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Topic 12 · 12.1–12.4

Experimental techniques

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Name apparatus for measuring time, temperature, mass and volume, and suggest advantages and disadvantages of methods and apparatus.
  • Use the terms solvent, solute, solution, saturated solution, residue and filtrate.
  • Describe an acid–base titration and how to find its end-point.
  • Describe paper chromatography and interpret chromatograms; use a locating agent and calculate Rf EXTENDED.
  • Describe and choose separation methods, and assess purity from melting and boiling points.

📚The chemistry

Apparatus

Seven pieces of measuring apparatus: a burette for a variable volume, a volumetric pipette for one fixed volume, a measuring cylinder for an approximate volume, a gas syringe for gas volume, a balance for mass, a thermometer for temperature and a stop-watch for time.
Choose by precision: a volumetric pipette delivers one volume very accurately, a burette measures any volume to 0.05 cm³, a measuring cylinder is quicker but less precise.

Solutions vocabulary

solvent: the substance that dissolves a solute
solute: the substance that is dissolved
solution: one or more solutes dissolved in a solvent
saturated solution: contains the maximum concentration of solute at a specified temperature
residue: the substance that remains after evaporation, distillation, filtration or a similar process
filtrate: the liquid or solution that has passed through a filter

Acid–base titration

A burette of acid above a conical flask containing alkali measured with a volumetric pipette, plus indicator, standing on a white tile. The tap is opened slowly and the flask swirled. An enlarged view of the burette scale shows the bottom of the meniscus at 23.45 cubic centimetres, read at eye level.
Burette readings go down the scale and are recorded to the nearest 0.05 cm³.
  1. Use a volumetric pipette to put a fixed volume (e.g. 25.0 cm³) of alkali in a conical flask, and add a few drops of a suitable indicator (methyl orange or thymolphthalein).
  2. Fill the burette with acid and record the starting reading.
  3. Add acid while swirling; near the end, add it drop by drop.
  4. The end-point is when the indicator just changes colour permanently (methyl orange: yellow to orange). Record the final reading.
  5. Repeat until two titres are concordant (within 0.10 cm³) and use their mean.

Chromatography

Paper chromatography separates a mixture of soluble substances. Spots are put on a pencil baseline (pencil does not dissolve), the paper is stood in a solvent whose level is below the baseline (or the spots would dissolve into it), and the solvent rises up the paper carrying each substance a different distance.

A chromatogram, drawn to scale. Four spots were placed on a pencil baseline: A, B, C and X. The solvent front is 7.0 centimetres above the baseline. A has moved 4.2 centimetres, B 2.1 centimetres and C 5.6 centimetres. X has two spots, level with A and with C. Rf of A = 4.2 divided by 7.0 = 0.60. X contains A and C.
A pure substance gives one spot; X gives two, so it is a mixture — of A and C, which travelled the same distances on the same paper.

EXTENDED Colourless substances are made visible by spraying the dried chromatogram with a locating agent. Each substance has an Rf value for a given solvent:

\[ R_f = \frac{\text{distance travelled by substance}}{\text{distance travelled by solvent}} \]

Both distances are measured from the baseline. Rf has no units and is always less than 1.

Separation and purification

  • A suitable solvent dissolves one substance but not another (water dissolves salt, not sand).
  • Filtration separates an insoluble solid (residue) from a liquid (filtrate).
  • Crystallisation gets a soluble solid from its solution: heat to the crystallisation point, then cool.
  • Simple distillation separates a liquid (the solvent) from a solution: pure water from salt water.
  • Fractional distillation separates miscible liquids with different boiling points, such as ethanol (78 °C) and water (100 °C). A fractionating column is fitted between the flask and the condenser; the liquid with the lower boiling point distils over first.
Simple distillation. A round-bottomed flask of the mixture is heated. A thermometer's bulb is level with the side arm. The vapour passes down a sloping condenser, cooled by water that enters at the lower end and leaves at the upper end, and the distillate collects in a conical flask.
Water enters the condenser at the bottom so that the jacket stays full. The thermometer reads the temperature of the vapour that is distilling.

Purity. A pure substance melts and boils at sharp, fixed temperatures. Impurities lower the melting point and make it melt over a range, and raise the boiling point. Comparing measured values with data identifies a substance and shows whether it is pure — which matters for food and medicines.

✏️Worked example

(a) Describe how to obtain dry salt from a mixture of sand and salt. [4] (b) Use the chromatogram above: is X pure? Which substances does it contain? [2] (c) EXTENDED Calculate the Rf value of C. [1] (d) Why is the baseline drawn in pencil and not in ink? [1]

(a) Add water and stir (salt dissolves, sand does not); filter — the sand is the residue and salt solution the filtrate; heat the filtrate to the crystallisation point and leave to crystallise (or evaporate to dryness); dry the crystals.

(b) No, it gives two spots. It contains A and C (spots at the same heights).

(c) \( R_f = 5.6 \div 7.0 = 0.80 \).

(d) Ink is a mixture of dyes that would dissolve and separate too; pencil (graphite) is insoluble.

Check it. Rf must be between 0 and 1: C travelled less far than the solvent, 5.6 cm < 7.0 cm ✓.
Measuring from the bottom of the paper. Both distances start at the baseline. Measuring the solvent from the paper’s edge gives 8.0 cm and a wrong Rf.

📝Practise

In the style of the multiple-choice, theory and practical papers. EXTENDED marks Supplement content.

1. (Multiple choice.) Which apparatus measures exactly 25.0 cm³ of a solution into a flask? A: beaker. B: measuring cylinder. C: volumetric pipette. D: gas syringe.
C. A volumetric pipette delivers one fixed volume accurately.
2. (Theory.) Muddy water is filtered. Name the residue and the filtrate. [2]
Residue: the mud (insoluble solid left in the filter paper). Filtrate: the water that passes through.
3. (Theory.) Suggest how to separate ethanol (boiling point 78 °C) from water, and explain why the method works. [3]
Fractional distillation. The liquids have different boiling points: ethanol, with the lower boiling point, evaporates first, reaches the top of the column and condenses, while water condenses in the column and runs back.
4. (Practical.) In a titration, why is the conical flask placed on a white tile, and why is the acid added drop by drop near the end? [2]
The white tile makes the colour change of the indicator easy to see. Adding drop by drop near the end means the end-point is not overshot, so the titre is accurate.
5. (Theory.) Pure aspirin melts at 136 °C. A sample melts between 128 °C and 133 °C. What does this show? [2]
The sample is impure: it melts below the melting point of pure aspirin, and over a range rather than at a sharp temperature.
6. (Theory.) EXTENDED On a chromatogram the solvent moved 6.0 cm from the baseline and a spot moved 3.3 cm. Calculate Rf. [1]
\( 3.3 \div 6.0 = 0.55 \).
7. (Theory.) EXTENDED A mixture of colourless amino acids is separated by chromatography. How are the spots made visible? [1]
The dried chromatogram is sprayed with a locating agent, which makes the spots coloured.
8. (Practical.) Suggest a method to obtain (a) pure water from seawater, (b) copper(II) sulfate crystals from its solution. [2]
(a) Simple distillation. (b) Crystallisation: heat to the crystallisation point, cool, filter and dry.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • Royal Society of Chemistry — the titration screen experiment, a free online simulation
  • Royal Society of Chemistry — chromatography of food colourings, a class practical