Practical Skills and Experimental Techniques
Practical skills and experimental techniques tests the accuracy of measurements, titration technique, separation methods, gas tests, data processing and evaluation of errors.
Core rules
- Read a burette to ±0.05 cm3; a 25 cm3 pipette and volumetric flask are more precise than a measuring cylinder.
- Rinse the burette with the titrant and the pipette with the solution it delivers; rinse the conical flask with distilled water. Record initial and final readings and use concordant titres within 0.10 cm3.
- Percentage uncertainty = (uncertainty/measurement) × 100. A burette reading has two readings per titre, so the uncertainty is ±0.10 cm3.
- Separation: filtration removes insoluble solids; crystallisation recovers a soluble salt; simple distillation separates a solvent from a non-volatile solute; fractional distillation separates miscible liquids with different boiling points; a separating funnel separates immiscible liquids.
- Gas tests: H2 gives a squeaky pop; O2 relights a glowing splint; CO2 turns limewater milky; Cl2 bleaches damp litmus; NH3 turns damp red litmus blue.
- Evaluation should distinguish random from systematic error and suggest a specific improvement, not 'be more careful'.
Worked example
A titre is 24.30 cm3 from a burette with an uncertainty of ±0.05 cm3 per reading. Two readings give ±0.10 cm3. Percentage uncertainty = (0.10/24.30) × 100 = 0.41%. If the rough titre is 24.60 cm3, it is excluded from the mean because it was not used to find the end-point. The mean uses only concordant accurate titres.
Traps
Including the rough titre in the mean; reading the top of the meniscus; using the final burette reading as the titre without subtracting the initial reading; putting the measurement rather than the uncertainty in the numerator; describing 'human error' without saying what was done; failing to control variables in a comparison.