At A-Level, practical marks hide in the words. Accurate and precise are not synonyms; repeatable and reproducible name two different tests of your data; and calling a systematic error “random” costs the mark even when everything else is right. This module drills the vocabulary, then puts it to work on a results table and a rate graph.
These skills are assessed across the GCSE required practicals — the same language carries into A-Level.
The assessed vocabulary
Eight terms, checked word by word in written papers and practical write-ups. Flip each card, then be honest: Got it removes the card, Again sends it to the back of the pile — the deck isn't cleared until every definition comes back without hesitation. (Keyboard: Space flips, G and A answer.)
A result close to the true value is…
accurate
Repeat readings that cluster closely together are…
precise
Same person, same method, same equipment → similar results. The measurement is…
repeatable
A different person — or different equipment — gets similar results. The measurement is…
reproducible
A measurement that actually answers the question being investigated is…
valid
The interval within which the true value can be expected to lie is the…
uncertainty
An error that scatters readings either side of the true value — reduced by repeating and averaging — is a…
random error
An error that shifts every reading the same way — and is NOT reduced by averaging — is a…
systematic error
Random or systematic?
The test is simple: would repeating and averaging help? If the error scatters both ways, yes — it's random. If every reading is shifted the same way, averaging just gives you a precisely wrong answer — it's systematic, and the fix is better equipment or a better method.
Drag each situation into a box — or tap it to step through the boxes. Then press Check.
Variables
Three words that structure every investigation: you change the independent variable, you measure the dependent variable, and you keep everything else the same — the control variables — so the test is fair.
In “how does acid concentration affect rate”, the acid concentration is the… Tick (✓) one box.
Fix the results table
A student investigates how temperature affects the time for a sodium thiosulfate solution to turn cloudy, and records this table. It contains five errors of good table practice.
| Temperature | Time, run 1 | Time, run 2 | Time, run 3 | Mean time |
|---|---|---|---|---|
| 20 °C | 98.0 | 96 | 97.55 | 97.2 |
| 30 °C | 71.0 | 70.5 | 71.5 | 71.0 |
| 40 °C | 49.5 | 21.0 | 50.5 | 40.3 |
| 50 °C | 35.0 | 34.5 | 35.5 | 35.0 |
| 60 °C | 24.0 | 24.5 | 23.5 | 24.000 |
Which five of these criticisms of the table are valid? Tick (✓) five boxes, then press Check.
Reading a tangent
On a curved graph the rate is changing constantly, so “the rate at 60 s” means the gradient of the tangent drawn at 60 s — the straight line that just touches the curve there. The tangent has been drawn for you; the skill is reading its gradient from the two labelled intercepts.
Reading a rate from a graph started at GCSE — see C6 — Calculating rates of reaction.
Use the tangent to find the rate at 60 s.
Worked steps
- Rate = gradient of the tangent = rise ÷ run, using the labelled intercepts.
- Rise = 1.80 mol/dm3; run = 150 s.
- 1.80 ÷ 150 = 0.012 mol dm−3 s−1
How did you do?
Work through the questions above and your score appears here.
One module left: the first genuinely new chemistry of Year 12 — shells splitting into sub-shells.
That's fixable before September — I take on a small number of A-Level students each year →