IB Chemistry HL Internal Assessment Example

The student clearly situates the investigation in an appropriate chemical and environmental context by linking salinity to dissolved oxygen and the tap-water setting. This works well because it gives the research question real-world relevance and justifies why sodium chloride is a meaningful independent variable.

The background still contains a hypothesis-direction inconsistency: the chemistry explanation describes salting-out, which should lower dissolved oxygen as salinity rises, yet the research framing risks implying the opposite. The student should align the stated hypothesis with the theory so the investigation is scientifically coherent from the start.

The student gives a strong conceptual explanation of the Winkler method, including oxidation of manganese hydroxide, acidification, iodide oxidation, and titration with thiosulfate. This is valuable because the chemistry is not merely named — it is linked to how dissolved oxygen is actually measured.

The independent variable is described as “% salinity,” but the procedure operationalises it as grams of NaCl per 100 cm³ of water. The student should use one consistent unit system throughout, or explicitly state the conversion, because ambiguity here weakens reproducibility and data interpretation.

The controlled-variables table is useful, especially for temperature and reagent volumes. To make the design more rigorous, the student should add pH monitoring and justify how it will be kept constant, since pH can affect dissolved oxygen measurement and interpretation.

The student should also specify a glassware rinsing and conditioning sequence for the bottles, burette, and pipette. This would reduce cross-contamination and improve validity, especially in an iodine/thiosulfate titration system where tiny residues can affect titres.

A stronger conclusion would briefly quantify the overall change in dissolved oxygen across the investigation, for example from the lowest to highest salinity. Including the numerical shift would make the final judgment more precise and more tightly linked to the processed data.

The comparison to literature remains qualitative. The student should compare the measured ppm values with published or expected values at similar conditions, because quantitative comparison would verify whether the results are plausible and would strengthen the scientific context section.

The student does make an attempt to connect the results to scientific literature on salinity and dissolved oxygen. This is useful because it shows awareness that the investigation should be interpreted against established chemical knowledge, not in isolation.

The conclusion relies on “R^2 = 0.7377” as proof, but R² alone does not prove causation or validate the hypothesis. The student should use the actual trend in the data, alongside the correlation coefficient, to make a more cautious and evidence-based claim.

The conclusion directly conflicts with the data by claiming the calculations “prove” a salinity increase in dissolved oxygen, even though the correlation is negative and the plotted trend decreases. The student must correct the direction of the claim and make the conclusion match the analysis.