Paper chromatography is a separation technique in which a mixture is spotted on paper (the stationary phase) and a solvent (the mobile phase) carries components up the paper at different rates based on the relative strength of their intermolecular interactions with each phase (AP Chem Topic 3.9).
Paper chromatography is the simplest chromatography setup you'll see in AP Chem. You spot a tiny drop of a mixture near the bottom of a strip of paper, then stand the paper in a shallow pool of solvent. The solvent climbs the paper by capillary action and drags the components of the mixture along with it. Here's the part the exam actually cares about, and it's all intermolecular forces. The paper is the stationary phase, and its surface is polar (cellulose is loaded with -OH groups that hydrogen bond). The solvent is the mobile phase. Each component in the mixture is in a constant tug-of-war between sticking to the paper and dissolving in the moving solvent.
A component that interacts more strongly with the paper (say, a very polar molecule that hydrogen bonds with cellulose) keeps getting grabbed, so it travels a short distance. A component that interacts more strongly with the mobile phase rides the solvent and travels far. The result is a chromatogram, a paper with separated spots at different heights, and per EK 3.9.A.1 you can read it like a polarity ranking. With a polar solvent like water on polar paper, the spot that traveled farthest is the one that preferred the solvent, and the spot stuck near the start is the one clinging to the paper.
This term lives in Unit 3: Properties of Substances and Mixtures, Topic 3.9 (Separation of Solutions and Mixtures) and directly supports learning objective 3.9.A: explain the results of a separation experiment based on intermolecular interactions. The deeper point is that Topic 3.9 is where Unit 3's IMF theory gets cashed in for lab results. You can't filter a true solution, because the components are dissolved at the particle level. So separation has to exploit differences in intermolecular interactions, and chromatography (paper, thin-layer, and column) is the CED's flagship example. On the exam, paper chromatography is rarely about memorizing the procedure. It's a vehicle for IMF reasoning, asking you to explain why spot X outran spot Y using hydrogen bonding, dipole-dipole forces, or London dispersion forces.
Keep studying AP Chemistry Unit 3
Polarity and Intermolecular Forces (Units 2-3)
Chromatography is basically an IMF competition you can watch. Everything you learned about hydrogen bonding, dipoles, and dispersion forces in Units 2 and 3 becomes the explanation for why one spot climbs higher than another. A chromatogram is IMF theory turned into evidence.
Mobile Phase and Stationary Phase (Unit 3)
These two terms are the vocabulary the CED expects in your answer. In paper chromatography, the paper is the stationary phase and the solvent is the mobile phase. A correct exam explanation names which phase each component interacts with more strongly, not just 'it's more polar.'
Rf Value (Retention Factor) (Unit 3)
Rf is how you turn a chromatogram into a number. Divide the distance a spot traveled by the distance the solvent front traveled. Matching Rf values is how you identify an unknown, which is exactly what the 2017 FRQ asked when a student had to figure out which of three dyes was in an unknown sample.
Distillation (Unit 3)
Distillation is the other Topic 3.9 separation technique, but it exploits a different property. Distillation separates by boiling point (which still traces back to IMF strength), while chromatography separates by relative attraction to two phases. Knowing which technique fits which mixture is a classic MCQ setup.
Paper chromatography shows up in two main ways. Multiple-choice questions give you a result, like 'compound X traveled farther than compound Y' or 'pigment A moved farthest, C moved least,' and ask you to pick the IMF explanation. The winning answer always compares interactions with both phases, for example 'X interacts more strongly with the mobile phase, while Y hydrogen bonds more strongly with the polar paper.' On the free-response side, the 2017 exam's Short FRQ Q4 had a student use paper chromatography to identify which of three pure dyes was in an unknown sample, which means comparing spot positions or Rf values and justifying the match. The skill being tested is the same every time: translate distance traveled into a claim about relative polarity and intermolecular interactions. One trap to avoid is writing 'it's more polar so it moves less.' That's only true when the paper is the more polar phase, so always anchor your reasoning to the actual phases given in the problem.
Both are Topic 3.9 separation techniques for mixtures that can't be filtered, but they exploit different things. Distillation separates components by differences in boiling point (you vaporize the more volatile one first). Paper chromatography separates by differences in how strongly components stick to a stationary phase versus a mobile phase. If an exam question involves heating a liquid mixture, think distillation; if it involves spots moving up paper or through a column, think chromatography and IMF tug-of-war.
In paper chromatography, the paper is the polar stationary phase and the solvent is the mobile phase, and components separate based on which phase they interact with more strongly.
A component that travels far up the paper interacts more strongly with the mobile phase, while a component that barely moves is held by stronger interactions (often hydrogen bonding) with the paper.
Per EK 3.9.A.1, a chromatogram lets you infer the relative polarities of the components in a mixture, which is the main thing the exam asks you to do with it.
A substance with stronger hydrogen bonding to the cellulose paper travels less because it keeps getting pulled out of the moving solvent.
Rf value equals the distance a spot traveled divided by the distance the solvent front traveled, and matching Rf values is how you identify an unknown component.
Solutions can't be separated by filtration, so techniques like chromatography and distillation work by exploiting differences in intermolecular interactions instead.
It's a separation technique from Topic 3.9 where a mixture is spotted on paper and a solvent carries the components up at different rates. Separation happens because each component has a different balance of intermolecular attractions to the paper (stationary phase) versus the solvent (mobile phase).
Not automatically. Paper is polar, so polar components often stick to it and travel less, but the result depends on the solvent too. If the mobile phase is also very polar, a polar component can travel far. Always reason from interactions with both phases, not from polarity alone.
Chromatography separates components by their differing attractions to a stationary phase and a mobile phase, while distillation separates by differences in boiling point. Both appear in Topic 3.9 as ways to separate solutions, since filtration can't separate dissolved components.
Cellulose paper is covered in -OH groups, so a component that hydrogen bonds strongly keeps binding to the paper instead of riding the solvent. More time stuck to the stationary phase means less distance traveled, which is exactly the reasoning AP multiple-choice answers reward.
Yes. The 2017 exam included a short FRQ where a student used paper chromatography to determine which of three pure dyes (A, B, or C) was present in an unknown sample, which required comparing spot positions and justifying the identification.