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5.1.2Tools

Measuring Variables in Biology — IB Biology SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 3 of 10

📚 IB Biology SL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 6Physical Quantities

Mass
  • Mass Use a balance, set to zero with the container on it, and record to the same number of decimal places each time;
    • the SI unit is kg but g is usual.
Volume of liquid
  • Volume of liquid Measuring cylinders suit moderate volumes, pipettes and burettes give greater precision,
    • and the reading is taken at the bottom of the meniscus.
Volume of gas
  • Volume of gas A gas syringe is suitable for gases, provided the gas does not dissolve in water.
Time
  • Time Stopwatches measure to 0.01 s, but human reaction time limits accuracy for short intervals.
Temperature
  • Temperature Thermometers read to about 0.5 or 1 °C, while digital probes can read to 0.1 °C.

Part 2 of 6Length and Counts

Length and Counts
Ruler
  • Ruler Measures lengths of a few centimetres to the nearest millimetre.
Graticule
  • Graticule An eyepiece graticule measures microscopic objects after calibration with a stage micrometer.
Cell counts
  • Cell counts A haemocytometer counts cells in a known volume, so the concentration can be calculated.
Population counts
  • Population counts Whole populations are rarely counted, so samples are taken with quadrats or transects.
Qualitative observations
  • Qualitative observations Attributes such as colour, shape and number of limbs are recorded without measuring and used for classification.

Part 3 of 6Precision and Uncertainty

All raw data in one table should be recorded to the same precision.

1
Precision is shown by the smallest scale division the instrument allows, such as 1 cm³ on a cylinder.
2
For an analogue instrument, the uncertainty is usually half of the smallest division, written with a ± sign.
3
For a digital instrument, the uncertainty is usually the smallest displayed unit.
4
Percentage uncertainty equals the absolute uncertainty divided by the measured value, multiplied by 100.
5
A smaller percentage uncertainty means a more reliable measurement, so large readings are measured with greater relative precision.

Part 4 of 6Choosing Apparatus

Match to the size
  • Match to the size Choose a cylinder that is close to the volume needed, since smaller scale divisions give lower uncertainty.
Units
  • Units Record the unit in column headings and keep it consistent.
Repeat
  • Repeat Repeat measurements to reduce the effect of random error.
Calibrate
  • Calibrate Check instruments such as balances, probes and graticules against a standard.

Part 5 of 6Worked Example

Question

  • A 25 cm³ measuring cylinder has 1 cm³ graduations.
  • A student measures 24 cm³ of solution.
  • State the absolute uncertainty and calculate the percentage uncertainty.

  1. For an analogue scale the uncertainty is half the smallest division.
  2. Half of 1 cm³ is 0.5 cm³, so the volume is 24 ± 0.5 cm³.
  3. Percentage uncertainty = 0.5 ÷ 24 × 100.
  4. 0.5 ÷ 24 = 0.0208, so the percentage is 2.08%, or 2.1% to two significant figures.

Answer

  • Absolute uncertainty ±0.5 cm³;
    • percentage uncertainty about 2.1%.

Part 6 of 6Exam Checklist & Tips

5.1.2 – Key Points for the Exam

  • Precision How close repeated measurements are to each other.
  • Accuracy How close a measurement is to the true value.
  • Meniscus The curved upper surface of a liquid in a tube.
  • Uncertainty The range within which the true value is expected to lie.
  • Quantitative data Numerical measurements.

Common Mistakes

  • Reading a measuring cylinder from the top of the meniscus.
  • Writing results to different numbers of decimal places in one table.
  • Confusing accuracy with precision.
  • Using a large cylinder to measure a small volume, which increases the relative uncertainty.

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