Experimental Design
IB Chemistry HLΒ· 35 min read
1. Classifying Experimental Variablesβ β ββββ± 10 min
All chemical experiments test the causal relationship between variables. Correct classification of variable types is the foundation of a valid experimental design. Misclassification leads to flawed conclusions and lost marks in exams and IA.
Core Variable Types
Three standard variable categories are used in all experimental design:
- Independent Variable (IV): The variable intentionally manipulated or changed by the investigator
- Dependent Variable (DV): The variable measured to test the effect of changes in the IV
- Controlled Variables: All other variables that could affect the DV, kept constant across all trials
Example:
In an experiment testing the effect of acid concentration on reaction rate, IV = acid concentration, DV = rate, controlled variables = temperature, acid volume, metal surface area
A student investigates how pH affects the solubility of calcium hydroxide. Classify all key variables for this experiment.
- 1
Step 1: Identify the variable the student intentionally changes: pH of the solvent. This is the independent variable.
- 2
Step 2: Identify the variable measured to detect the effect of pH change: mass of dissolved calcium hydroxide per unit volume. This is the dependent variable.
- 3
Step 3: List other variables that could affect solubility that need to be controlled: temperature, pressure, purity of calcium hydroxide, total volume of solvent. These are all controlled variables.
Exam tip:
1 mark questions asking to identify variables are common in Paper 3: always remember IV = you change it, DV = you measure it.
2. Validity and Reliabilityβ β β βββ± 15 min
Validity and reliability are the two key criteria used to evaluate experimental design. They are often confused in exams, but have distinct, separate meanings that examiners expect you to distinguish.
Validity vs Reliability
- Validity: An experiment is valid if it actually measures what it claims to measure. This requires all confounding variables to be controlled, so any change in the DV can only be caused by the IV.
- Reliability: An experiment is reliable if repeating it gives consistent, repeatable results. This is achieved through repeated trials and reduced random error.
A student tests the effect of lead nitrate concentration on the growth of duckweed in water. They do 5 repeats for each concentration and get very consistent growth measurements, but keep the solutions near an open window where temperature varies with sunlight. Comment on the validity and reliability of this design.
- 1
Step 1: Evaluate reliability: 5 repeated trials per concentration produce consistent results. The experiment is reliable.
- 2
Step 2: Evaluate validity: Temperature affects plant growth, and it is not controlled. Changes in duckweed growth could be caused by temperature, not lead concentration. The experiment is not valid.
- 3
Conclusion: A reliable experiment is not automatically valid, but a valid experiment must always be reliable.
3. Controlling Confounding Variablesβ β β βββ± 15 min
A confounding variable is any uncontrolled variable that correlates with both the IV and DV, leading to false causal conclusions. Good experimental design uses specific methods to eliminate or minimize confounding.
Common Control Methods
- Constant holding: Keep the confounding variable at the same value for all trials
- Randomization: Randomize the order of trials to spread systematic error evenly across all IV levels
- Control groups: Include a trial with no treatment to isolate the effect of the IV
A student investigates how enzyme concentration affects the rate of breakdown of hydrogen peroxide by catalase. Identify two key confounding variables and describe how to control each.
- 1
- Confounder: Temperature. Higher temperature increases enzyme activity, independent of concentration. Control: Carry out all trials in a thermostatically controlled water bath held at 25Β°C for all trials.
- 2
- Confounder: pH. pH changes alter enzyme shape and activity. Control: Add a pH 7 buffer to all reaction mixtures to keep pH constant across all enzyme concentration levels.
4. Replication and Samplingβ β β β ββ± 10 min
Replication (running repeated trials for each level of the IV) and random sampling reduce the impact of random error on your results. IB requires a minimum of 3-5 repeated trials per IV level for internal assessment to demonstrate reliability.
A student wants to test how solvent polarity affects the Rf value of different amino acids in paper chromatography. How should they incorporate replication?
- 1
Step 1: For each solvent polarity (each level of the IV), run at least 3 separate chromatography plates, not just one. This produces 3 separate Rf measurements for each amino acid.
- 2
Step 2: Calculate the mean Rf value and standard deviation across the three repeats. A low standard deviation confirms the results are reliable.
- 3
Step 3: Any outlier results that are far from the others can be identified and tested or discarded if a clear error occurred.
5. Common Pitfalls
Wrong move:
Confusing validity and reliability in exam answers
Why:
Examiners require clear distinction between the two terms, and will award no marks if you mix them up
Correct move:
Memorize: Reliability = consistency of repeated results, Validity = experiment measures what it claims to measure
Wrong move:
Claiming all variables must be controlled in an experiment
Why:
This misrepresents variable classification: the independent variable is intentionally changed, not controlled
Correct move:
Only variables that could affect the dependent variable (other than the IV) need to be controlled
Wrong move:
Doing only one trial per level of the independent variable
Why:
You cannot identify random error or outliers, so your results cannot be proven reliable, leading to lost IA/Paper 3 marks
Correct move:
Complete a minimum of 3 repeated trials for every level of the IV
Wrong move:
Selecting an irrelevant controlled variable in Paper 3 answers
Why:
Examiners only award marks for controlled variables that actually affect the dependent variable
Correct move:
Always pick a variable that is chemically relevant to the experiment, e.g., temperature for reaction rate experiments
6. Quick Reference Cheatsheet
Term | Definition | Example |
|---|---|---|
Independent Variable | Intentionally changed by investigator | Reactant concentration |
Dependent Variable | Measured to test IV effect | Rate of reaction |
Controlled Variable | Kept constant across all trials | Reaction temperature |
Validity | Measures what it claims to measure | All confounders controlled = valid |
Reliability | Consistent results on repetition | 3+ trials, low spread = reliable |
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2025 Β· 3
Classify variables in rate experiment
- 2023 Β· 3
Evaluate validity of design
- 2021 Β· IA
Required for all internal assessments
Going deeper
What's Next
Experimental design is the foundation of all practical work in IB Chemistry, forming the core of your internal assessment investigation and frequent short-answer questions in Paper 3. A well-designed experiment simplifies later data processing and analysis, and makes it much easier to draw evidence-based conclusions that earn high marks. After mastering experimental design, you can build on this knowledge by learning to process and visualize your experimental data, calculate and propagate errors and uncertainty, and evaluate your own results for your internal assessment.