Lesson 1 of 8

Read unfamiliar data

Select evidence and connect it to a chemical principle.

O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)

I can explain investigatory problems.Syllabus
Syllabus K326 / K328, Practical(d). Investigatory problems

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.

From information to an answer

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.

Answer

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, the more concentrated acid gives a higher measured reaction rate. More reacting particles per unit volume cause more frequent collisions with the metal surface." State the controlled conditions and explain the observed trend, rather than merely repeating that a graph is higher.

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?

Temperature, concentration and reaction time, after checking which one was changed
Every column, since the examiner put each one there to be used in the answer
Reaction time alone, since rate is calculated from time and nothing else

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.

Exam-style question

Data-based question
A fizzy vitamin tablet contains sodium hydrogencarbonate, NaHCO₃, and a solid acid called citric acid. When the tablet is dropped into water, both substances dissolve and react. The gas that forms bubbles out of the water.
3NaHCO₃(aq) + C₆H₈O₇(aq) -> Na₃C₆H₅O₇(aq) + 3H₂O(l) + 3CO₂(g)
A student placed a flask containing 100 cm³ of water at 25 °C on a balance. She added one whole tablet, plugged the flask with cotton wool and recorded the loss in mass every 20 s. She repeated the experiment with an identical tablet that had been crushed into a powder. Relative atomic masses: H 1, C 12, O 16, Na 23.
Table 1. Loss in mass of the flask
time / s 0 20 40 60 80 100 120
whole tablet: loss in mass / g 0 0.18 0.33 0.45 0.54 0.60 0.62
crushed tablet: loss in mass / g 0 0.40 0.56 0.61 0.62 0.62 0.62
(a) Name the gas that bubbles out, and describe a test for it with the result. [2]
(b) Calculate the average rate of loss in mass for the whole tablet over the first 40 s. Give the unit. [2]
(c) Use the data to compare how the whole tablet and the crushed tablet react. [2]
(d) Explain, in terms of particles, why the crushed tablet reacts faster. [2]
(e) Assume that all the loss in mass is carbon dioxide. Calculate the smallest mass of sodium hydrogencarbonate that the tablet could contain. [3]
(f) Suggest why the real mass of carbon dioxide produced may be a little larger than the loss in mass. [1]
[12]

Table 1. Loss in mass of the flask
time / s	0	20	40	60	80	100	120
whole tablet: loss in mass / g	0	0.18	0.33	0.45	0.54	0.60	0.62
crushed tablet: loss in mass / g	0	0.40	0.56	0.61	0.62	0.62	0.62

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