Revise Resolution, Precision & Significant Figures for Biology 3 (IAL) WBI13 (AS Level) — revision notes and instant AI marking. Free to start.
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Edexcel IAL Biology • Unit 3: Practical Skills in Biology I
Resolution, Precision & Significant Figures
📏 Big idea: The instrument you choose decides how many digits you are allowed to write down. Recording 24.63 cm³ from a cylinder marked every 1 cm³ is not extra precision — it is a claim the apparatus cannot support, and it loses marks.
Summary — What This Topic Covers
What resolution means and how to read it off an instrument
Why resolution sets the uncertainty of a single reading
How many significant figures to record — and why more is not better
Carrying significant figures correctly through a calculation
Choosing apparatus that makes the percentage uncertainty small
1. Resolution
Key Term
Resolution is the smallest change an instrument can detect — the size of one division on its scale, or the last digit on a digital display.
A ruler marked in mm has a resolution of 1 mm
A balance reading to 0.01 g has a resolution of 0.01 g
A measuring cylinder marked every 2 cm³ has a resolution of 2 cm³
A stopwatch reading to 0.01 s has a resolution of 0.01 s
Higher resolution means finer detail — but resolution alone does not make a measurement accurate. A balance reading to 0.001 g with a zero error is high-resolution and wrong.
Analogy
Resolution is the number of pixels in a photograph. More pixels show finer detail — but if the lens is pointed at the wrong thing, a sharper picture of the wrong thing is all you get.
2. From Resolution to Uncertainty
The rule
For a single reading, uncertainty = ± half the resolution. For a measurement needing two readings — a burette, or a thermometer read at the start and the end — uncertainty = ± the whole resolution, because you take the uncertainty twice.
ruler, 1 mm divisions, single length → ± 0.5 mm
balance, 0.01 g, mass by difference → ± 0.02 g (two weighings)
burette, 0.05 cm³, start and end reading → ± 0.10 cm³
thermometer, 1 °C, temperature rise → ± 2 °C (two readings)
Common mistake
Using ± half a division for a temperature rise or a burette titre. Both are differences between two readings, so the uncertainty doubles.
3. Significant Figures
The number of significant figures you write is a claim about how well you measured. Write only what the instrument justifies.
The rule
Record raw data to the resolution of the instrument — no more, no fewer. A cylinder marked every 1 cm³ gives 24 cm³ or, if you estimate between marks, 24.5 cm³ — never 24.63 cm³.
0.0250 g → 3 s.f. leading zeros never count; the trailing zero does
1500 mm → ambiguous — write 1.50 × 10³ mm to mean 3 s.f.
20.0 °C → 3 s.f. the .0 says you measured to a tenth
20 °C → 2 s.f. a different claim entirely
What examiners check
Every value in a column should carry the same number of decimal places, set by the instrument. A column reading 21, 22.5, 20.25 tells the examiner you were not thinking about resolution.
4. Significant Figures in Calculations
The rule
A calculated answer carries the same number of significant figures as the least precise measurement that went into it. Round once, at the end — never at each step.
volume = 24.5 cm³ (3 s.f.)
time = 30 s (2 s.f.)
rate = 24.5 ÷ 30 = 0.81666…
= 0.82 cm³ s⁻¹ ← 2 s.f., set by the time
Easy marks
When a question says "give your answer to an appropriate number of significant figures", it is telling you a mark is available for exactly this. Find the least precise input and match it.
5. Choosing Apparatus That Reduces Uncertainty
You are often asked to improve a method. The reliable answer is to reduce the percentage uncertainty, and there are only two ways to do it.
Use finer resolution — a 0.1 cm³ syringe instead of a 1 cm³ cylinder cuts the uncertainty tenfold
Measure a larger quantity — the same ±0.5 cm³ is 10 % of 5 cm³ but only 1 % of 50 cm³
25 cm³ in a 1 cm³ cylinder (0.5 ÷ 25) × 100 = 2.0 %
5 cm³ in a 1 cm³ cylinder (0.5 ÷ 5) × 100 = 10.0 % ← same kit, worse method
5 cm³ in a 0.1 cm³ syringe (0.05 ÷ 5) × 100 = 1.0 % ← the fix
What mark schemes look for
Name the apparatus and its resolution. "Use a more accurate cylinder" earns nothing; "use a 10 cm³ graduated pipette reading to 0.05 cm³" earns the mark.
Practice Questions
Practice Question 1
A thermometer has 1 °C divisions. A student records the temperature rise of a water bath as 18 °C. State the uncertainty in this value and explain your answer.
Practice Question 2
A student measures a leaf as 6.4 cm using a ruler with millimetre divisions, and writes the area as 20.48 cm². Comment on the number of significant figures used.
Practice Question 3
Explain why measuring 4 cm³ of solution in a 100 cm³ measuring cylinder is poor technique, and suggest a better method.
Practice Question 4
Define resolution and explain why an instrument with high resolution can still give inaccurate results.
What to Memorise
Resolution = one divisionSingle reading → ± half a divisionTwo readings → ± whole divisionRecord raw data to the instrument's resolutionAnswer = least precise inputRound once, at the end% uncertainty = (unc ÷ value) × 100Small quantity in big apparatus = big % uncertainty
Concepts Checklist
Exam Tips
What mark schemes look for
Name the apparatus and its resolution when suggesting an improvement. Vague comparatives — "more accurate", "better" — score nothing on their own.
The trap
Temperature rises and burette titres are differences between two readings. The uncertainty doubles; it does not halve.
Easy marks
"Give your answer to an appropriate number of significant figures" is a mark for matching the least precise input. Find it, match it, move on.
Worth remembering
Percentage uncertainty is the number that lets you argue one method is better than another. Whenever a question asks you to improve a method, reach for it.