Resolution, Precision & Significant Figures

Revise Resolution, Precision & Significant Figures for Biology 3 (IAL) WBI13 (AS Level) — revision notes, 2 practice questions and instant AI marking.

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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

improve

Use finer resolution

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Also in the full note

Practice Questions
What to Memorise
Concepts Checklist
Exam Tips
5. Choosing Apparatus That Reduces Uncertainty

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