This lab is really about one question: why do some molecules stick around longer than others? Chromatography separates the components of a mixture based on how strongly each molecule interacts with two competing environments. The molecules that "stick" more to one environment travel differently than molecules that don't. Your job is to connect what you see on the chromatography paper (or plate) to the intermolecular forces happening at the molecular level.

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Why This Lab Matters for the AP Exam

The AP exam will absolutely ask you to predict or explain solubility, miscibility, and separation behavior using intermolecular forces. This lab gives you a physical, visible example of those forces in action. When you watch pigments separate on paper, you're watching intermolecular forces compete in real time.

You'll also need to write claim-evidence-reasoning responses that connect macroscopic observations (how far a spot traveled) to molecular-level explanations (what kinds of attractions are present). This lab builds exactly that skill.

CED Connections

This lab connects directly to Unit 3: Properties of Substances and Mixtures.

Topic 3.1 - Intermolecular and Interparticle Forces (Learning Objective 3.1.A)

The core of this lab is 3.1.A: explaining how chemical structure determines the relative strength of intermolecular forces. Every separation you observe is a result of differences in those forces. Specifically:

3.1.A.1: London dispersion forces explain why larger, more polarizable molecules tend to interact more strongly with nonpolar stationary phases.
3.1.A.2: Dipole-dipole interactions and ion-dipole forces explain why polar molecules interact more strongly with polar solvents and polar stationary phases.
3.1.A.4: Hydrogen bonding explains why molecules with O-H or N-H groups interact especially strongly with polar stationary phases like paper (which is made of cellulose, full of -OH groups).

Topic 3.10 - Solubility (Learning Objective 3.10.A)

The "like dissolves like" principle is the backbone of chromatography. Essential knowledge 3.10.A.1 states that substances with similar intermolecular interactions tend to be miscible or soluble in one another. In this lab, that principle determines which molecules dissolve into the mobile phase and which ones stay stuck to the stationary phase.

What You Need to Be Able to Do

By the end of this lab, you should be able to:

Calculate Rf values for separated components and use them to compare relative interactions with the stationary and mobile phases.
Identify variables: the stationary phase, mobile phase (solvent), and the mixture being separated are all distinct parts of the system.
Connect structure to behavior: given a molecule's structure, predict whether it will travel far or stay near the origin based on its intermolecular forces.
Explain separation at the molecular level: use specific intermolecular force vocabulary (hydrogen bonding, dipole-dipole, London dispersion) to justify why two components separated.
Design a variation: predict what would happen if you changed the polarity of the solvent or the stationary phase, and explain your reasoning using intermolecular forces.
Write a CER response: make a claim about which component has stronger interactions with the stationary phase, support it with Rf data, and explain using intermolecular force reasoning.

Core Concepts

Intermolecular Forces vs. Intramolecular Forces

These two terms get confused constantly, so let's clear it up right away.

Intramolecular forces are the bonds within a molecule, like covalent bonds holding atoms together. These don't break during chromatography.

Intermolecular forces (also called intermolecular interactions) are the attractions between separate molecules. These are what chromatography exploits. They're weaker than intramolecular bonds, but they're what determines how a molecule behaves in a mixture or solution.

All intermolecular forces are rooted in Coulombic interactions, which are electrostatic attractions and repulsions between charged or partially charged particles. The different types of intermolecular forces are really just different flavors of Coulombic interactions.

Types of Intermolecular Forces (Weakest to Strongest)

London dispersion forces exist between all molecules, polar or not. They arise from temporary, fluctuating dipoles in electron clouds. At any instant, electrons aren't perfectly evenly distributed, which creates a brief partial charge. That temporary dipole can induce a dipole in a neighboring molecule, and the two attract each other.

The key factors that increase London dispersion forces:

More electrons in the molecule (larger electron cloud = more polarizable)
Greater molecular surface area (more contact between molecules)
Presence of pi bonds (pi electrons are more loosely held and more polarizable)

One important note: London dispersion forces are not the same thing as van der Waals forces. Van der Waals forces is a broader term that includes dispersion forces, dipole-dipole forces, and dipole-induced dipole forces.

