IGCSE Biology (0610)

Controlling variables for a fair test

What this topic tests, the mistakes that cost marks, and one exam-style question worked through step by step.

Key ideas and common mistakes

Identifying control variables means naming other factors (besides the independent variable) that could affect the outcome if not kept constant.
Common mistake: Students often mistakenly include the independent variable or irrelevant factors.
Maintaining a constant temperature in an experiment requires using a water-bath set to the desired temperature and monitoring it with a thermometer, rather than relying on room temperature or direct heating, which can fluctuate or overheat.
Maintaining a constant temperature requires both monitoring (e.g. with a thermometer) and actively adjusting (adding hot or cold water), not just measuring or setting up the water-bath initially.
Common mistake: Students often forget the need for ongoing adjustment.
Identifying a controlled variable requires distinguishing between the independent variable (what is changed, here pH) and other factors (like temperature) that must be kept the same to ensure a fair test.
Common mistake: Students often mistakenly name the independent variable or an uncontrolled factor.
To ensure a fair test of temperature's effect, solutions must equilibrate to the water-bath temperature before adding yeast.
A control lacking the variable being tested (here, the enzyme) is needed to show that any observed effect is due to that variable, not to other factors like water, temperature, or the substrate itself.
Common mistake: Students often forget the purpose of such controls.

Worked example

State two variables, other than temperature, that the student should keep constant in this investigation.

Try it first, then show the working

Any 2 of these:

Volume / concentration of yeast suspension · Volume / concentration of sugar (glucose) solution · Time over which bubbles are counted · same apparatus / same size of tube / same pH · volume of total mixture

Answer: Two of: the volume (or concentration) of yeast suspension used; the volume (or concentration) of the sugar/glucose solution; the time over which bubbles are counted; the same apparatus / same diameter of delivery tube.

Try one yourself

Describe how the student could keep the milk and lipase mixture at a constant temperature of 35 °C.Answer it and get it marked → Describe how the temperature of the water-bath could be maintained at 40 °C.Answer it and get it marked → State two variables the farmers should have kept the same in this investigation.Answer it and get it marked →

15 exam-style questions on Controlling variables for a fair test, each marked to the scheme.

Controlling variables for a fair test

Key ideas and common mistakes

Worked example

Try one yourself

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