Full chapter
Data and Experimental Skills
Read unfamiliar evidence, make useful measurements and explain what a result can support.
A-Level 8873, revised syllabus (2026-2027)
Read unfamiliar data with a purpose
Separate the question, the evidence and the chemistry that connects them.
Start with the quantity or claim the question asks you to establish. Then locate the relevant measurements, conditions and any new definitions in the stem. An unfamiliar substance name does not make its data unusable: use the supplied information together with a familiar idea such as conservation of atoms, collision frequency or intermolecular attraction.
- Name the task
For a calculation, identify the required quantity and unit. For an explanation, identify the observation that needs explaining.
- Select the evidence
Extract the relevant rows, axes and conditions. Distinguish a measurement from a proposed explanation; do not use every number just because it is present.
- Connect a principle
State the chemical relationship that makes the evidence relevant. Check that the comparison holds other important variables constant.
- State the result and its limit
Give a numerical answer with units, or a conclusion supported by a specific comparison. State an assumption where the inference depends on it.
Worked exampleAssess a claim about a catalyst
Two runs use the same amount of reactant and the same temperature. Run A gives 40 cm3 of gas in 20 s; run B gives 40 cm3 in 50 s. Both eventually give 60 cm3. A report says: "A produces more product because it is faster."
Try it first. Then show the worked stepsHide the worked steps
- The time to reach the same volume is smaller for A: its average rate to 40 cm3 is 2.0 cm3 s-1, compared with 0.80 cm3 s-1 for B.
- The final volumes are the same under the same collection conditions, so these data do not show more final gaseous product in A.
- The evidence supports faster production, not a greater final amount. Identifying the cause as a catalyst also requires the stated experimental difference between the runs.
Compare rate using a time interval and final amount using the plateau. They answer different questions.
A useful explanation joins evidence and mechanism: "At the same temperature and reactant concentration, this catalyst gives a higher measured rate; it provides an alternative pathway with a lower activation energy." Merely repeating "the graph is higher" does not explain the chemistry.
Check your understanding
A table gives temperature, concentration, reaction time and the colour of the laboratory bench. Which data belong in an explanation of the rate trend?
Show the answer
Temperature, concentration and reaction time, after checking which one was changed
Use the variables that can affect rate, and find out which one was varied. Bench colour has no chemical role, so do not invent a link just because a detail is given.
Turn measurements into a graph or calculation
Keep the meaning of each axis, ratio and unit visible.
Put the independent variable on the horizontal axis and the measured or derived dependent variable on the vertical axis. Label each axis with a quantity and unit, use a clear uniform scale, and plot the actual points. A best-fit line or smooth curve represents a trend; do not force it through every point or through the origin without a reason.
| Time / s | Gas volume / cm3 |
|---|---|
| 0 | 0 |
| 20 | 24 |
| 40 | 39 |
| 60 | 45 |
| 80 | 48 |
An interval rate is a gradient between two points
Between 20 and 40 seconds, gas volume increases from 24 to 39 cubic centimetres. The horizontal change is 20 seconds and the vertical change is 15 cubic centimetres. The curve becomes less steep later.
Worked exampleCalculate and interpret a gradient
Find the average gas-production rate between 20 and 40 s.
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- Change in gas volume = 39 - 24 = 15 cm3.
- Time interval = 40 - 20 = 20 s. Average rate = 15 / 20 = 0.75 cm3 s-1.
- This is an interval average. A rate at one instant is the gradient of a tangent to the curve at that instant, where required.
0.75 cm3 s-1. The graph later becomes flatter, so the gas-production rate decreases.
Interpolation estimates a value within the measured range. Extrapolation extends a trend outside it and needs an extra assumption that the relationship continues. If a graph is linear, choose two well-separated points on the best-fit line for a gradient, not two close points chosen because they happen to be measurements.
Keep guard digits during working. Round the final answer to a precision justified by the inputs and any question instruction. A unit conversion changes the numerical value, not the physical quantity: 25.0 cm3 = 0.0250 dm3. Check dimensions before inserting numbers into a formula.
Check your understanding
A gas-volume curve is high but nearly horizontal. Does its height show a high rate at that moment?
Show the answer
No. Height shows gas made so far; the flat gradient shows a low rate now.
The rate at a moment is the gradient of the curve. A high, flat curve means a lot of gas has been made but very little is being made now.
Evaluate evidence and explain the direction of an error
An improvement is useful when it targets a specific limitation.
| Issue | Effect on evidence | Useful response |
|---|---|---|
| Random variation | Repeated results scatter unpredictably | Repeat independent measurements; compare their spread and use a justified mean |
| Systematic bias | Results are displaced consistently, for example by an incorrect calibration | Check against a suitable standard or correct the apparatus/method; repetition alone does not remove the bias |
| Uncontrolled variable | Two possible causes change together | Hold that variable constant or design an additional comparison |
| Limited range or sample | A relationship may be supported only locally | Collect appropriate additional values; restrict the conclusion to the evidence |
Accuracy concerns closeness to a valid reference value; precision concerns the closeness of repeated measurements. A tightly grouped set can still be inaccurate. An anomalous point is a reason to investigate the method and repeat a measurement, not permission to remove inconvenient evidence silently. Record an exclusion and its reason.
| Run | Treatment and temperature | Time to the same gas volume |
|---|---|---|
| A | Without M, 25 °C | 48 s |
| B | With M, 35 °C | 24 s |
| C | Without M, 35 °C | 25 s |
Worked exampleBuild an argument, then choose the next comparison
A supplier claims that adding unfamiliar material M doubles the rate of a gas-forming reaction. All runs use the same reactant amounts and apparatus. The table gives one run per condition; a time difference of 1 s is within the observed timing variation.
