Separation techniques split mixtures by exploiting differences in intermolecular forces between components. For the AP Chemistry exam, focus on chromatography (paper, thin-layer, and column) and distillation, and be ready to explain why a component moves faster, boils first, or sticks to a surface based on polarity and IMF strength.

more resources to help you study

practice multiple choice FRQ practice & scoring cheatsheets score calculator key terms

Chromatography in AP Chemistry

Chromatography separates components of a mixture based on how strongly each component interacts with the mobile phase compared with the stationary phase. In paper and thin-layer chromatography, a component that has a stronger attraction to the mobile phase travels farther, while a component that has a stronger attraction to the stationary phase travels less.

For AP Chemistry, explain chromatography results with intermolecular forces and polarity. With a polar stationary phase, polar substances tend to stick more and move less, while less polar substances usually travel farther with the solvent. A chromatogram can therefore give evidence about the relative polarities of the components in a mixture.

Why This Matters for the AP Chemistry Exam

This topic connects directly to a skill the AP exam tests often: identifying experimental procedures that match a question, sometimes including a sketch of a lab setup. You should be able to explain the results of a separation experiment using the intermolecular interactions between the components and the phases involved.

Both multiple-choice and free-response questions can ask you to interpret a chromatogram, predict which component travels farther, or reason about which substance distills off first. The key move is always the same: link a macroscopic result (distance traveled, boiling order, what gets trapped) to the strength and type of intermolecular forces. This builds on what you learned about IMFs in earlier Unit 3 topics and on polarity from Unit 2.

Key Takeaways

The components of a liquid solution cannot be separated by filtration; you need methods that use differences in intermolecular interactions.
Chromatography (paper, thin-layer, and column) separates species based on how strongly each interacts with the mobile phase versus the stationary phase.
A chromatogram can be used to infer the relative polarities of components in a mixture.
In normal-phase chromatography with a polar stationary phase, polar components stick more and travel less, while nonpolar components move farther with the solvent.
Distillation separates components by differences in vapor pressure, which depend on IMF strength; the substance with the weakest IMFs (lowest boiling point) vaporizes first.
Filtration only removes insoluble solids; a dissolved solute passes through with the solvent.

Separating Mixtures Using Intermolecular Forces

When you have a solute (or several solutes) dissolved in a solvent, you often need to separate them, especially after a reaction. The method you choose depends on the physical properties and the intermolecular forces of the components.

One important limit to know: the components of a liquid solution cannot be separated by simple filtration. Filtration only traps insoluble solids. To pull apart species that are actually dissolved, you need techniques that take advantage of differences in their intermolecular interactions, mainly chromatography and distillation.

Filtration and Its Limit

Filtration runs a mixture through a porous barrier. Liquid and dissolved species pass through, while undissolved solids get trapped in the filter.

A real-life example is making coffee: you pour hot water and ground beans through a filter, the coffee runs into your mug, and the grounds stay behind.

Imagine filtering a mixture of salt, water, and sand. Only the sand gets caught, because the salt is dissolved in the water and passes through with it. The filtrate is still salt water. So filtration alone cannot fully separate a true solution; the dissolved solute stays with the solvent.

Chromatography

Chromatography separates chemical species by taking advantage of the different strengths of intermolecular interactions between the components of the mixture (carried in the mobile phase) and the surface of the stationary phase. Because separation depends on these interactions, the resulting chromatogram can be used to infer the relative polarities of the components.

The three types you should know are paper, thin-layer, and column chromatography. They share the same core idea but differ in setup.

Paper Chromatography

In paper chromatography, a small spot of the sample is applied near one end of a strip of chromatography paper, and the paper is placed in a solvent. The solvent travels up the paper by capillary action, carrying the components with it. Each component interacts differently with the paper and the solvent, so they move at different rates and separate.

Want to review capillary action and the forces behind it? Check out the study guide on solids, liquids, and gases.

Picture the solvent traveling up the paper and "dragging" each component along based on differences in intermolecular forces and polarity.

The stationary phase in paper chromatography is a cellulose material, which is polar. Polar components have a stronger attraction to the paper, so they move more slowly, while nonpolar components have a weaker attraction and move farther with the solvent.

Remember: like dissolves like when it comes to polarity.

Thin-Layer Chromatography (TLC)

Thin-layer chromatography uses a stationary phase coated onto a thin layer on glass, plastic, or aluminum. Like paper chromatography, it separates components by their differing affinity for the stationary phase based on polarity.

Most TLC plates use polar silica as the stationary phase, with the solvent in the chamber acting as the mobile phase. The general rule: polar components are more strongly attracted to the polar stationary phase and move less, while nonpolar components are carried farther by the solvent. The idea matches paper chromatography, but the stationary phase is silica rather than cellulose.

Column Chromatography

Column chromatography is tested less often than paper and TLC, but it is still part of the AP Chemistry course. It works on the same principle, except the stationary phase is packed into a column.

The stationary phase is usually silica or alumina, both solid materials. As the components pass through the column, they interact with the stationary phase and move at different rates, separating based on their affinity for it. This method is more complex and slower than the other two, but it can handle larger quantities and reach higher purity of the separated components.

A nice application: you can use this technique to separate the different pigments in spinach.

