Separation of Solutions and Mixtures
After a long Fourth of July weekend, a pool tech tests water that has sand, leaves, dissolved salt, and a little algae in it. How do you get each part out? You can't pick it all out by hand, and you shouldn't need to. Every method in this page works the same way: find a physical property where the components differ, then use it. At the end, you'll see why one trick can't make 100% pure ethanol from a drink-strength ethanol–water mixture.
Use a difference in properties
A mixture keeps the identity of its components. No bonds break when you mix sand and salt, so you can separate them using physical properties (not chemical reactions).
| Property that differs | Method |
|---|---|
| Particle size (solid vs. liquid or dissolved) | Filtration |
| Boiling point | Distillation |
| Solubility in a solvent | Dissolving, then filtering or evaporating |
| Attraction to a stationary phase vs. a solvent | Chromatography |
The key idea is particle-level. Substances with stronger intermolecular forces (hydrogen bonding, dipole–dipole, London dispersion) hold on to each other and to polar surfaces more tightly. That shows up as a higher boiling point, or as slower movement in chromatography.
Name a physical property that differs between the components of each mixture and that can be used to separate them: (a) ethanol and water, (b) sand and salt dissolved in…
Name a physical property that differs between the components of each mixture and that can be used to separate them: (a) ethanol and water, (b) sand and salt dissolved in water, (c) the colored pigments in a marker ink.
Distillation
Distillation separates liquids with different boiling points. You heat the mixture; the component with the lower boiling point has the higher vapor pressure at a given temperature, so it vaporizes first. The vapor travels into a condenser, cools, and drips into a collection flask as a liquid.
Why do boiling points differ? A liquid boils when its particles have enough energy to overcome the attractions to their neighbors. Stronger intermolecular forces mean more energy needed, so a higher boiling point.
A mixture of hexane ($\text{C}6\text{H}{14}$, nonpolar) and ethanol ($\text{C}2\text{H}5\text{OH}$, hydrogen bonding) is separated by simple distillation. Which…
A mixture of hexane ($\text{C}_6\text{H}_{14}$, nonpolar) and ethanol ($\text{C}_2\text{H}_5\text{OH}$, hydrogen bonding) is separated by simple distillation. Which substance distills first? Explain in terms of intermolecular forces and boiling point.
A mixture contains water (boiling point 100 °C), ethanol (78 °C), and dissolved sodium chloride. (a) Describe a sequence of separation steps to isolate pure ethanol and…
A mixture contains water (boiling point 100 °C), ethanol (78 °C), and dissolved sodium chloride. (a) Describe a sequence of separation steps to isolate pure ethanol and recover the salt. (b) Explain, in terms of particle-level forces, why ethanol boils at a lower temperature than water and why the salt does not distill. (c) Explain why a single simple distillation of an ethanol–water mixture does not give 100% pure ethanol.
Paper chromatography and R_f
Chromatography separates components by how strongly each is attracted to two phases:
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The stationary phase stays put (paper, which is polar).
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The mobile phase moves (the solvent), carrying the components along as it rises.
A component that sticks more to the paper moves less. A component that is more attracted to the solvent moves farther. Each component travels at its own pace, so the mixture spreads into separate spots.
To compare spots, use the retention factor:
Why a ratio? The same compound always gives the same $R_f$ in the same paper and solvent, no matter how long the paper is.
In paper chromatography, what are the stationary phase and the mobile phase? Which component of a mixture moves farther up the paper: the one more attracted to the paper…
In paper chromatography, what are the stationary phase and the mobile phase? Which component of a mixture moves farther up the paper: the one more attracted to the paper or the one more attracted to the solvent?
In a paper chromatography experiment, a dye spot traveled 3.0 cm while the solvent front traveled 6.0 cm from the starting line. Calculate the retention factor ($R_f$)…
In a paper chromatography experiment, a dye spot traveled 3.0 cm while the solvent front traveled 6.0 cm from the starting line. Calculate the retention factor ($R_f$) of the dye.
Emma runs a paper chromatogram of a green ink. The solvent front moves 8.0 cm. The yellow component moves 6.4 cm and the blue component moves 2.0 cm. (a) Calculate the…
Emma runs a paper chromatogram of a green ink. The solvent front moves 8.0 cm. The yellow component moves 6.4 cm and the blue component moves 2.0 cm. (a) Calculate the $R_f$ value of each component. (b) Which component has stronger attractions to the polar paper? Explain.
Column chromatography and TLC
The same logic works in other setups.
Column chromatography: a tube is packed with a polar solid such as silica gel (the stationary phase). You pour the mixture on top and wash it down with a solvent (the mobile phase). Components leave the bottom at different times.
Thin-layer chromatography (TLC): a thin coating of silica on a plate acts like the paper. You calculate $R_f$ exactly as before.
With polar silica and a nonpolar solvent:
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A nonpolar compound has weak attractions to silica and dissolves well in the solvent. It moves fast, has a high $R_f$, and exits the column first.
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A polar compound hydrogen bonds or forms dipole attractions with silica. It moves slowly, has a low $R_f$, and exits last.
If you switch to a more polar solvent, it competes with the silica for the polar compounds and carries them along. All $R_f$ values increase, and the polar compounds speed up the most, so the spots bunch together and separate less.
In column chromatography, a polar silica gel column is used to separate a nonpolar compound A from a polar compound B using a nonpolar solvent. Which compound exits the…
In column chromatography, a polar silica gel column is used to separate a nonpolar compound A from a polar compound B using a nonpolar solvent. Which compound exits the column first? Explain, and describe what would happen if the solvent were changed to a much more polar one.
A student separates three compounds on a polar silica TLC plate using a nonpolar solvent. The solvent front travels 7.5 cm. Compound P travels 6.0 cm, compound Q travels…
A student separates three compounds on a polar silica TLC plate using a nonpolar solvent. The solvent front travels 7.5 cm. Compound P travels 6.0 cm, compound Q travels 3.0 cm, and compound R travels 0.75 cm. (a) Calculate each $R_f$. (b) Rank the compounds from least polar to most polar and justify using intermolecular forces. (c) Predict how the $R_f$ values would change if a more polar solvent were used, and explain.
Combining methods
Real mixtures often need more than one step. Think about the sequence by asking: what differs most between the components?
Back to the opening: the pool water gets sand out by filtration, leaves by skimming, and dissolved salt would need evaporation or distillation. Each step uses a different property.