5.1Theme A — Unity and diversity

Tool 1: Experimental Techniques — IB Biology SL Revision Notes & Topical Questions

10 min read 13 parts Lesson 2 of 11 Unit quiz

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Lesson summary

5.1 Tool 1: Experimental Techniques — IB BIOLOGY SL notes: Select and use suitable apparatus and techniques to make measurements with stated uncertainty, and keep practical work safe, ethical and fair.Hazard = a feature of a procedure that has inherently harmful propertiesA chemi…

Risk assessment
Justified precautions
Hazard warning symbols
Human participants
Field work
Range and repeats

Part 1 of 11Hazard, risk and risk assessment

Hazard

Hazard a feature of a procedure that has inherently harmful properties

A chemical may be an irritant to the skin.
A rocky shore may have slippery rocks.
A Bunsen burner may cause burns.

Risk

Risk the level of danger posed by a hazard, decided by how likely the harm is and how severe it would be

Risk assessment

Risk assessment identifies the hazards, assesses the risk and changes the method to reduce it
Rocky shore exampleslippery rocks are a hazard and slipping is quite likely.
The procedure is altered so that footwear has good grip, students avoid seaweed-covered rocks and walk slowly.

Justified precautions

Justified precautions name the hazard and explain why the precaution reduces the risk
Exampleiodine is an irritant, so wear safety goggles and wash any skin it touches at once.

Part 2 of 11Hazard symbols, ethics and environment

Hazard symbols, ethics and environment

Hazard warning symbols

Hazard warning symbols nine standard symbols, usually in a red-bordered diamond on labels and safety data sheets
Examples are health hazard, corrosive, flammable, acute toxicity, oxidising and hazardous to the environment.
Check them for stains, enzyme reagents and chromatography solvents.

Animals

IB policy says to use animals only if no alternative exists
Investigations must not be cruel and must remove causes of distress.
Keep experiments short, handle animals as little as possible and release them to their habitat afterwards.

Human participants

Human participants any investigation, such as measuring breathing with a spirometer, needs fully informed consent

Laboratory waste

No toxic chemicals down the sink.
Sharps go into a sharps bin.
Biological waste, such as cultures or dissected organs, is sterilised or disposed of correctly.

Field work

Field work work in small groups, avoid sensitive habitats and take away all equipment and litter

Part 3 of 11Designing a fair test

Fair test

Fair test changes one independent variable, measures one dependent variable and keeps all other significant variables constant
Name each controlled variable and the method used, such as a water bath at 30 °C.

Range and repeats

Range and repeats use at least five values of the independent variable and at least three repeats of each
This lets trends and anomalies be identified.

Control

Control a treatment identical to the others except that the independent variable is absent or at a baseline level

Independent variable

Independent variable deliberately changed and plotted on the x-axis

Dependent variable

Dependent variable measured in response and plotted on the y-axis

Part 4 of 11Uncertainty and error

Instrument uncertainty
- Analogue scales are read to ± half the smallest division.
- Digital displays are read to ± the last digit.
- Record the uncertainty in the table heading, for example Volume / cm³ ± 0.05.

Feature	Random error	Systematic error
Effect	Scatter about the true value	All values shifted one way
Affects	Precision	Accuracy
Reduced by	Repeats and a mean; a more sensitive instrument	Calibration, zeroing, improving the method
Example	Judging a colour end-point by eye	A balance not tared

Accuracy

Accuracy how close a measurement is to the true value

Precision

Precision how close repeated measurements are to each other

Part 5 of 11Measuring instruments

Quantity	Instrument	Units	Notes
Mass	Electronic balance	g, mg	Tare before use
Volume	Pipette, syringe, measuring cylinder, burette	cm³, ml	Read the bottom of the meniscus at eye level
Time	Stopwatch or data logger	s, min	Human reaction time adds about 0.2 s of random error
Temperature	Thermometer, probe or water bath	°C	Let solutions equilibrate before mixing
pH	pH meter or universal indicator	none	Calibrate the meter with buffers
Length	Ruler or eyepiece graticule	mm, µm	Recalibrate the graticule at each magnification
Concentration or colour	Colorimeter	Absorbance	Zero with a blank

Choosing volume apparatus

A measuring cylinder gives approximate volumes.
A volumetric pipette gives one fixed volume, such as 25 cm³, most accurately.
A burette gives accurate variable volumes up to 50 cm³ and reads from zero at the top.

Gases

Gases collect in a gas syringe, or in an inverted cylinder of water if the gas is insoluble

Part 6 of 11Dilution and serial dilution

Serial dilutions create known reducing-sugar standards for comparison.

Dilution equation

C₁V₁ = C₂V₂, where C is concentration and V is volume

Dilution factor

Dilution factor final volume divided by the volume of stock added

Serial dilution

Serial dilution each step uses the same dilution factor and the concentrations change logarithmically
It is useful for bacterial counts or sucrose solutions for osmosis.

Make a 1 mol dm^-3 stock solution and label it.

Set out labelled tubes each holding 9.0 cm³ of distilled water.

Transfer 1.0 cm³ of stock to tube 1 with a clean pipette and mix; this gives 0.1 mol dm^-3.

With a fresh pipette transfer 1.0 cm³ from tube 1 to tube 2 and mix to give 0.01 mol dm^-3.

Repeat for further tubes to give 10^-3, 10^-4 mol dm^-3.

Use each dilution in the investigation, with at least three repeats per concentration.

Part 7 of 11Microscopy and magnification

Temporary mount

Temporary mount cut a thin section and add a drop of water or stain such as iodine or methylene blue
Lower the coverslip at an angle to avoid air bubbles.
Focus with low power first, then use only the fine focus on higher power.

Magnification

M = I / A,
- so actual size A = I / M
  What the symbols mean: M = magnification (no unit) · I = image size · A = actual size; use the same unit for both (1 mm = 1000 μm)
I and A must be in the same units and M has no units.
1 mm = 1000 µm and 1 µm = 1000 nm.

Resolution

Resolution the minimum distance between two points that can still be seen as separate

It is about 0.2 µm for a light microscope and 0.2 nm for an electron microscope.

Place the stage micrometer (1 division = 10 µm) on the stage and focus at the chosen objective.

Turn the eyepiece so the graticule scale lies alongside the micrometer scale.

Find two points where lines coincide and count the divisions on each scale between them.

Calculate 1 eyepiece division = (stage divisions × 10 µm) / eyepiece divisions.

Replace the micrometer with the specimen, measure in eyepiece divisions and multiply by the calibration value.

Recalibrate for every change of objective lens.

Part 8 of 11Chromatography and colorimetry

Chromatography

Chromatography separates photosynthetic pigments by their solubility in the solvent and attraction to the paper or TLC plate
Mark the origin in pencil and keep the spot above the solvent level.
Mark the solvent front immediately when the paper is removed.

Rf distance moved by the pigment divided by distance moved by the solvent front

It has no units and is always between 0 and 1.

Colorimetry

Colorimetry measures the absorbance of light by a coloured solution
Make a calibration curve from standards of known concentration.
Read the unknown concentration from the curve.
Zero the colorimeter with a blank of solvent.

Part 9 of 11Rates, counts and field sampling

Measuring rates

Collect gas in a gas syringe or inverted measuring cylinder rather than counting bubbles.
A respirometer with KOH or soda lime to absorb CO₂ measures O₂ uptake.
A potometer measures water uptake as an estimate of transpiration.
Rate = amount of product formed (or substrate used) divided by time.

Cell counts

Cell counts a haemocytometer is a slide with a grid of known volume
Counting cells in the grid allows the concentration of the whole culture to be calculated.
Direct counts need no specialist equipment but are slow.

Random quadrats

Random quadrats use random number coordinates to compare two areas
Population density = mean number of individuals per quadrat divided by the area of one quadrat.

Transects

Transects line or belt transects show change along a gradient

Mark-release-recapture

Mark-release-recapture population size N = (M × C) / R
M is the number first marked, C the total caught second time and R the number recaptured marked.
It assumes no births, deaths or migration and that marks do not affect survival or get lost.

Part 10 of 11Aseptic technique and observation

Aseptic technique

Aseptic technique procedures that prevent contamination of cultures and protect the worker
Sterilise loops by flaming until red hot.
Open lids as little as possible.
Tape Petri dishes without sealing them fully.
Incubate at 25 °C, not 37 °C, to avoid culturing human pathogens.
Dispose of cultures by autoclaving.

Biological drawings

Biological drawings labelled line drawings show the specific features seen in the specimen

Qualitative data

Qualitative data describe attributes that are observed but not measured, such as fur colour, sex or number of limbs
They are used when identifying and classifying organisms.

Required techniques

Required techniques chromatography, colorimetry and serial dilution, modelling, light microscopy, temporary mounts, sampling, karyotyping and cladogram analysis

Part 11 of 11More from the specification

Safety in biology

Safety in biology risk assessment identifies hazards such as sharp tools, microorganisms and chemicals and the steps to reduce them

Ethical considerations

Ethical considerations involve respect for living organisms, minimising harm and justifying any animal or human use

Environmental issues

Environmental issues disposal of waste and effects on habitats must be considered when making observations in the field

Applying techniques

Applying techniques experimental techniques involve careful measurement, controls and repeats, for example for a liquid culture or sampling

Key terms

Term	Meaning
Accuracy	How close a measured value is to the true value.
Precision	How close repeated measurements are to each other.
Controlled variable	A factor other than the independent variable that is kept constant.
Serial dilution	A stepwise series of dilutions that each use the same dilution factor.
Resolution	The minimum distance between two points at which they can still be seen as separate.
Retention factor (Rf)	Distance moved by the pigment divided by distance moved by the solvent front.
Quadrat	A square frame of known area used to sample organisms and estimate density.
Aseptic technique	Procedures that prevent unwanted microorganisms contaminating cultures.

Avoid these mistakes

Confusing hazard and risk — a hazard is something that can cause harm and risk is the likelihood and severity of that harm

Stating only wear a lab coat and goggles as a precaution — name the hazard and justify the precaution

Mixing units in M = I / A — convert I and A to the same units first

Thinking repeats remove systematic error — repeats reduce random error and only calibration removes systematic error

Last updated: 25 October 2026