Apparatus, Measurement & Uncertainty

Revise Apparatus, Measurement & Uncertainty for Biology 3 (IAL) WBI13 (AS Level) — revision notes and instant AI marking.

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Edexcel IAL Biology • Unit 3: Practical Skills in Biology I

Apparatus, Measurement & Uncertainty

⚗️ Big idea: Every mark in the measurement half of Unit 3 comes down to one question: can this apparatus support the number you wrote down? Pick the instrument to fit the quantity, read it properly, and quote the uncertainty it carries. This is the overview — the topics that follow take each piece apart.

Summary — What This Topic Covers

Choosing the right instrument for the quantity being measured
Reading scales without introducing parallax
Where uncertainty comes from, and how to quote it
The apparatus that recurs across Core Practicals 1–9
How this topic connects to the ones that follow

1. Choosing the Instrument

The instrument must be matched to the size of what you are measuring. The same apparatus can be excellent or hopeless depending on the quantity.

QUANTITY TYPICAL APPARATUS RESOLUTION ────────────────────────────────────────────────────────────────── volume, 1-10 cm³ graduated pipette/syringe 0.05-0.1 cm³ volume, 10-100 cm³ measuring cylinder 1 cm³ volume of gas gas syringe 0.5-1 cm³ mass electronic balance 0.01 g temperature thermometer 0.5-1 °C time stopwatch 0.01 s length ruler 1 mm colour intensity colorimeter arbitrary units pH pH meter (not paper) 0.1

The principle

Choose apparatus whose resolution makes the percentage uncertainty small for the quantity you are measuring. A 100 cm³ cylinder is the wrong tool for 4 cm³ — not because it is inaccurate, but because ±0.5 cm³ is 12.5 % of 4.

Worth remembering

pH paper gives a colour band, not a number, and its resolution is roughly one whole pH unit. Where a question asks for accurate pH, the answer is a pH meter.

2. Reading a Scale Properly

Key Term

Parallax error is the shift in apparent reading caused by viewing a scale from an angle rather than straight on. It is a systematic error if you always read from the same wrong angle.

Read at eye level, straight on to the scale
For a liquid in glass, read the bottom of the meniscus — consistently
On a digital display, wait for the reading to stabilise before recording
Zero or tare a balance before every set of measurements

reading from above → value appears LOWER than it is reading from below → value appears HIGHER than it is reading at eye level → correct always the same wrong angle → systematic error a different angle each time → random error

Common mistake

Describing parallax as random error. If you habitually read from above, every reading is shifted the same way — that is systematic, and repeats will not help.

3. Quoting Uncertainty

The rule

Single reading: ± half the resolution.
Difference of two readings: ± the whole resolution.
Percentage uncertainty = (uncertainty ÷ measured value) × 100.