Experimental Design, Variables and Risk
Planning valid chemical investigations with a testable hypothesis, controlled variables, appropriate controls and repeats, ethical practice and proportionate risk management.
How to study A-level Chemistry
Represent the species and process correctly, conserve atoms, charge and energy, calculate with units, then test the result against observations and chemical evidence.
Core concepts
Concept 1
A valid design changes or compares the independent variable, measures the dependent variable and controls relevant confounding variables.
Exam cue: State how and why each controlled variable could affect the result.
Concept 2
Accuracy, precision, repeatability, reproducibility and validity describe different qualities of experimental evidence.
Exam cue: Link every method improvement to a specific source of uncertainty, bias or risk.
Concept 3
Risk assessment identifies hazards, exposure routes, likelihood and severity before selecting control measures and disposal methods.
Exam cue: Choose apparatus from the required range, precision, chemical compatibility and safety constraints.
Risk pitfalls and guardrails
Calling a measurement accurate because repeated values are close together.
Guardrail: Do not substitute a memorised equation, mechanism or trend until you have identified the species, conditions and chemical model required.
Assuming extra repeats remove systematic error.
Guardrail: Do not substitute a memorised equation, mechanism or trend until you have identified the species, conditions and chemical model required.
Naming a hazard without assessing the risk under the actual conditions.
Guardrail: Do not substitute a memorised equation, mechanism or trend until you have identified the species, conditions and chemical model required.
Memory anchors
Independent Variable
The independent variable is deliberately changed or used to define comparison groups.
Dependent Variable
The dependent variable is the measured response.
Precision
Precision describes how closely repeated measurements agree.
Accuracy
Accuracy describes closeness to the accepted or true value.
Risk Assessment
Identify hazards, assess likelihood and severity, then apply proportionate controls.
Checkpoint rule
Do the check-up only after you can summarize each concept in one sentence and identify one dangerous pitfall from memory.
Knowledge Check (after reading)
Short check-up to confirm understanding of this module.
Check-up Questions
A student compares how chloride and iodide ions affect the rate of persulfate decomposition. Which variable should be deliberately changed?
Two groups obtain closely clustered titres, but both use a pipette that delivers 24.70 cm³ instead of 25.00 cm³. How should their data be described?
Answer all questions to submit.
Next step personalized recommendations
Continue learning
Move forward only after this module is stable.
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