Distillation

Distillation separates liquids by taking advantage of differences in their vapor pressures, which depend on the strength of their intermolecular forces. The solution is carefully heated so one liquid boils off and then recondenses into a separate container. The first liquid to boil is the one with the weakest IMFs (and lowest boiling point); as IMF strength increases, so does boiling point.

Two common versions to compare are simple distillation and fractional distillation:

Difference in boiling points: Simple distillation works when components have a large difference in boiling points. Fractional distillation is used when the boiling points are close or when simple distillation cannot separate them cleanly.
Number of vaporization and condensation steps: Simple distillation uses one vaporization and condensation step, while fractional distillation uses multiple steps, allowing a more refined separation.
Purity of separated components: Fractional distillation generally gives higher purity than simple distillation because of those extra vaporization and condensation steps.

How to Use This on the AP Chemistry Exam

Free Response

When asked to explain the results of a separation, name the specific intermolecular forces involved and compare their strengths. For chromatography, connect how far a component traveled to how strongly it interacts with the stationary phase versus the mobile phase. For distillation, connect boiling order to IMF strength and vapor pressure.

Problem Solving

Reading a chromatogram: a component that traveled far had a weaker attraction to a polar stationary phase, so it is likely less polar. A component that barely moved is likely more polar.
Predicting boiling order in distillation: the component with the weakest IMFs has the highest vapor pressure and boils off first.
Choosing a method: if a substance is truly dissolved, filtration will not separate it; reach for chromatography or distillation instead.

Common Trap

When you explain a result, do not just say one force is "strong" or "weak." Identify the actual force (for example, hydrogen bonding, dipole-dipole, or London dispersion) and explain its strength relative to the other forces at play. That comparison is what earns credit.

Common Misconceptions

Filtration can separate any mixture. Filtration only removes insoluble solids. A dissolved solute passes right through with the solvent, so it cannot separate a true solution.
Polar substances always travel farther in chromatography. With a polar stationary phase (like silica or cellulose), polar components stick more and travel less. The nonpolar components are the ones that move farther with the solvent.
Distillation separates by mass or size. Distillation separates by differences in vapor pressure and boiling point, which come from IMF strength, not from how heavy the molecules are.
The first liquid to boil has the strongest IMFs. It is the opposite: the component with the weakest IMFs has the lowest boiling point and boils off first.
Calling a force "strong" or "weak" is a full explanation. Name the specific intermolecular force and compare it to the others present; vague strength language loses points.
Chromatography only tells you that components separated. A chromatogram can also be used to infer the relative polarities of the components based on how far each one traveled.

Vocabulary

The following words are mentioned explicitly in the College Board Course and Exam Description for this topic.

Term	Definition
chromatogram	The visual result of a chromatography separation showing the separated components as distinct spots or bands.
chromatography	A separation technique that separates chemical species based on differences in their intermolecular interactions with a mobile phase and a stationary phase.
column chromatography	A chromatographic separation technique using a column filled with a stationary phase material through which a mobile phase flows.
distillation	A separation technique that separates chemical species based on differences in their vapor pressures and boiling points.
filtration	A separation technique that uses a physical barrier to separate solid particles from a liquid or gas based on particle size.
intermolecular interactions	Forces between molecules, such as hydrogen bonding, dipole-dipole forces, and London dispersion forces, that affect the physical and chemical properties of substances.
liquid solution	A homogeneous mixture where a solute is dissolved in a liquid solvent, forming a single phase.
mobile phase	In chromatography, the solvent or gas that moves through the stationary phase and carries the components of a mixture.
paper chromatography	A chromatographic separation technique using paper as the stationary phase and a liquid solvent as the mobile phase.
polarity	The distribution of electric charge in a molecule, determining its ability to interact with polar and nonpolar substances.
stationary phase	In chromatography, the solid or liquid material that remains fixed and interacts with the components of a mixture to separate them.
thin-layer chromatography	A chromatographic separation technique using a thin layer of absorbent material on a solid support as the stationary phase.
vapor pressure	The pressure exerted by a vapor in equilibrium with its liquid or solid phase at a given temperature.

Frequently Asked Questions

What is chromatography in AP Chemistry?

Chromatography is a separation technique that separates components based on how strongly they interact with the mobile phase versus the stationary phase. It uses differences in intermolecular forces and polarity.

What are the mobile phase and stationary phase?

The mobile phase moves through the system and carries components of the mixture. The stationary phase stays in place and interacts with those components, causing them to move different distances or at different speeds.

In paper chromatography, why does one component travel farther?

A component travels farther if it has a stronger attraction to the mobile phase than to the stationary phase. With a polar stationary phase, less polar components usually travel farther than more polar components.

Can filtration separate a liquid solution?

No. Filtration can separate an insoluble solid from a liquid, but dissolved solutes pass through the filter with the solvent. Liquid solutions need methods like chromatography or distillation.

How does distillation separate mixtures?

Distillation separates components by differences in vapor pressure and boiling point, which come from differences in intermolecular forces. The component with weaker IMFs and a lower boiling point vaporizes first.

How does AP Chemistry test chromatography?

AP Chemistry questions often ask you to interpret a chromatogram or choose a separation method. Explain results by comparing intermolecular forces between the component, mobile phase, and stationary phase.