Separation of Solutions and Mixtures: Study Guide (Chemistry 3.9)

Why a filter cannot separate a solution

Filtration works like a sieve. Filter paper has tiny holes, or pores, typically a few micrometres to about 25 micrometres across. A solid that has not dissolved, such as sand in water, is made of grains far larger than the pores, so it stays on the paper while the liquid runs through.

A solution is different. In Topic 3.7 you saw that a solution is a homogeneous mixture: the solute is spread out as single ions or molecules, each well under a nanometre across. That is thousands of times smaller than the pores. When you pour salt water through filter paper, the ions go through with the water. The filtrate is still salt water.

So the parts of a solution have to be separated in another way. Every method in this topic uses the same idea:

The components of a solution differ in the strength of their intermolecular forces. A good separation method turns that difference into a difference in where each component goes.

Two methods do this in the course: chromatography, which uses differences in attraction to a surface, and distillation, which uses differences in vapor pressure.

Chromatography: a tug-of-war

Every kind of chromatography has two phases:

• the stationary phase, which stays still: the paper, a thin coating on a plate, or a packed column of solid;
• the mobile phase, a solvent that moves through or over the stationary phase and carries the mixture with it.

Each molecule of a component is pulled two ways. Its attractions to the mobile phase carry it along. Its attractions to the stationary phase hold it back. A molecule spends some of its time stuck to the surface and some of its time moving with the solvent. A component that is attracted more strongly to the stationary phase spends more time stuck, so it moves more slowly. Different components therefore spread out into separate bands or spots.

Type	Stationary phase	Mobile phase	What you see
Paper	Cellulose fibres in the paper, which are polar (many –OH groups)	A solvent that rises up the paper	Spots at different heights
Thin-layer (TLC)	A thin layer of a polar solid, such as silica gel, on a plate	A solvent that rises up the plate	Spots at different heights
Column	A tube packed with a polar solid, such as silica gel	A solvent poured through from the top	Bands that leave the bottom one after another

The surface of silica gel is covered in Si–O–H groups, and cellulose is covered in C–O–H groups. Both surfaces can form hydrogen bonds and dipole–dipole attractions. So on paper or silica, the more polar a component is, the more strongly it is held, and the more slowly it moves. A nonpolar component has only weak London dispersion attractions to the polar surface, so it travels close behind the solvent.

Reading a chromatogram: the Rf value

The pattern of spots is called a chromatogram. To compare spots, measure distances from the starting line (the origin) and work out the retention factor:

Rf = distance moved by the centre of the spot ÷ distance moved by the solvent front

Both distances are measured from the origin. An Rf value has no unit and always lies between 0 and 1. A spot that does not move from the origin has Rf = 0. A spot that moves with the solvent front has Rf = 1.

Changing the mobile phase

The mobile phase competes with the stationary phase for each component. If you switch to a more polar solvent, the solvent itself is attracted more strongly to the polar surface and attracts polar components more strongly. Every spot then moves further, and the Rf values go up. The order of the spots usually stays the same, because the order is set by how polar each component is compared with the others.

Some labs use the reverse arrangement: a nonpolar stationary phase with a polar solvent. Then the order flips, and the most polar component moves fastest. Always check what the stationary phase is before you reason about polarity.

Worked example 1: ranking polarity from a chromatogram

Question. A student spots a mixture of three dyes on a silica TLC plate and develops it with a solvent that is less polar than silica. Figure 1 shows the result. The data are invented for this example. (a) Calculate the Rf value of each dye. (b) Rank the dyes from most polar to least polar. (c) Explain the ranking in terms of intermolecular interactions.

(a) The solvent front moved 8.0 cm from the origin.

Dye	Distance moved (cm)	Rf
P	6.4	6.4 ÷ 8.0 = 0.80
Q	3.6	3.6 ÷ 8.0 = 0.45
R	1.2	1.2 ÷ 8.0 = 0.15

(b) Most polar: R, then Q, then P (least polar).

(c) Silica’s surface is covered in polar Si–O–H groups. A polar dye forms strong dipole–dipole attractions or hydrogen bonds with these groups, so it spends more of its time held on the surface and moves only a short distance. Dye R moved least, so it has the strongest attractions to the stationary phase and is the most polar. Dye P moved furthest because its attractions to the surface are weakest, so it is the least polar.

Check. All three Rf values lie between 0 and 1. A common slip is to measure R from the bottom edge of the plate instead of from the origin. If the origin is 1.0 cm above the edge, that gives (1.2 + 1.0) ÷ (8.0 + 1.0) = 0.24, which is wrong.

Column chromatography: collecting the parts

A TLC plate tells you what is in a mixture. A column lets you collect the parts. The mixture is added to the top of a column of silica, and solvent is poured through. The component least attracted to the silica travels fastest and leaves the bottom first. The liquid that drips out is collected in a series of tubes, called fractions. Each fraction then holds mostly one component, and evaporating the solvent recovers it.

The reasoning is the same as on a plate: the first component to leave the column is the one with the highest Rf on a silica TLC plate run with the same solvent.

Distillation: using vapor pressure

In Topic 3.2 you saw that a liquid with weaker intermolecular forces has a higher vapor pressure and a lower boiling point, because its molecules escape into the gas more easily. Distillation uses this.

When you heat a mixture of two liquids, the vapor above it is richer in the component with the higher vapor pressure. That vapor is cooled in a condenser, turns back into liquid and drips into a separate flask. The first liquid collected, the distillate, is therefore rich in the component with the weaker intermolecular forces.

Three practical details are worth knowing for a lab sketch:

• The thermometer bulb sits level with the side arm, so it reads the temperature of the vapor that is leaving, not of the liquid.
• Cooling water enters the condenser at the lower end so the jacket stays full.
• If the boiling points are close, a fractionating column packed with glass beads or rings is fitted between the flask and the still head. Vapor condenses and boils again many times on its way up, which gives a much better separation.

Worked example 2: distilling hexane and heptane

Question. A student distils a mixture of 50 mL of hexane, C₆H₁₄, and 50 mL of heptane, C₇H₁₆, using a fractionating column. She records the vapor temperature as the distillate collects (Figure 3; the data are idealized for this example). (a) Which liquid collects first? (b) Explain your answer in terms of intermolecular forces.

(a) Hexane collects first: the first flat section is at about 69 °C, the boiling point of hexane. Heptane boils at about 98 °C.

(b) Both molecules are nonpolar, so the only intermolecular forces in either liquid are London dispersion forces.

1. Heptane has more electrons per molecule (58) than hexane (50), and a longer chain with more surface for contact.
2. A larger, more polarizable electron cloud gives stronger dispersion forces between heptane molecules.
3. Weaker forces between hexane molecules mean they escape into the vapor more easily: hexane has the higher vapor pressure at any temperature.
4. So the vapor, and the distillate, is richest in hexane at the start. Only after most of the hexane has gone does the temperature rise to the boiling point of heptane.

Interpretation. The steep rise between about 45 and 55 mL is where the distillate is a mixture of both. A real curve is less sharp than this one, and without a fractionating column the two plateaus blur together.

Worked example 3: choosing a method

Question. For each mixture, choose a separation method and justify it. (a) Sand stirred into salt water. (b) The colored dyes in a drop of ink. (c) Ethanol dissolved in water.

(a) Two steps. Filtration first: sand has not dissolved, so its grains are trapped by the paper. The filtrate is a salt solution, which filtering cannot separate. Then evaporate the water: water has a far higher vapor pressure than the ionic solid, so the salt is left behind.

(b) Paper or thin-layer chromatography. The dyes are dissolved and present in tiny amounts. Each dye has a different balance of attractions to the polar stationary phase and to the solvent, so each moves a different distance.

(c) Distillation, with a fractionating column. Both liquids form hydrogen bonds, but ethanol’s molecules have only one O–H group and a nonpolar part, so ethanol’s intermolecular forces are weaker overall. Ethanol has the higher vapor pressure and boils at about 78 °C, so the distillate is rich in ethanol.

Background, beyond the course. Ethanol and water cannot be fully separated this way. At about 96 % ethanol by mass, the mixture boils at a constant temperature with no further change in composition.

Common misconceptions

• “A fine enough filter will remove dissolved salt.” Ordinary filter paper cannot: dissolved ions are thousands of times smaller than its pores.
• “The spot that travels furthest is the most polar.” On a polar stationary phase it is the reverse: the most polar component is held back most and has the lowest Rf.
• “A spot moves because it dissolves in the solvent, and that is all.” Attraction to the mobile phase is only half the story. Separation happens because components differ in how strongly the stationary phase holds them back.
• “Rf can be bigger than 1.” No spot can move further than the solvent that carries it. If you get Rf > 1, you have inverted the ratio.
• “Measure from the bottom of the paper.” Both distances are measured from the origin line, and to the centre of the spot.
• “Draw the origin line in pen.” Ink is a mixture of dyes and would run with the solvent. Use pencil, and keep the origin above the solvent level in the tank so the sample does not dissolve into the pool of solvent.
• “Boiling a liquid breaks its covalent bonds.” Boiling overcomes only the intermolecular forces. The molecules in the vapor are the same molecules as in the liquid.
• “The liquid with the lower molar mass always distils first.” Compare intermolecular forces, not mass. Water (18 g mol⁻¹) boils at a higher temperature than hexane (86 g mol⁻¹) because of hydrogen bonding.

Where this leads

You have used intermolecular forces to pull a mixture apart. Next, Topic 3.10, Solubility, uses the same forces to explain why some substances mix in the first place. Before that, it helps to look back at the particulate drawings in Topic 3.8, Representations of Solutions. Try the practice questions now, then use the revision notes and the checklist to consolidate.