Cambridge O Level Chemistry · Syllabus 5070 · Experimental Techniques and Chemical Analysis
Solvent
What is Solvent?
A substance that dissolves a solute. In a salt solution the water is the solvent. Which component is the solvent depends on the mixture, not on the substance alone: a liquid that dissolves nothing present is not acting as a solvent there.
This definition is part of the Experimental Techniques and Chemical Analysis chapter in Cambridge O Level Chemistry.
Solvent in context
Experimental techniques and chemical analysis cover how a chemist gathers and interprets evidence: choosing the right apparatus for a measurement, running a titration to find a reacting volume, using paper chromatography to separate and identify substances, separating and purifying mixtures by filtration, crystallisation or distillation, and running the qualitative analysis tests that identify anions, cations, gases and metal ions. A titration uses a volumetric pipette to measure a fixed volume and a burette to add a variable volume until an indicator shows the end-point, giving a titre as final reading minus initial reading. Chromatography separates soluble substances by how far they travel in a solvent, measured as the retention factor, Rf.
Common mistakes with Solvent
- 6. Solute and solvent Fix The solute is dissolved; the solvent does the dissolving. In salt water the solute is salt and the solvent is water. Say which is which explicitly — a lot of definition marks are lost by writing the pair the wrong way round under time pressure.
- 8. The baseline and the solvent front Fix The baseline is the pencil line where the spots start. The solvent front is how far the solvent reached, marked as soon as the paper comes out of the tank before it evaporates. Both \(R_f\) distances are measured from the baseline, and the solvent must start below the baseline, never above it.
- 9. Spot distance and solvent distance Fix \(R_f\) puts the substance distance on top and the solvent distance underneath. Measure the substance distance to the centre of the spot. Getting a value above 1 means you divided the wrong way round or measured from the bottom edge of the paper.
- 4. “A saturated solution is one where no more will dissolve.” Why it fails Warm it and more will dissolve, so the statement describes something that changes as soon as the temperature does. Correct A solution containing the maximum concentration of a solute dissolved in the solvent at a specified temperature. Check Why do crystals form when a hot saturated solution cools? Only the full definition, with temperature in it, can answer that.
- 5. “Fill the tank so the solvent covers the spots.” Why it fails The spots dissolve straight into the solvent in the tank instead of being carried up the paper, so nothing separates. Correct The solvent level must be below the pencil baseline. Check A diagram shows the baseline \(5\ \mathrm{mm}\) above the bottom edge and the solvent \(10\ \mathrm{mm}\) deep. Two faults follow from one measurement.
- 6. “Draw the baseline in pen so it is easy to see.” Why it fails Ink dissolves in the solvent and travels up the paper, producing spots of its own and contaminating the chromatogram. Correct Pencil, which does not dissolve, so the line stays where it was drawn and can still be measured from at the end. Check Why is that a problem specifically for interpreting the result? Because extra spots would be counted as substances in the sample.
- 8. “My \(R_f\) is \(0.45\) and the data book says \(0.45\), so it is that substance.” Why it fails \(R_f\) depends on the solvent and the conditions as well as on the substance. A match between values obtained under different conditions is not evidence. Correct Run a known sample alongside the unknown, on the same paper in the same solvent, and compare the positions directly. Check Two students use the same solvent but let it run different distances. Can they compare? Their \(R_f\) values, yes; their raw distances, no.
- 9. “One spot proves the sample is pure.” Why it fails Two substances that travel the same distance in that solvent would also give one spot. The evidence supports purity; it does not prove it. Correct Give the exam interpretation — one spot means pure — and use “supports” rather than “proves”. Running the sample again in a different solvent tests the conclusion. Check The same caution applies to the sharp melting point: it supports purity, it does not settle it beyond all doubt.
- 22. “Statement 12.1.3 is about collecting and drying gases.” Why it fails Statement 12.1.3 is the six definitions — solvent, solute, solution, saturated solution, residue and filtrate. Gas collection and drying methods are not part of Topic 12 for this cycle. Correct Learn the six definitions for 12.1.3, in the official wording. Check If a revision resource lists upward delivery and drying agents under 12.1.3, it is describing an older specification.
- 27. “Use a separating funnel to separate the mixture.” Why it fails A separating funnel is not one of the five methods named in statement 12.4.1, so it is not the expected answer — and it only works at all for liquids that do not mix. Correct Choose from a suitable solvent, filtration, crystallisation, simple distillation and fractional distillation, and justify the choice from the property data. Check Ethanol and water are miscible, so no layers form; fractional distillation is the method.
Questions students ask about Solvent
How do you find the Rf value from a chromatogram, and why does it have no unit?
Rf is the distance travelled by the substance divided by the distance travelled by the solvent, both measured from the same pencil baseline. Because it is a ratio of two lengths, the units cancel and Rf has no unit, with values between 0 and 1 for an ordinary chromatogram. Rf depends on the solvent and conditions, so values are only compared between chromatograms run under the same conditions.
How should you choose a separation technique on an unfamiliar substance?
Match the technique to the property difference the question gives you: use a suitable solvent or filtration when solubility differs, crystallisation to recover a dissolved solid, simple distillation to separate a solvent from a dissolved solid or non-volatile component, and fractional distillation when two or more miscible liquids have different boiling points. A full-credit answer names the method, states the property difference that makes it work, and says what is kept.