source) — Exact topic-numbered learning objectives, essential knowledge, science practices and AP exclusions; supplied PDF SHA256 b5dfe8677ef3d88c613865d2e2a3e8d6125d652e2b24c71ef1e8ce4e011094f0.
Glossary and deeper reading · 13 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Intermolecular force
Intermolecular forces are interactions between molecules. They affect properties such as boiling point and solubility and are distinct from the bonds within each molecule. For example, vaporizing water separates molecules while largely preserving their O–H bonds.
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Molecular polarity
Molecular polarity concerns a molecule's overall separation of charge. Both bond polarity and molecular geometry determine whether individual bond dipoles cancel. For example, carbon dioxide has polar bonds but is nonpolar overall because its linear bond dipoles cancel.
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Filtration
Filtration separates suspended particles from a fluid using a barrier that retains appropriate particle sizes. It generally does not remove individually dissolved ions or molecules. For example, a filter can separate an undissolved precipitate from the surrounding solution.
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Distillation
Distillation separates mixture components using differences in their vaporization behavior. Vapor is collected and condensed, with composition depending on the mixture and conditions. For example, a vapor can be enriched in the more volatile component of a liquid mixture.
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Solubility
Solubility describes how much of a substance can dissolve in a specified solvent under stated conditions. It depends on the solute, solvent, temperature, and other relevant conditions and is distinct from how fast dissolution occurs. For example, two salts can have different equilibrium solubilities in water at the same temperature.
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Ion
An ion is an atom or group of atoms with a net electric charge. Ordinary ion formation changes electron count without changing the element's proton count. For example, a sodium atom losing one electron becomes a sodium ion with charge +1.
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Mobile phase
The mobile phase is the fluid that carries a sample through a chromatographic system. A component's movement depends on its interactions with this phase and the stationary phase. For example, a solvent can carry dissolved dyes along a chromatographic strip.
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Stationary phase
The stationary phase is the material that remains in place in a chromatographic system. Components differ in how strongly they interact with it relative to the mobile phase. For example, stronger stationary-phase retention can make one dye move less far than another under the same conditions.
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Solute
A solute is a component dispersed in a solvent to form a solution. It need not be a solid, and a solution need not be liquid. For example, carbon dioxide is a gaseous solute in carbonated water.
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Solvent
A solvent is the component used as the dissolving medium in a solution. It is often present in the greater amount, though the designation also depends on the mixture's context. For example, water is the solvent in a dilute aqueous sugar solution.
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Mixture
A mixture contains two or more substances whose proportions can vary. Its components retain their chemical identities until a chemical reaction changes them. For example, water and ethanol can form mixtures with many different compositions.
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Vaporization
Vaporization is a phase change from liquid to gas. It requires energy to separate particles against their attractive interactions without ordinarily breaking the molecules' internal bonds. For example, boiling liquid water produces water vapor while preserving the water molecules.
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Condensation
Condensation is a phase change from gas to liquid. It releases energy to the surroundings as particles form more closely associated liquid arrangements. For example, water vapor condensing on a cooler surface forms liquid droplets.