5.1.2Tools
Measuring Variables in Biology — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 3 of 10
Part 1 of 5Choosing Apparatus
Mass
- A balance measures the mass of tissue or organisms in grams, and the resolution should suit the size of the sample being weighed.
Volume of liquids
- Measuring cylinders, pipettes and burettes measure liquid volumes in cm³ or dm³,
- and a burette or pipette is more precise than a beaker.
Volume of gases
- A gas syringe measures a gas directly, while an inverted measuring cylinder in water collects gas by displacement.
Time
- A stopwatch measures time in seconds or minutes, and human reaction time limits the real accuracy to a fraction of a second.
Temperature
- Thermometers or temperature probes measure in degrees Celsius, and a water bath keeps temperature steady.
Part 2 of 5Other Common Measurements
Other Common Measurements
Length
- A ruler measures to the nearest millimetre, while an eyepiece graticule calibrated against a stage micrometer measures microscopic specimens.
pH
- A pH meter or probe gives a numerical value, while indicators and paper give only an approximate result.
Light and colour
- A light meter measures light intensity, and a colorimeter measures how much light a coloured solution absorbs.
Counting
- Population sizes are estimated by sampling with quadrats, since a complete count of a large population is rarely possible.
Units
- Always record the unit with each measurement and use SI-based units, such as g, cm³, s and °C.
| Variable | Instrument | Unit |
|---|---|---|
| Mass | Balance | g |
| Liquid volume | Measuring cylinder | cm³ |
| Time | Stopwatch | s |
| Temperature | Thermometer | °C |
Part 3 of 5Resolution and Uncertainty
1
The resolution of an instrument is the smallest change it can show, such as 1 cm³ on a measuring cylinder scale.
2
For an analogue scale the uncertainty is taken as plus or minus half of the smallest division.
3
For a digital reading the uncertainty is taken as plus or minus the smallest digit displayed.
4
Percentage uncertainty = absolute uncertainty ÷ measured value × 100, so a small reading has a larger percentage uncertainty.
5
Choose an instrument whose uncertainty is small compared with the change you expect to measure.
Part 4 of 5Worked Example
Question
- A 50 cm³ measuring cylinder is marked in 1 cm³ divisions and reads 24 cm³.
- State the uncertainty and the percentage uncertainty.
- The smallest division is 1 cm³, so the uncertainty is half of this: ±0.5 cm³.
- The reading is therefore 24.0 ± 0.5 cm³.
- Percentage uncertainty = 0.5 ÷ 24.0 × 100 = 2.08%.
Answer The measurement is 24.0 ± 0.5 cm³, a percentage uncertainty of about 2.1%.
Part 5 of 5Exam Checklist & Tips
5.1.2 – Key Points for the Exam
- Resolution The smallest change in a quantity that an instrument can detect.
- Uncertainty The range within which the true value is expected to lie.
- Graduated Marked with a scale to allow measurement.
- Calibration Checking or adjusting an instrument against a known standard.
Common Mistakes
- Quoting a reading to more decimal places than the instrument allows.
- Leaving out units from the data table.
- Taking the uncertainty of a digital meter to be half the last digit.
- Using a beaker's markings to measure volumes that need precision.
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