Unit 3 · Topic 3.9 Beta

Separation of Solutions and Mixtures

Mixtures are separated by physical properties that trace back to particle forces.

Practice 2: Question and MethodPractice 6: Argumentation

Question set for this topic

Part 1 · Hook

Why this matters

Forensic chemists can tell which pen wrote a note by separating the dyes in its ink. Refineries split crude oil into gasoline and diesel. Each separation works because the parts of a mixture are held by different forces.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. Which liquid has the higher vapor pressure at 25 °C?

  1. The one with weaker intermolecular forces
  2. The one with stronger intermolecular forces
  3. The one with more mass
Show the answer

Weaker attractions let more molecules escape.

  • Correct: The one with weaker intermolecular forces:
  • The one with stronger intermolecular forces:
  • The one with more mass:

2. Which molecule can hydrogen-bond to an –OH group?

  1. CH₃OH
  2. CH₄
  3. C₆H₁₄
Show the answer

Methanol has an O–H group and lone pairs on O.

  • Correct: CH₃OH:
  • CH₄:
  • C₆H₁₄:

Part 4 · See it

See it first

A paper chromatogram. From a start line, the solvent has climbed 9.0 cm. A blue dye moved 6.3 cm (Rf = 0.70) and a red dye moved 2.7 cm (Rf = 0.30), so the red dye was held more strongly by the paper.
Rf compares how far each spot traveled with how far the solvent traveled. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. Components of a mixture differ in size, attractions or volatilityphysical methods can separate them without changing them
  2. In chromatography each component is pulled by a stationary and a mobile phasethe one held more by the stationary phase moves more slowly and has a smaller Rf
  3. In distillation the component with weaker intermolecular forces has the higher vapor pressureit boils off first and is collected first
  4. Undissolved solids are larger than filter pores; dissolved particles are notfiltration removes solids but cannot separate a solution

Part 6 · Key ideas

Key ideas

  • Filtration: undissolved solid from liquid, by size.
  • Chromatography: stronger attraction to the stationary phase means slower movement. Rf = spot distance ÷ solvent distance.
  • Paper is polar: polar components lag; a more polar solvent carries them farther.
  • Distillation: weaker intermolecular forces, higher vapor pressure, vaporizes first.

Part 7 · Misconception

A common mistake

The wrong idea: In chromatography, heavier molecules move more slowly because they are harder to carry.

What actually happens: Separation follows attractions, not mass. A component held more strongly by the stationary phase moves more slowly, whatever its mass.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

Data table

Identifying the dyes in a drink

A student spots a sports drink and four known food dyes on one strip of chromatography paper and develops it with a salt-water solvent. The solvent front ends 8.0 cm above the start line.

Spot positions above the start line
SampleSpot distance (cm)Rf
Unknown6.4, 2.0?
Dye A6.40.80
Dye B4.00.50
Dye C2.00.25
Dye D1.20.15

1. What is the Rf of the drink's higher spot?

Type a number.

Show the answer

Rf = 6.4 cm ÷ 8.0 cm = 0.80. The distances share units, so Rf has none.

  • Answer: 0.80

2. Which dyes are present in the drink? Select all that apply.

  1. Dye A
  2. Dye B
  3. Dye C
  4. Dye D
Show the answer

The drink's spots have Rf = 6.4/8.0 = 0.80 and 2.0/8.0 = 0.25, matching Dye A (0.80) and Dye C (0.25).

  • Correct: Dye A: Right: Rf 0.80 matches.
  • Dye B: Dye B has Rf 0.50; the drink has no spot there.
  • Correct: Dye C: Right: Rf 0.25 matches.
  • Dye D: Dye D has Rf 0.15; the drink has no spot there.

3. Dye D has the smallest Rf. Which conclusion is supported?

  1. Dye D is held most strongly by the paper relative to the solvent.
  2. Dye D has the most mass, so it is too heavy to be carried far up the paper.
  3. Dye D interacts most strongly with the solvent, so it stays near the start line.
  4. Dye D was spotted last, so it had the least time to travel up the paper.
Show the answer

A small Rf means the component spent most of its time attached to the stationary phase. That reflects stronger attractions to the paper than to the solvent.

  • Correct: Dye D is held most strongly by the paper relative to the solvent.: Right: held most by the stationary phase.
  • Dye D has the most mass, so it is too heavy to be carried far up the paper.: Separation follows attractions to the two phases, not mass.
  • Dye D interacts most strongly with the solvent, so it stays near the start line.: Strong attraction to the solvent would carry it farther, giving a large Rf.
  • Dye D was spotted last, so it had the least time to travel up the paper.: All samples develop together for the same time.

