All subjects · Biology · WBI13 Practical Skills in Biology I

Variables, controls and fair tests

Edexcel IAL Biology · WBI13 Practical Skills in Biology I

Every investigation changes one thing, measures one thing and keeps everything else the same. Knowing which is which, and what a control is, earns easy marks in every paper.

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Types of variable

Independent variable (IV): the one you deliberately change, for example temperature in CP4.
Dependent variable (DV): the one you measure, for example time for the starch-iodine colour to disappear, or absorbance of the solution in CP3.
Controlled (standardised) variables: other factors that could affect the DV and are kept constant, for example pH, enzyme concentration, substrate concentration and volume.

For each controlled variable say how it is controlled: 'pH kept at 7 using a buffer solution', 'temperature kept at 30 °C in a thermostatically controlled water bath'.

The January 2025 report says nearly half of candidates gave the dependent variable when asked for the independent variable: read the question twice.

A control is not a controlled variable

A control is an extra set-up in which the independent variable is absent (or the factor being tested cannot act), used as a baseline for comparison. It shows that any change is due to the IV and not something else.

Enzyme experiment: boiled (denatured) enzyme, or the solvent the enzyme was dissolved in instead of enzyme.
CP9 antimicrobial plants: a disc soaked in the solvent only (for example ethanol that has evaporated).
CP3 beetroot: no alcohol, only water.

The June 2024 report stresses the difference: describing what you keep the same does not describe the control.

Choosing relevant controlled variables

Give variables that would really affect this DV. In the June 2024 squid-meat enzyme question, pH was the DV, so 'keep pH constant' scored nothing; temperature and time were credited.

Useful checklist: temperature, pH, concentration, volume, time, surface area or size of tissue, age or source of organisms, light. Then cross out anything that is the IV or DV, or that is already fixed by the design.

Practical	IV	DV	Key controlled variables
CP3 beetroot	Temperature or alcohol concentration	Absorbance of the solution (colorimeter)	Size and surface area of beetroot pieces, rinsing, volume of water, time
CP4 enzyme	Temperature, pH, enzyme or substrate concentration	Initial rate	The other three factors, volumes
CP8 fibres	Type or length of fibre	Mass needed to break the fibre	Fibre length (if not IV), how masses are added

Worked example

Students cut potato cylinders and soak them in sucrose solutions of different concentrations for 30 minutes, then measure their change in mass. Identify the IV, the DV and two controlled variables, with how each is controlled.

IV: sucrose concentration.
DV: percentage change in mass of the cylinders.
Controlled: same length and diameter of cylinder (cut with the same cork borer and ruler); same temperature (water bath); same volume of solution; same time (30 minutes); blot dry the same way.

Answer: IV sucrose concentration; DV percentage change in mass; control, for example, cylinder size with the same borer and ruler and temperature in a water bath.

Key idea: IV changed, DV measured, controlled variables kept constant (say how). A control is a separate set-up without the IV, giving a baseline. Choose controlled variables that really affect this DV.

Check yourself

In CP4, students measure the time taken for starch to disappear at five temperatures. What is the independent variable?
- Time for starch to disappear
- Temperature
- Volume of amylase
- pH of the buffer
Show answer
Temperature. Temperature is deliberately changed.
In CP3, beetroot pieces are put in water at different temperatures and the absorbance of the water is measured. What is the dependent variable?
- Temperature of the water
- Size of the beetroot pieces
- Volume of water
- Absorbance of the water around the beetroot
Show answer
Absorbance of the water around the beetroot. Absorbance measures how much pigment leaked out.
Which is a suitable control for an experiment testing whether a plant extract (dissolved in ethanol) kills bacteria?
- A paper disc soaked in a stronger extract
- A paper disc soaked in ethanol only, with the ethanol allowed to evaporate
- Keeping all plates at 25 °C
- Using the same volume of agar in each plate
Show answer
A paper disc soaked in ethanol only, with the ethanol allowed to evaporate. The solvent-only disc shows any effect is due to the extract, not the ethanol.
What is the purpose of a control?
- To make the results more precise
- To keep the temperature constant
- To give a baseline so any effect can be attributed to the independent variable
- To increase the number of repeats
Show answer
To give a baseline so any effect can be attributed to the independent variable. Comparing with a control shows that the IV, not something else, caused the change.
In CP4 the IV is temperature. Which is a controlled variable, with a suitable method?
- pH, kept the same using the same buffer solution
- Temperature, kept the same using a water bath
- Time for the colour to disappear, kept the same
- The colour of the iodine, kept the same
Show answer
pH, kept the same using the same buffer solution. pH affects enzyme activity, so it is controlled with a buffer; temperature is the IV here.
An investigation measures the pH of a solution as protein is digested. Why would 'keep pH constant' not be credited as a controlled variable?
- pH has no effect on enzymes
- pH cannot be measured
- pH is the dependent variable being measured
- pH is the independent variable
Show answer
pH is the dependent variable being measured. You cannot keep the DV constant; the June 2024 report made this point.
State how temperature would be controlled in an enzyme experiment where the independent variable is substrate concentration.
Show answer
Use a thermostatically controlled water bath set to one temperature, for example 30 °C, for every tube.. Naming the apparatus (a thermostatically controlled water bath) and a value shows how it is controlled.

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Last updated 8 November 2026. Spotted a mistake? Tell us.