Worked example · Science · Year 10
DCF: Producing → Creating digital content
The actual report, written out in full, as a secure Year 10 pupil would hand it in. Every number in it comes from the raw results on the stimulus sheet, so you can check any cell with a calculator. Show it after pupils have processed their own table — the processing is the part that carries the marks.
The effect of hydrochloric acid concentration on the rate of reaction with calcium carbonate
1. Aim
To find out how the concentration of hydrochloric acid affects the rate at which carbon dioxide is produced when the acid reacts with marble chips.
2. Hypothesis
As the concentration increases, the rate will increase. A more concentrated acid contains more hydrogen ions in the same volume of solution, so there are more successful collisions with the surface of the marble chips every second.
3. Variables
4. Method
Meets criterion 1: six numbered sections, each one a real heading rather than a bold sentence, so a contents list can be generated automatically. The method is in the past tense with no "you", and it is detailed enough to repeat.
5. Results
Table 1: processed times and rates of gas production
| Concentration (mol/dm³) | Repeats used (s) | Excluded | Mean time (s) | Rate (cm³/s) |
|---|---|---|---|---|
| 0.5 | 96, 99, 94 | — | 96.3 | 0.208 |
| 1.0 | 51, 48, 50 | — | 49.7 | 0.403 |
| 1.5 | 34, 33, 35 | — | 34.0 | 0.588 |
| 2.0 | 25, 26 | 71 s (repeat 3) | 25.5 | 0.784 |
| 2.5 | 20, 21, 19 | — | 20.0 | 1.000 |
Means were calculated with =AVERAGE(B2:D2), except at 2.0 mol/dm³ where only the two trusted repeats were used: =AVERAGE(B5:C5) gives 25.5. Rates were calculated with =ROUND(20/E2,3).
Meets criteria 2 and 4. The table is captioned and numbered, units sit in the headings and not in every cell, and the processing is shown — including the excluded reading, named in its own column rather than quietly dropped.
Figure 1: rate of gas production against acid concentration. Points are the five processed rates from Table 1; the dashed line is the line of best fit, drawn through the origin.
Meets criterion 3: numbered figure, both axes labelled with units, a caption that says what the line is, and a single line of best fit rather than dot-to-dot.
6. Conclusion and evaluation
The hypothesis was supported. Increasing the concentration from 0.5 to 2.5 mol/dm³ increased the rate of gas production from 0.208 cm³/s to 1.000 cm³/s, so the reaction became almost five times faster. Figure 1 shows the five points lying close to a straight line through the origin, which means the rate is directly proportional to concentration: doubling the concentration from 1.0 to 2.0 mol/dm³ roughly doubled the rate, from 0.403 to 0.784 cm³/s. Dividing each rate by its concentration gives 0.416, 0.403, 0.392, 0.392 and 0.400, which are all within about 6 per cent of each other — further evidence of proportionality. This happens because a more concentrated solution has more acid particles in the same volume, so there are more collisions with the marble surface per second.
The evidence is repeatable. Ignoring the anomaly, the three repeats at each concentration lay within 5 s of each other at 0.5 mol/dm³ and within 2 s at 1.5 and 2.5 mol/dm³, so the means can be trusted. The third repeat at 2.0 mol/dm³ took 71 s, nearly three times the other two, and the lab book records that the bung was not pushed in fully so gas escaped for the first 30 seconds. That is a method failure rather than natural variation, so it was excluded and the mean was taken from two repeats instead of three — which is the weakest row in the table and the run I would repeat first.
Two limitations remain. Reaction time on the stop clock is roughly ±0.5 s, which is only 0.5 per cent of the 96 s run at 0.5 mol/dm³ but 2.5 per cent of the 20 s run at 2.5 mol/dm³, so the fastest result is the least precise one. The reaction is also exothermic — the flask felt warm at 2.5 mol/dm³ — so temperature was not perfectly controlled at the top end. A data logger on a mass balance, recording every second, would remove the human reaction time and let me measure the gradient at the start instead of timing a fixed volume.
Meets criterion 5: the conclusion answers the aim with specific values, the evaluation judges repeatability with ranges in seconds and converts the timing uncertainty into percentages, and it names the weakest row instead of claiming the experiment "went well".