Data table

Distilling a mixture of two liquids

A student distills a mixture of two liquids, acetone and water, and records the temperature of the vapor at the top of the apparatus as liquid collects in the receiving flask.

Vapor temperature during the distillation
Volume collected (mL)Vapor temperature (°C)
056
1057
2058
3061
3578
4097
50100
60100

4. Which liquid is collected first?

  1. Water, which boils at about 100 °C
  2. Acetone, which boils at about 56 °C
  3. Both at once, in equal amounts
  4. Neither, until 100 °C is reached
Show the answer

The vapor stays near 56–61 °C for the first 30 mL: that is acetone, the lower-boiling liquid, vaporizing first.

  • Water, which boils at about 100 °C: Water comes over later, at the 100 °C plateau.
  • Correct: Acetone, which boils at about 56 °C: Right: the first plateau is near acetone's boiling point.
  • Both at once, in equal amounts: The two plateaus show they come over one after the other.
  • Neither, until 100 °C is reached: Liquid starts collecting at 56 °C.

5. Which statement best explains why acetone distills first?

  1. Acetone has weaker intermolecular forces, so its vapor pressure is higher.
  2. Acetone molecules are heavier than water molecules, so they rise into the vapor first.
  3. Water molecules break apart at 56 °C, so they stay behind in the boiling flask.
  4. Acetone hydrogen-bonds more strongly than water, so it leaves the liquid together.
Show the answer

Acetone has dipole-dipole and dispersion forces but cannot hydrogen-bond to itself; water hydrogen-bonds extensively. Weaker attractions give acetone the higher vapor pressure and the lower boiling point.

  • Correct: Acetone has weaker intermolecular forces, so its vapor pressure is higher.: Right: weaker attractions, more volatile.
  • Acetone molecules are heavier than water molecules, so they rise into the vapor first.: Heavier molecules would not escape more easily; and attractions, not mass, decide.
  • Water molecules break apart at 56 °C, so they stay behind in the boiling flask.: Water molecules stay intact; no covalent bonds break.
  • Acetone hydrogen-bonds more strongly than water, so it leaves the liquid together.: Acetone has no H bonded to O, so it cannot hydrogen-bond with itself.

6. Why does filtration not separate the salt from salt water?

  1. Dissolved ions are far smaller than the pores of the paper.
  2. Salt ions stick to the filter paper and block it before any water passes.
  3. Filtration works on gases, not on liquids such as salt water.
  4. The salt reacts with the paper and forms a new dissolved compound.
Show the answer

A solution is mixed down to individual particles. Ions are about 10⁻¹⁰ m across, thousands of times smaller than filter pores, so they pass through with the water.

  • Correct: Dissolved ions are far smaller than the pores of the paper.: Right: dissolved particles pass through.
  • Salt ions stick to the filter paper and block it before any water passes.: The ions pass through freely; they do not clog the filter.
  • Filtration works on gases, not on liquids such as salt water.: Filtration is used for liquids all the time.
  • The salt reacts with the paper and forms a new dissolved compound.: No reaction occurs with filter paper.

7. Hexane (C₆H₁₄) and 1-propanol (C₃H₇OH) are separated on a column packed with polar silica (covered in –OH groups) using a nonpolar solvent. Which comes out of the column first, and why?

  1. Hexane, because it is barely attracted to the polar silica.
  2. 1-Propanol, because it hydrogen-bonds to the solvent and is carried along.
  3. Hexane, because it is the heavier molecule and falls through the column faster.
  4. 1-Propanol, because it has fewer carbon atoms and fits through the packing.
Show the answer

The polar silica holds 1-propanol by hydrogen bonds; nonpolar hexane interacts only weakly with it and stays in the mobile phase, so it leaves first.

  • Correct: Hexane, because it is barely attracted to the polar silica.: Right: weakest attraction to the stationary phase moves fastest.
  • 1-Propanol, because it hydrogen-bonds to the solvent and is carried along.: A nonpolar solvent cannot hydrogen-bond; 1-propanol is held by the silica instead.
  • Hexane, because it is the heavier molecule and falls through the column faster.: Separation follows attractions, not falling speed.
  • 1-Propanol, because it has fewer carbon atoms and fits through the packing.: Size is not the main factor here; polarity is.

Part 9 · Summary

Summary

Mixtures are separated by physical properties that trace back to particle forces. Filtration removes undissolved solids by size. Chromatography separates components by their attraction to a stationary phase versus a mobile phase, measured by Rf. Distillation separates liquids by vapor pressure: weaker intermolecular forces vaporize first.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections