Short on time? This page is the recap. For explanations, diagrams and worked examples, use the full study guide.
Recap
- Filtration traps undissolved solid. It cannot separate a solution, because dissolved ions and molecules are far smaller than the filter’s pores.
- To separate the parts of a solution, use a difference in their intermolecular forces.
- Chromatography (paper, thin-layer, column): a mobile phase (moving solvent) carries the mixture over a stationary phase (paper, silica gel). Components held more strongly by the stationary phase move more slowly.
- Paper (cellulose) and silica are polar: they form hydrogen bonds and dipole–dipole attractions with polar components.
- Distillation: heat the mixture, condense the vapor. The component with weaker intermolecular forces has the higher vapor pressure and collects first.
Key relationships
| Relationship | Meaning | Use it to |
|---|---|---|
| Rf = spot distance ÷ solvent-front distance | both from the origin; 0 ≤ Rf ≤ 1, no unit | compare spots on a chromatogram |
| Polar stationary phase: lower Rf ↔ more polar | the polar component is held back | rank relative polarities |
| More polar solvent → all Rf values rise | solvent competes for the surface | predict the effect of changing solvent |
| First out of a silica column = highest Rf on silica TLC | least attracted to the stationary phase | predict elution order |
| Weaker IMFs → higher vapor pressure → lower boiling point | from Topic 3.2 | predict which liquid distils first |
Assumptions
- The stationary phase is polar (paper or silica). If it is nonpolar, every polarity conclusion reverses.
- Spots are compared on the same plate with the same solvent. Rf values from different solvents cannot be compared directly.
- In distillation, the vapor temperature is read with the thermometer bulb level with the side arm.
Mistakes to avoid
- “Furthest spot = most polar.” Wrong on paper or silica: the furthest spot is the least polar.
- Measuring from the paper’s edge. Measure from the origin line, to the centre of the spot.
- Inverting the ratio. Rf can never be more than 1.
- Origin line in ink, or below the solvent. Use pencil, and keep the origin above the solvent level.
- “Boiling breaks bonds.” Boiling overcomes intermolecular forces only.
- Comparing molar masses instead of IMFs. Water (18 g mol⁻¹) boils at a higher temperature than hexane (86 g mol⁻¹) because of hydrogen bonding.
- “Distillation always gives pure liquids.” Close boiling points need a fractionating column, and some mixtures (ethanol–water) cannot be fully separated.
Quick self-check
- A spot moves 2.8 cm while the solvent front moves 7.0 cm. What is its Rf? (0.40)
- On a silica plate, spot X has Rf 0.20 and spot Y has Rf 0.70. Which is more polar? (X: it is held more strongly by the polar silica)
- Why can you not filter the sugar out of sugar water? (The dissolved sugar molecules are far smaller than the filter’s pores, so they pass through with the water.)
- Pentane and octane are mixed and distilled. Which collects first, and why? (Pentane: fewer electrons, weaker London dispersion forces, higher vapor pressure.)
Next: practice questions.