8.1.3Additional lessons
Controlling Variables in Chemistry — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 169 of 172
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.
Section 1 of 5Types of Variable
A fair test changes only one factor at a time.
Independent
- Independent The factor you change on purpose, such as the temperature of the reaction mixture.
Dependent
- Dependent The factor you measure to see the effect, such as the time taken for a colour change.
Controlled
- Controlled Every other factor that could affect the result must be kept the same in each trial.
Why control
- Why control If a second factor changes together with the independent variable, you cannot tell which one caused the change in the result.
Section 2 of 5Controlling Common Variables
Controlling Common Variables
Temperature
- Temperature Use a thermostatically controlled water bath, and allow solutions to reach the bath temperature before mixing.
Concentration and volume
- Concentration and volume Measure volumes with the same glassware each time and prepare solutions from the same stock.
Surface area
- Surface area Use solid pieces of the same size or the same mass of powder in every run.
Stirring and timing
- Stirring and timing Stir at the same rate and start the timer at the same point, for example when the last reagent is added.
Section 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.
Section 4 of 5Variables That Cannot Be Fully Controlled
Room temperature
- Room temperature This drifts during an experiment, so it should be recorded and its possible effect should be discussed in the evaluation.
Impurities
- Impurities Reagents of different purity or age change results, so use the same batch throughout.
Human factors
- Human factors Reaction time at a stopwatch varies, so use sensors or the same person for all timings where possible.
Reporting
- Reporting State how each controlled variable was kept constant, and what was uncontrollable, in the method.
Section 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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