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- A and B change both temperature and the presence of M. Their 48 s versus 24 s times cannot isolate an effect of M.
- B and C use the same temperature. Their 24 s and 25 s times differ by an amount within the stated variation, so these results do not establish an improvement caused by M.
- Higher temperature is a plausible alternative explanation: at 35 °C a larger fraction of collisions can overcome the activation energy. This explains a rate change without assuming M is a catalyst.
- Recommend repeated runs with and without M at the same controlled temperature, keeping quantities, total solution volume and the measured gas-volume endpoint fixed. Compare the spread as well as the mean times.
Do not adopt M on this evidence alone. The claim is not established; that is different from proving that M has no effect. A controlled, repeated comparison can distinguish the explanations.
Worked exampleFollow an error through the calculation
An acid in a burette neutralises a known amount of alkali in a flask, in a 1:1 ratio. The burette tip initially contains an air bubble that fills with acid during the titration. What happens to the calculated acid concentration?
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- Some liquid leaving the graduated barrel fills the tip instead of entering the flask. The recorded titre is larger than the acid volume delivered to the flask.
- The known alkali amount still determines the acid amount needed for neutralisation.
- Calculated acid concentration = known reacting amount / recorded acid volume. A denominator that is too large gives a concentration that is too low.
The calculated acid concentration is too low in this arrangement. Fill the tip before taking the initial reading. If the unknown were instead in the flask, the calculation and error direction would need to be reconsidered.
Write evaluation as a causal chain: limitation -> affected measurement -> effect on the calculated result -> targeted improvement. "Human error" and "use better equipment" do not identify what happened. For example, gas escaping before collection lowers measured gas volume; assembling a closed collection system before starting the reaction addresses that loss.
Check your understanding
A thermometer reads 2 degrees too high at every temperature. Will averaging five readings remove that error?
Show the answer
No. Averaging reduces random scatter, but a fixed offset stays in every reading.
Every reading is 2 degrees high, so their average is too. Calibrate the thermometer against a reference. A temperature change measured with the same thermometer can cancel a constant offset.
Quick revision
Revisit the essentials, then return to an explanation when you need it.
Read the requested quantity or claim first. Select the relevant evidence, state the chemical connection and give a result with its unit or a justified conclusion.
| Question | Check |
|---|---|
| What do the axes mean? | Distinguish accumulated amount from its rate of change |
| Is the comparison fair? | Check temperature, amounts and other potentially changing variables |
| What does the uncertainty affect? | Trace its effect through the calculation; do not assume every error raises the answer |
| How far can I conclude? | Distinguish interpolation from extrapolation and evidence from an untested explanation |
Check your understanding
With a catalyst, a reaction reaches the same final volume of gas sooner. Which conclusions are correct?
Show the answer
The reaction is faster, and the final amount of product is unchanged.
Reaching the plateau sooner shows a faster rate. The plateau is at the same height, so the same amount of product formed.
Scope
Learning outcomes
Assessment Objective B: handling, applying and evaluating information. Use the outcome map to find the explanation for a particular syllabus requirement.
These labels map assessment skills and practical techniques, separately from the numbered theory outcomes. P, MMO, PDO and ACE are practical skill areas; T labels follow the listed techniques, and B labels follow H1 Assessment Objective B.
See the learning outcome map
B1 Organise relevant information
- Locate and present information from supplied sources
Read unfamiliar data with a purposeTurn measurements into a graph or calculation
B2 Select what matters
- Distinguish relevant evidence from extraneous detail
B3 Manipulate and translate data
- Move between numerical, symbolic and graphical forms
- Use units and suitable precision
B4 Find patterns and draw inferences
- Analyse trends and report conclusions supported by measurements
Turn measurements into a graph or calculationEvaluate evidence and explain the direction of an error
B5 Explain relationships
- Connect a measured pattern to a chemical mechanism or principle
B6 Apply chemistry to unfamiliar situations
- Use supplied definitions and familiar principles without relying on a familiar substance name
Read unfamiliar data with a purposeEvaluate evidence and explain the direction of an error
B7 Integrate knowledge
- Combine rate, reacting amount and measurement conditions in one argument
Read unfamiliar data with a purposeEvaluate evidence and explain the direction of an error
B8 Evaluate information and hypotheses
- Separate claim and evidence
- Recognise confounding variables, scatter and systematic error
B9 Justify a conclusion or action
- Use specific evidence and a causal chain
- Target an improvement to a real limitation
B10 Recognise model limitations
- State conditions and assumptions
- Limit a conclusion to the measured range and supported comparison
Turn measurements into a graph or calculationEvaluate evidence and explain the direction of an error