The investigation · Concentration and rate of reaction
How does the concentration of sodium thiosulfate (0.05 to 0.25 mol dm⁻³) affect the rate of its reaction with 1.0 mol dm⁻³ hydrochloric acid at 25 °C, measured by the disappearing-cross method?
Chemistry worked example: what becomes more explicit?
The comparison below is instructional, not examiner-scored. Use it to see which evidence or reasoning is missing, then check the current criterion wording for your session.
How does the example make research design evidence more explicit? (6-mark criterion)
The more explicit design shows ranges, apparatus uncertainty, controls, and the safety consideration specific to the simulated investigation.
Under-specified
I will change the concentration of thiosulfate and time how long the cross takes to disappear.
More explicit
Independent variable: Na₂S₂O₃ concentration, five values from 0.05 to 0.25 mol dm⁻³, made by dilution from a 0.25 mol dm⁻³ stock with a 10.0 cm³ graduated pipette (±0.05 cm³). Dependent variable: time for the cross to be obscured, converted to rate as 1/t. Controlled: total volume 50.0 cm³, HCl volume and concentration, temperature (25 ± 1 °C water bath), same observer and cross. Three trials per concentration; a fume cupboard is used because SO₂ is released.
How does the example make data analysis evidence more explicit? (6-mark criterion)
Raw times are processed into a rate, the relationship is tested quantitatively, and uncertainty is propagated rather than mentioned.
Under-specified
As the concentration went up, the time went down, so the reaction got faster.
More explicit
Rate (1/t) was plotted against concentration. The best-fit line through the origin has R² = 0.987, consistent with a first-order dependence on thiosulfate under these conditions. Percentage uncertainty in time was propagated from reaction-time judgement (±0.5 s), giving error bars of 2 to 6%; the steepest and shallowest lines give a gradient uncertainty of ±8%.
How does the example make conclusion evidence more explicit? (6-mark criterion)
It answers the question with the numbers, connects to theory, and states the limits of the claim.
Under-specified
In conclusion, higher concentration makes the reaction faster because there are more particles.
More explicit
Within 0.05 to 0.25 mol dm⁻³, rate increased linearly with thiosulfate concentration, supporting a first-order dependence. This fits collision theory: more particles per unit volume means more frequent successful collisions. The ±8% gradient uncertainty does not overlap with a second-order fit, but the conclusion only holds for this range and temperature.
How does the example make evaluation evidence more explicit? (6-mark criterion)
Each weakness is tied to the specific data it affects, with a realistic improvement.
Under-specified
It was hard to see when the cross disappeared. Using a sensor would be better.
More explicit
Judging the end point by eye was the largest random error, worst at low concentrations where the solution clouds slowly. A light sensor and data logger measuring absorbance against time would remove observer judgement. Temperature fell by up to 1 °C during slow runs, which would slightly underestimate rate at 0.05 mol dm⁻³.
How do you evaluate each criterion?
Compare the strength and relevance of evidence, reasoning, and evaluation against the criterion's mark-band descriptors.
- Changing two variables at once (concentration and total volume).
- Reporting times without converting to a rate.
- Mentioning uncertainty without propagating it.
- Ignoring the safety issue specific to the chemicals used.
- Going past the 3,000-word maximum.
Where can you find official IB Chemistry IA exemplars?
Marked Chemistry exemplars with moderator comments are in the IB Programme Resource Centre, which your school accesses. Ask your teacher for the pack matching the first-assessment-2025 criteria.
Sources & verification
Primary-source references used to verify key assessment facts on this page. Last source check: September 24, 2026.
This Chemistry example is original simulated iBacalao writing, not an official exemplar or real student work. Figures and scenarios shown here are teaching data, not sources for a student assessment. Check the current subject guide for your assessment session.
- IB — Diploma Programme assessment and examsOfficial overview of criterion-related internal assessment.
- IB — current curriculum updatesOfficial hub for current and upcoming assessment changes.
See where your own draft lands on each criterion.
iBacalao reviews your Scientific Investigation (IA) criterion by criterion using the Chemistry criteria for your session. The first 3 lifetime checks are free.
Open the Chemistry IA grader →