AP® Chemistry review sheet from Aim for Five (aimforfive.com/chem/units/3/3-9)
Unit 3 · Topic 3.9
3.9 Separation of Solutions and Mixtures
Filtering can't separate the parts of a solution, but differences in intermolecular forces can. Chromatography separates substances by how strongly they're attracted to a stationary phase versus a moving solvent, and distillation separates them by vapor pressure.
Key terms
- chromatography
- stationary phase
- mobile phase
- distillation
- vapor pressure
- polarity
Why filtering doesn't work
In a solution, the solute particles are individual ions or molecules mixed among the solvent particles. They pass right through filter paper. Filtration only separates an undissolved solid, like a precipitate or sand, from a liquid. To separate the components of a solution, you need to use differences in how strongly each component is attracted to things.
Chromatography
Chromatography has two parts. The stationary phase stays put: paper in paper chromatography, a coated plate in thin-layer chromatography (TLC), or packed material in column chromatography. The mobile phase is the solvent that moves through the stationary phase, carrying the sample along.
Each component is pulled two ways: it's attracted to the stationary phase, and it's attracted to (dissolved in) the mobile phase. A component more strongly attracted to the stationary phase spends more time stuck to it and travels a shorter distance. A component more strongly attracted to the mobile phase travels farther.
Paper and silica plates are very polar (they have many O–H groups). So with a polar stationary phase, polar components stick and move less, while nonpolar components travel farther. If the stationary phase were nonpolar, the order would reverse. Always check what each phase is before you decide.
The result, a chromatogram, can be described with Rf = distance traveled by the component ÷ distance traveled by the solvent front, both measured from the starting line where the sample was spotted. Rf is between 0 and 1; a larger Rf means the component moved farther.
Distillation
Distillation separates liquids by heating a mixture and collecting the vapor, which is then cooled back into a liquid. The component with weaker IMFs has a higher vapor pressure, so it makes up more of the vapor and comes off first.
In a mixture of ethanol (boils at about 78 °C) and water (100 °C), the vapor is richer in ethanol, because ethanol has weaker IMFs overall. The first liquid collected has a higher fraction of ethanol than the original mixture. Repeated distillation concentrates it further.
Explaining a separation
A good explanation names the IMFs involved and compares them. For chromatography: 'Component A is more polar, so it forms stronger dipole-dipole attractions (or hydrogen bonds) with the polar paper and travels a shorter distance.' For distillation: 'Liquid B has weaker London dispersion forces, so it has a higher vapor pressure and a larger share of B is in the vapor.'
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Interpreting a paper chromatogram
A mixture of two dyes is separated on paper (a polar stationary phase) using a fairly nonpolar solvent. The solvent front moves 8.0 cm. Dye A moves 6.0 cm and dye B moves 2.0 cm. Calculate each Rf and decide which dye is more polar.
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- Step 1: Rf(A) = 6.0 ÷ 8.0 = 0.75. Rf(B) = 2.0 ÷ 8.0 = 0.25.
- Step 2: Dye B moved less, so it's more strongly attracted to the stationary phase.
- Step 3: The paper is polar, so the dye with stronger attraction to it is the more polar one.
Answer: Rf(A) = 0.75, Rf(B) = 0.25; dye B is more polar.
- Example 2
Changing the phases (classic trap)
The same two dyes are run on a nonpolar stationary phase with a polar solvent. Predict which dye travels farther.
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- Step 1: Now the polar dye (B) is more strongly attracted to the polar mobile phase.
- Step 2: Dye A, the less polar dye, is more attracted to the nonpolar stationary phase.
- Step 3: So B travels farther this time. Memorizing 'polar moves less' without checking the phases gives the wrong answer here.
Answer: Dye B (the more polar dye) travels farther, because it is more attracted to the polar mobile phase.
- Example 3
Distillation
A mixture of hexane (C₆H₁₄, bp 69 °C) and octane (C₈H₁₈, bp 126 °C) is distilled. Which compound is enriched in the first distillate? Explain in terms of IMFs.
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- Step 1: Both are nonpolar, so both have only London dispersion forces.
- Step 2: Hexane has fewer electrons and a smaller, less polarizable electron cloud, so its dispersion forces are weaker.
- Step 3: Weaker IMFs mean a higher vapor pressure, so hexane makes up more of the vapor at any temperature.
Answer: Hexane, because its weaker dispersion forces give it a higher vapor pressure.
Common mistakes
- Saying you can filter salt out of salt water.
- Assuming polar substances always travel less in chromatography. It depends on the polarity of both phases.
- Explaining distillation only with boiling points. Connect it to IMFs and vapor pressure.
- Calculating Rf using the distance from the bottom of the paper instead of from the starting line.
On the exam
- Expect a chromatogram or distillation setup in a lab-based question. Name the IMFs in each component, compare their strengths and connect to the observed result.
- Questions may ask you to choose a technique: filtration for an insoluble solid, chromatography or distillation for dissolved components.
Connected topics
Videos
Check yourself
4 questions on 3.9 Separation of Solutions and Mixtures. Pick an answer to see if you got it, and why.
A student uses paper chromatography to separate a mixture of two dyes. The paper (the stationary phase) is very polar, and the solvent (the mobile phase) is much less polar than the paper.
When the run is stopped, the solvent front has moved 10.0 cm from the starting line. Dye 1 has moved 8.0 cm and dye 2 has moved 3.0 cm.
Described laboratory experiment
What is the retention factor (Rf) of dye 2?
Which of the following is the best claim about the two dyes, based on the results?
A mixture of hexane, C₆H₁₄, and octane, C₈H₁₈, is separated by distillation. Which of the following correctly identifies the substance that is more concentrated in the first liquid collected, and gives the reason?
A mixture of a polar compound and a nonpolar compound is separated by column chromatography. The column is packed with a nonpolar stationary phase, and a polar solvent mixture is used as the mobile phase. Which of the following correctly predicts the result?
0 of 4 answered