Dipole-dipole forces occur between polar molecules. A polar molecule has a dipole moment, meaning it has a partial positive end (delta+) and a partial negative end (delta-) due to unequal sharing of electrons in its bonds. When two polar molecules are near each other, the positive end of one is attracted to the negative end of the other. The strength depends on the magnitude of the dipoles and how the molecules are oriented relative to each other.

Dipole-induced dipole interactions happen between a polar molecule and a nonpolar molecule. The polar molecule's dipole temporarily distorts the electron cloud of the nonpolar molecule, inducing a dipole. These are always attractive.

Ion-dipole forces occur between an ion (fully charged) and a polar molecule. Because ions carry a full charge rather than a partial one, these interactions are stronger than dipole-dipole forces. This is why ionic compounds dissolve in water: water molecules orient their partial negative oxygen end toward cations, and their partial positive hydrogen ends toward anions.

Hydrogen bonding is a particularly strong type of dipole-dipole interaction. It happens when a hydrogen atom is covalently bonded to a highly electronegative atom (nitrogen, oxygen, or fluorine) and is attracted to a lone pair on another electronegative atom (N, O, or F) in a different molecule (or a different part of the same molecule). The hydrogen in an O-H bond, for example, carries a significant partial positive charge because oxygen pulls the electrons so strongly. That makes it a good hydrogen bond donor. The electronegative atom with a lone pair is the hydrogen bond acceptor.

Hydrogen bonds are directional and stronger than typical dipole-dipole forces, which is why water has such a high boiling point for its size.

Polarity and "Like Dissolves Like"

Polarity refers to the uneven distribution of electron density in a molecule, creating partial positive and negative regions. Polar molecules have significant dipole moments. Nonpolar molecules don't.

The "like dissolves like" principle says that polar solutes dissolve well in polar solvents, and nonpolar solutes dissolve well in nonpolar solvents. This works because the solute-solvent interactions need to be strong enough to pull solute molecules away from each other and solvent molecules away from each other. If a nonpolar solute is placed in a polar solvent like water, the water molecules interact much more strongly with each other (through hydrogen bonding) than they do with the nonpolar solute, so the solute doesn't dissolve well.

Two liquids that dissolve in each other in all proportions are called miscible. Two liquids that don't mix (like oil and water) are immiscible.

A homogeneous mixture is one where the composition is uniform throughout, like a solution. Chromatography starts with a homogeneous mixture and separates it into its components.

Stationary Phase and Mobile Phase

Every chromatography system has two phases competing for the molecules you're separating.

The stationary phase is the material that stays in place. In paper chromatography, it's the paper itself (cellulose, which is very polar and capable of hydrogen bonding). In thin-layer chromatography (TLC), it's often silica gel, also very polar.

The mobile phase is the solvent that moves through the stationary phase, carrying molecules with it. The polarity of the mobile phase is a key variable. A more polar solvent will carry polar molecules farther. A less polar solvent will carry nonpolar molecules farther.

Each molecule in your mixture is constantly partitioning between the two phases: spending some time dissolved in the moving solvent and some time attracted to the stationary phase. The more time a molecule spends in the mobile phase, the farther it travels.

How the Lab Works

The investigation logic is straightforward once you understand the two-phase competition.

You spot a mixture onto the stationary phase (paper or a TLC plate) near the bottom. You place it in a container with a small amount of solvent (the mobile phase) at the bottom, making sure the spot is above the solvent level. The solvent slowly wicks upward through the stationary phase by capillary action.

As the solvent front moves up, it carries the molecules in the mixture with it. But here's where intermolecular forces take over: molecules that interact more strongly with the stationary phase spend more time stuck to it and travel a shorter distance. Molecules that interact more strongly with the mobile phase spend more time dissolved in it and travel farther.

The result is separation. Different components end up at different heights on the paper or plate, visible as distinct spots (especially if the mixture contains colored pigments, or if you use a visualization method for colorless compounds).

The polarity of the solvent you choose matters enormously. If you use a very polar solvent, it competes more effectively with the stationary phase for polar molecules, so polar molecules travel farther. If you use a nonpolar solvent, nonpolar molecules travel farther while polar ones stay near the bottom.

This is the guided-inquiry part: you're investigating how changing the solvent (mobile phase polarity) or the mixture affects separation, and you're explaining the results using intermolecular force reasoning.

