Topic 4 of 7
Measurements and recording
Keep raw readings, temperature changes and observations interpretable.
SEC G2 Science K223 / K225 (2027)
Record measurements that another person can interpret
Use the instrument scale, a clear table and observations that preserve the sequence.
Choose apparatus for the needed measurement: a volumetric pipette delivers a fixed accurate volume, a burette delivers a variable accurately measured volume, a measuring cylinder is suitable for less exact volume work, and a balance measures mass. Read the scale at eye level to avoid parallax; use the bottom of the meniscus for a clear aqueous solution unless the instructions specify otherwise.
| Trial | Initial burette / cm3 | Final burette / cm3 | Titre / cm3 |
|---|---|---|---|
| Rough | 0.00 | 24.80 | 24.80 |
| 1 | 1.10 | 25.50 | 24.40 |
| 2 | 0.20 | 24.65 | 24.45 |
| 3 | 0.30 | 24.70 | 24.40 |
For Secondary Chemistry, the syllabus normally expects burette readings to the nearest 0.05 cm3, and sufficient titrations to obtain, for a good end-point, two titres within 0.20 cm3. Record both readings and the difference. Follow the question if it gives a particular agreement criterion; do not include the rough titre in a precise mean.
Worked example
Select a justified mean titre
Use the precise titrations in the table; their maximum separation is 0.05 cm3.
- Trials 1, 2 and 3 form a close group. The rough trial was for locating the end-point.
- Mean = (24.40 + 24.45 + 24.40) / 3 = 24.4166... cm3.
- Record a suitable rounded mean of 24.42 cm3; retain the unrounded value through subsequent calculations where useful.
24.42 cm3, with the selected trials clearly identified.
Separate raw observations from interpretation. "A white precipitate forms after three drops and dissolves in excess" preserves information that "positive test" loses. Record colour, physical state, changes on standing or warming, and any confirmed gas test. Put units in table headings; do not attach a unit to a pure ratio.