Practical Techniques: Revision Notes

Condensed for revision. For the full explanation, use the Practical Techniques study guide, then test yourself with the practice questions.

Tier note: items marked (0620 Extended, 5070 required) are in the Supplement column of the 0620 syllabus: 12.3 outcomes 3–4 (colourless substances with a locating agent, and the Rf equation). Everything else is 0620 Core. 5070 has no tiers, so O Level candidates need all of it.

Syllabus: Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028, subtopics 12.1 Experimental design, 12.2 Acid–base titrations, 12.3 Chromatography and 12.4 Separation and purification.

Apparatus (12.1)

Measuring	Apparatus
Time	stop-watch
Temperature	thermometer
Mass	balance
Volume of liquid	burette, volumetric pipette, measuring cylinder
Volume of gas	gas syringe (or an upturned measuring cylinder over water)

Accuracy of volume apparatus: a volumetric pipette delivers one fixed volume very accurately; a burette delivers variable volumes accurately; a measuring cylinder is quicker but less accurate.

When asked for advantages and disadvantages of a method or apparatus, think about: accuracy of the reading, speed, safety, whether gas escapes or is lost, and whether the method gives a pure product.

Solution vocabulary (12.1)

Term	Meaning
Solvent	a substance that dissolves a solute
Solute	a substance that is dissolved in a solvent
Solution	a mixture of one or more solutes dissolved in a solvent
Saturated solution	a solution containing the maximum concentration of a solute dissolved in the solvent at a specified temperature
Residue	a substance that remains after evaporation, distillation, filtration or any similar process
Filtrate	a liquid or solution that has passed through a filter

Acid–base titration (12.2)

1. Use a volumetric pipette (with a pipette filler) to transfer a fixed volume, such as 25.0 cm³, of one solution into a conical flask.
2. Add a few drops of a suitable indicator.
3. Fill a burette with the other solution and record the starting reading.
4. Add the solution from the burette, swirling the flask, and add it drop by drop near the end.
5. Stop at the end-point: the moment the indicator changes colour permanently. Record the final reading. The titre is final reading minus initial reading.
6. Repeat until you have concordant titres (close agreement, for example within 0.10 cm³).

Indicator colours (from 7.1):

Indicator	In acid	In alkali
Litmus	red	blue
Methyl orange	red	yellow
Thymolphthalein	colourless	blue

Litmus is not used for titrations because its colour change is not sharp.

Paper chromatography (12.3)

Method:

1. Draw a baseline in pencil near the bottom of the paper (pencil does not dissolve and run).
2. Put small spots of the mixture and of known substances on the baseline.
3. Stand the paper in a suitable solvent, with the solvent level below the baseline (so the spots do not dissolve into the solvent in the beaker).
4. Let the solvent rise up the paper; the substances separate because they travel different distances.
5. Remove the paper and mark the solvent front before it evaporates.

Interpreting a chromatogram:

• An unknown is identified by comparison with known substances: spots at the same height (on the same chromatogram) are the same substance.
• A pure substance gives one spot; an impure substance gives two or more spots.
• A spot that stays on the baseline is insoluble in that solvent.

5070 wording covers soluble substances in general; 0620 Core describes the method for soluble coloured substances.

Colourless substances and Rf (0620 Extended, 5070 required)

• Colourless substances are made visible by spraying the dried chromatogram with a locating agent. You do not need to name a specific locating agent.

• The Rf value is:

  Rf = distance travelled by substance ÷ distance travelled by solvent

• Both distances are measured from the baseline, to the centre of the spot and to the solvent front. Rf has no units and lies between 0 and 1.

Worked example: the solvent front is 8.0 cm above the baseline and a spot is 3.2 cm above it. Rf = 3.2 ÷ 8.0 = 0.40.

Separation and purification (12.4)

Method	Separates	Key point
Suitable solvent	one soluble solid from an insoluble one	dissolve the soluble one, then filter
Filtration	an insoluble solid from a liquid	solid = residue; liquid = filtrate
Crystallisation	a dissolved solid from its solution	heat to evaporate some solvent until saturated, then cool; filter off crystals, wash with a little cold solvent and dry
Simple distillation	a solvent from a dissolved solid (for example, water from salt water)	the solvent evaporates and is condensed in a condenser
Fractional distillation	miscible liquids with different boiling points (for example, ethanol and water; petroleum)	a fractionating column; the liquid with the lower boiling point distils first

Choosing a method: look at the information given.

• Insoluble solid in a liquid → filtration.
• Want the solid from a solution → crystallisation (or evaporation).
• Want the liquid from a solution → simple distillation.
• Two liquids that mix, with different boiling points → fractional distillation.
• Several dissolved coloured substances → chromatography.

Purity from melting and boiling points:

• A pure substance has a sharp, fixed melting point and boiling point that match the data-book values. For example, pure water melts at 0 °C and boils at 100 °C at atmospheric pressure.
• An impure substance melts over a range of temperatures, lower than the pure substance, and boils at a higher temperature (often over a range).

Exam traps

• A measuring cylinder is not accurate enough for the fixed volume in a titration: use a volumetric pipette.
• The chromatography baseline is drawn in pencil, not ink, and must be above the solvent level.
• (0620 Extended, 5070 required) Rf is substance ÷ solvent, measured from the baseline, not from the bottom of the paper. An Rf above 1 means you divided the wrong way.
• In filtration, the solid left in the filter paper is the residue; do not call it the filtrate.
• Crystallisation: do not evaporate to dryness if you want crystals of a hydrated salt.
• Impurities lower the melting point and raise the boiling point.

Self-test

1. Name the apparatus used to measure (a) the volume of gas given off, (b) a fixed 25.0 cm³ volume of solution accurately.
2. Define a saturated solution.
3. What is the difference between a residue and a filtrate?
4. Give the colour of methyl orange in acid and in alkali.
5. How do you recognise the end-point of a titration?
6. Why is the baseline on a chromatogram drawn in pencil?
7. A sample of a solid melts between 118 °C and 123 °C. The pure compound melts at 125 °C. What does this show?
8. (0620 Extended, 5070 required) A spot travels 4.5 cm and the solvent travels 7.5 cm from the baseline. Calculate the Rf value.
9. (0620 Extended, 5070 required) How can the positions of colourless substances on a chromatogram be seen?

Answers:

1. (a) Gas syringe; (b) volumetric pipette.
2. A solution containing the maximum concentration of a solute dissolved in the solvent at a specified temperature.
3. The residue is the substance left behind (for example, in the filter paper); the filtrate is the liquid or solution that has passed through the filter.
4. Red in acid; yellow in alkali.
5. The indicator changes colour permanently.
6. Pencil does not dissolve in the solvent, so it does not run up the paper and interfere with the spots.
7. The sample is impure: it melts over a range, below the melting point of the pure compound.
8. Rf = 4.5 ÷ 7.5 = 0.60.
9. Spray the dried chromatogram with a locating agent.

These are original notes written for revision. Check the full syllabus wording in the official 0620 syllabus and official 5070 syllabus.