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8.1.3Additional lessons

Controlling Variables in Chemistry — IB Chemistry HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 169 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Part 1 of 5Types of Variable

8.1.3 Controlling Variables in Chemistry

A fair test changes only one factor at a time.

Independent
  • The factor you change on purpose, such as the temperature of the reaction mixture.
Dependent
  • The factor you measure to see the effect, such as the time taken for a colour change.
Controlled
  • Every other factor that could affect the result must be kept the same in each trial.
Why control
  • If a second factor changes together with the independent variable, you cannot tell which one caused the change in the result.

Part 2 of 5Controlling Common Variables

Controlling Common Variables
Temperature
  • Use a thermostatically controlled water bath, and allow solutions to reach the bath temperature before mixing.
Concentration and volume
  • Measure volumes with the same glassware each time and prepare solutions from the same stock.
Surface area
  • Use solid pieces of the same size or the same mass of powder in every run.
Stirring and timing
  • Stir at the same rate and start the timer at the same point, for example when the last reagent is added.

Part 3 of 5Control Experiments

1
Decide what the result would look like if the independent variable had no effect at all.
2
Prepare a control sample that is identical except that the independent variable is absent or at its normal value.
3
Run the control alongside the test samples under the same conditions.
4
Compare the test results with the control to show that the observed change is caused by the independent variable.
5
Use a blank in colorimetry or titration to correct for background readings.

Part 4 of 5Variables That Cannot Be Fully Controlled

Room temperature
  • This drifts during an experiment, so it should be recorded and its possible effect should be discussed in the evaluation.
Impurities
  • Reagents of different purity or age change results, so use the same batch throughout.
Human factors
  • Reaction time at a stopwatch varies, so use sensors or the same person for all timings where possible.
Reporting
  • State how each controlled variable was kept constant, and what was uncontrollable, in the method.

Part 5 of 5Exam Checklist & Tips

8.1.3 – Key Points for the Exam

  • Controlled variable A factor that is deliberately kept constant in every trial.
  • Control experiment A comparison run in which the independent variable is absent or unchanged.
  • Valid result A result that measures the effect of the independent variable only.
  • Thermostat A device that holds a temperature constant.
  • Blank A sample with no analyte, used to correct background readings.

Common Mistakes

  • Listing 'temperature' as controlled when the room temperature was never measured or kept constant.
  • Changing the volume of solution and its concentration in the same trial, which confuses two effects.
  • Starting the timer at different stages in different runs.
  • Using a control that differs from the test samples in more than one way.

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