Data and Analysis Moves

Calculating Rf Values

The Rf value (retention factor) is the key quantitative measurement in this lab.

$R_f = \frac{\text{distance traveled by spot}}{\text{distance traveled by solvent front}}$

Both distances are measured from the original spot location (the origin). Rf values are always between 0 and 1. A high Rf (close to 1) means the molecule traveled nearly as far as the solvent, so it has stronger interactions with the mobile phase. A low Rf (close to 0) means the molecule barely moved, so it has stronger interactions with the stationary phase.

Comparing Rf Values Across Conditions

When you change the mobile phase polarity, Rf values shift. If you switch to a more polar solvent and a component's Rf increases, that tells you the component is polar: the more polar solvent is better at pulling it away from the stationary phase.

You can also compare Rf values of different components in the same run to rank their relative polarities or their relative affinities for the stationary phase.

Identifying Variables

Independent variable: the mobile phase (solvent) used, or the mixture being tested
Dependent variable: the Rf values of the separated components
Controlled variables: the stationary phase, the amount of mixture spotted, the distance the solvent is allowed to travel, temperature

Connecting Data to Intermolecular Forces

This is the most important analysis move. For every Rf value you calculate, you should be able to explain it:

A low Rf on polar paper with a nonpolar solvent suggests the molecule is polar and is hydrogen bonding or forming dipole-dipole interactions with the stationary phase.
A high Rf under the same conditions suggests the molecule is nonpolar and prefers the nonpolar mobile phase through London dispersion forces.

When writing your analysis, be specific. Don't just say "the molecule is more polar." Say which type of intermolecular force is responsible and why the molecular structure supports that.

Sources of Error to Consider

Letting the solvent reach or pass the top of the paper (you lose your solvent front measurement)
Spotting too much mixture (spots overlap and are hard to measure accurately)
Uneven solvent front (paper not hanging straight, or container not level)
Evaporation of solvent during the run (changes the mobile phase composition)

Common Mistakes

Confusing stationary phase affinity with solubility. A molecule with a low Rf isn't insoluble in the solvent. It's just more attracted to the stationary phase than to the mobile phase. Solubility and chromatographic behavior are related but not the same thing.

Saying "London dispersion forces don't apply to polar molecules." They apply to everything. Polar molecules have London dispersion forces plus dipole-dipole forces. Nonpolar molecules only have London dispersion forces. This distinction matters when comparing interaction strengths.

Calling all weak intermolecular forces "van der Waals forces." On the AP exam, van der Waals is a broad category. London dispersion forces are a specific type. Using them interchangeably will cost you points.

Forgetting that hydrogen bonding requires N, O, or F. A molecule with a C-H bond does not form hydrogen bonds, even though hydrogen is present. The hydrogen must be bonded to nitrogen, oxygen, or fluorine to be a hydrogen bond donor.

Mixing up mobile and stationary phase. The mobile phase moves. The stationary phase doesn't. If you get these backwards in an explanation, your entire reasoning falls apart.

Assuming more polar always means lower Rf. This is only true when the stationary phase is polar. If the stationary phase were nonpolar (like in reverse-phase chromatography), the relationship flips. Always anchor your reasoning to the specific system you're working with.

Describing intermolecular forces as "bonds breaking." Intermolecular forces are not bonds in the same sense as covalent or ionic bonds. During chromatography, no intramolecular bonds break. The molecules stay intact. Only the intermolecular interactions between molecules and the phases are changing.

Quick Review Checklist

Rf = distance traveled by spot / distance traveled by solvent front; high Rf means stronger affinity for the mobile phase.
The stationary phase stays put; the mobile phase carries molecules through it.
Molecules separate because they have different relative affinities for the two phases, based on their intermolecular forces.
"Like dissolves like" means polar molecules interact more with polar phases, and nonpolar molecules interact more with nonpolar phases.
Hydrogen bonding requires H bonded to N, O, or F interacting with a lone pair on N, O, or F in another molecule.
London dispersion forces act between all molecules; larger and more polarizable molecules have stronger dispersion forces.
London dispersion forces are not the same as van der Waals forces.
Changing the mobile phase polarity shifts Rf values and changes the separation pattern, which you should be able to predict and explain using intermolecular force reasoning.