Deducing reactivity from supplied experimental results
Turn each stated comparison into an ordering constraint and combine only what the evidence supports.
Section 9.4 · Reactivity series
Exam scope and assessment notes
Core and Extended · Chemistry 0971, 2026–2028. Cambridge states that IGCSE 0971 has the same subject content as IGCSE 0620, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Read an above/below reactivity statement.
- Distinguish a controlled comparison from a raw rate measurement.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does a supplied A>B relation mean here?
What you will learn
- Deduce a reactivity order from a set of supplied experimental comparisons.
- Identify missing comparisons, contradictions and confounding conditions.
Make one supported relation from each result
A supplied result that metal A displaces metal B from its compound supports A>B in the stated comparable model. A stated series of controlled reaction results can similarly support an ordering when the question explicitly links the outcomes with relative reactivity. A, B and C here are labels for unknown metals, not chemical formulas for real elements.
If the evidence gives A>B and B>C, combine them into A>B>C. If it only gives A>B and A>C, A is above both but the order of B versus C is not established. Do not fill the missing comparison from alphabetical labels, colour intensity or an imagined value. The Extended lesson teaches the detailed aqueous-ion displacement mechanism; here the task is interpreting supplied ordering evidence.
Check whether the observations really compare the same question
A comparison based only on gas volume per second needs controlled surface area, amounts, temperature, concentration and apparatus; a protective coating can also affect contact. Equal final gas amounts do not give an order of reaction speed. Likewise no visible change can mean an unsuitable method or limited observation, rather than proof of universal unreactivity.
If a result set says A>B, B>C and C>A under the same intended model, it cannot give one consistent simple order. Investigate conditions, recording errors or interpretation instead of forcing a cycle into a list. Explain exactly what is known and what comparison is missing or contradictory. A justified partial order is better than an unsupported complete ranking.
Pause and explain
Which order follows from B>A and A>C?
Worked example
A supplied comparison says A displaces B, and B displaces C under suitable comparable conditions. Deduce the order and predict A versus C in the same model.
Show the worked solution
- The first result gives A>B.
- The second gives B>C, so together A>B>C.
- A is predicted to displace C under the same suitable comparison model.
Answer A > B > C; A is predicted to displace C in the stated model.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Supplied results give A>B and B>C. Deduce the order and state whether C is predicted to displace A in the same model.
Give me a hint
Link the two relations through B.
Compare my reasoning
- A>B and B>C combine into A>B>C.
- C is below A.
- C is not predicted to displace A under the stated comparable conditions.
A>B>C; C does not displace A in the same model.
Look for these in your work
- Both observations are used.
- The prediction follows the combined order.
Results give B>A and B>C with no A-versus-C comparison. State the supported ordering and the missing information.
Give me a hint
Both relations share the highest label B, without comparing the lower pair.
Compare my reasoning
- B is above A and above C.
- The relative order of A and C remains unknown.
- A suitable A/C comparison would be needed for a unique complete order.
B is above both A and C; their mutual order is not determined.
Look for these in your work
- Known and unknown comparisons are separated.
- No unsupported complete ranking is invented.
A table claims A>B, B>C and C>A under the same comparable simple model. Explain why it cannot produce a consistent linear order and what should be checked.
Give me a hint
Follow the first two implications before accepting the third.
Compare my reasoning
- A>B>C implies A>C.
- C>A contradicts that implication in one consistent linear order.
- Check the experimental conditions, transcription and interpretation rather than silently dropping a result.
The results are contradictory for one linear order; check conditions, recording and interpretation.
Look for these in your work
- The contradiction is demonstrated.
- The response preserves the evidence rather than changing it silently.
Common mistakes
- Inventing the order of two metals that were not constrained relative to each other.
- Using alphabetical order of labels as chemistry evidence.
- Treating different masses or temperatures as a fair rate comparison.
- Forcing contradictory cyclic results into a single order.
What can you explain now?
Results give A>B and A>C only. Is the relative order of B and C known?
Compare with the explanation
No; B versus C is not established.
Both are below A, but that does not compare them with each other.
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Make it stick
Turn a fresh result set into relations, combine the supported ones and state missing or contradictory comparisons explicitly.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.