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Separating mixtures

You can pull a mixture apart without any chemistry, just by exploiting how its components differ. Chromatography, distillation, and filtration each grab a different difference — and reading a chromatogram means knowing what its distances actually measure.

§1

Separating by physical differences.

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Because a mixture's components keep their own properties, they can be separated by physical means. Filtration separates by particle size (a filter traps large particles, passes small ones). Distillation separates by boiling point (the lower-boiling component vaporizes first).

Chromatography separates by how strongly each component is attracted to a moving solvent (the mobile phase) versus a stationary material (like paper). Components that cling to the paper move slowly; those that prefer the solvent travel far.

So on a chromatogram, the distance a component travels reflects the balance of attractions between it, the solvent, and the paper — not how much of it is present.

UNIT 3 TOPIC 3.9 • SEPARATION OF SOLUTIONS AND MIXTURES SEPARATION TOOLS Different tools separate the components of a mixture by exploiting different physical properties. 1 PAPER CHROMATOGRAPHY Separates components by how far each travels up the paper. solvent front separated components starting line solvent WHY IT WORKS Spots more soluble in the mobile phase (solvent) and less attracted to the stationary paper travel farther — set by polarity / IMFs. 2 SIMPLE DISTILLATION Separates components by differences in boiling point (volatility). thermometer condenser water out water in receiving flask boiling mixture WHY IT WORKS The more volatile (lower–boiling) component vaporizes first, then condenses in the water-cooled condenser and collects in the flask. HOW THEY COMPARE PAPER CHROMATOGRAPHY Relies on differences in polarity & intermolecular interactions (IMFs). VS. SIMPLE DISTILLATION Relies on differences in boiling point / volatility (vapor pressure). AP Chemistry · Unit 3 · Properties of Substances & Mixtures
Fig. 3.9.1 Mixtures separate by physical differences. In paper chromatography, components travel different distances based on their attraction to the paper versus the solvent — the less-adsorbed, more soluble components move farther toward the solvent front.
§2

Reading a chromatogram.

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Interpret distance as attraction, not amount.

  1. Identify the phases. The mobile phase (solvent) carries components; the stationary phase (paper) holds them back.
  2. Relate distance to attraction. A component that interacts more with the solvent (and less with the paper) travels farther.
  3. Do not read distance as quantity. How far a spot travels tells you about its attractions, not how much of it there is.
  4. Compare components. Different travel distances mean different components; matching a distance can help identify a substance.
§3

The pieces you'll meet.

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A quick map of the separation methods.

chromatography
Chromatography
Separates by relative attraction to mobile vs stationary phase.
distillation
Distillation
Separates by boiling point; lower-boiling component vaporizes first.
filtration
Filtration
Separates by particle size; a filter traps large particles.
mobile
Mobile phase
The moving solvent that carries components along.
stationary
Stationary phase
The fixed material (paper) that holds components back.
distance
Travel distance
Reflects attraction balance, not the amount of a component.
§4

Worked example: reading a paper chromatogram.

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Question. On a chromatogram, spot A traveled far up the paper and spot B stayed near the start. What does that tell you?

Spot A. It traveled far, so it is strongly attracted to the moving solvent and only weakly held by the paper.

Spot B. It barely moved, so it is strongly attracted to the paper (stationary phase) and less to the solvent.

Caution. This says nothing about how much of A or B is present — a faint spot that traveled far is still a component that prefers the solvent. Distance measures attraction, not quantity.

§5

Mistakes that cost real points.

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Pitfall · 01

"The component that travels farthest in chromatography is the most abundant one."

Travel distance reflects a component's relative attraction to the solvent versus the paper, not how much of it there is. A small amount of a solvent-loving substance can travel farther than a large amount of a paper-loving one.

Fix. Read distance as attraction balance. To judge amount, look at spot size or intensity, not how far it moved.

Pitfall · 02

"Separation methods change the chemical identity of the components."

Chromatography, distillation, and filtration are physical separations: the components keep their chemical identity throughout. Distilling water from salt gives back the same water and the same salt, unchanged.

Fix. Treat these as physical processes that sort components by a physical property, never as reactions that transform them.

Pitfall · 03

"Distillation separates substances by their particle size."

Distillation separates by boiling point, not size — the lower-boiling component vaporizes and is collected first. Size-based separation is filtration. Matching each method to the right property is the whole skill.

Fix. Match method to property: filtration → size, distillation → boiling point, chromatography → attraction.

§6

Skill Check.

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