Who this is for: Cambridge IGCSE Chemistry (0620) students who want experimental design — variables, controls and fair tests — to become structured Paper 6 answers instead of vague “repeat and average” statements.
What query it owns: how to understand and revise experimental design in Cambridge IGCSE Chemistry.
Why this is safe: this page owns the experimental design revision-guide angle, while Tutopiya’s Experimental Design subtopic page owns the learning resource.

Experimental design is tested heavily in Cambridge IGCSE Chemistry (0620) Paper 6 (Alternative to Practical). Examiners expect you to identify independent, dependent and control variables, design a fair test, and suggest improvements for accuracy and reliability. This guide covers the variable types, how to plan an investigation, and the wording that earns marks in planning questions.

Key takeaways

• Independent variable = the one you deliberately change.
• Dependent variable = the one you measure (the result).
• Control variables = all others kept constant for a fair test.
• Reliability improves by repeating and calculating a mean; discard anomalous results.
• Accuracy improves with better apparatus (e.g. burette instead of measuring cylinder).

What is experimental design in Cambridge IGCSE Chemistry?

Experimental design is the planning framework for a chemical investigation. You must state what you change, what you measure, what you keep constant, and how you ensure results are trustworthy. Paper 6 often presents an incomplete method and asks you to identify flaws or suggest improvements.

Read the full notes on Tutopiya’s Experimental Design subtopic page before attempting questions.

The core ideas you must master

Term	Definition	Example (rate of reaction)
Independent variable	Deliberately changed	Concentration of acid
Dependent variable	Measured / observed	Time for gas production / volume of gas
Control variables	Kept constant	Temperature, volume of acid, mass of metal
Fair test	Only one variable changed	Same temperature, same apparatus
Repeat / replicate	Do the experiment again	Repeat 3 times, calculate mean
Anomalous result	Result far from others	Discard before calculating mean

Accuracy vs reliability vs fair test

Term	What it means	How to improve
Accuracy	How close to the true value	Better apparatus (burette, balance to 2 dp)
Reliability	Consistency of repeated results	Repeat and calculate mean; discard anomalies
Fair test	Only independent variable changes	List and control all other variables
Validity	Test measures what it claims	Ensure method actually tests the hypothesis

Experimental design in past-paper wording: command words that matter

Command word	What the question wants	Typical design stem
Identify	Name a variable type	”Identify the independent variable.”
State	Control variable or improvement	”State one variable that must be kept constant.”
Suggest	Improvement to method	”Suggest how to improve the reliability of the results.”
Explain	Why a control is needed	”Explain why temperature must be kept constant.”
Plan	Full investigation outline	”Plan an experiment to investigate…”

Worked exam-style stems (how to answer the wording)

1. “Identify the independent and dependent variables in an investigation into how temperature affects the rate of reaction.” Independent: temperature. Dependent: rate of reaction (e.g. time for a fixed volume of gas, or volume of gas in a fixed time). Reward: both variables correctly named.
2. “Suggest how to improve the reliability of the results.” Repeat the experiment at least three times and calculate the mean; discard anomalous results before averaging. Reward: repeat + mean + anomalies.
3. “State two control variables in this investigation.” Any two from: concentration of reactants, volume of solution, mass/surface area of solid, apparatus used. Reward: two valid controls named.

Work through examples on Tutopiya’s Experimental Design subtopic page.

How experimental design connects to the rest of the course

Experimental design underpins practical work across the syllabus — Rate of Reaction, Acid–Base Titrations and Electrolysis. The Cambridge IGCSE Chemistry resource hub links all units.

Common mistakes students make

• Swapping independent and dependent variables.
• Saying “repeat once” improves reliability — you need at least three repeats.
• Confusing accuracy (closer to true value) with reliability (consistent repeats).
• Listing the dependent variable as a control variable.
• Forgetting to keep temperature constant in rate investigations.

When you need more support

If variable identification and method-improvement questions keep costing marks, work through the Experimental Design subtopic page, then get focused help from a Cambridge IGCSE Chemistry tutor.

Frequently asked questions

What is the independent variable? The variable you deliberately change in an investigation.

What is the difference between accuracy and reliability? Accuracy is how close a result is to the true value; reliability is how consistent repeated results are.

How do you improve reliability? Repeat the experiment at least three times, discard anomalous results, and calculate the mean.

How do I revise experimental design effectively? Practise identifying variables in past Paper 6 questions, learn accuracy vs reliability definitions, then apply to planning stems.

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