How does changing the salinity of water by adding 0%, 10%, 20%, 30%, 40%, and 50% of Sodium Chloride affect the amount of dissolved oxygen in water, measured by Winkler's method?HL
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6
Official IB Result
17/24
General feedback
17/24
0
12
24
No overall summary is available for this report.
3.1·Suggestion
Page 6
Consider adding a brief explanation of why the chosen method is suitable for salinity work specifically, not just for DO generally. That extra line of justification would strengthen the methodological rationale by showing why the technique matches the research question.
3.2·Question
Page 11
What evidence was used to justify the salt range up to 50 g per 100 cm³, given that the later results show incomplete dissolution? A tighter justification here would help the student show that the chosen range was experimentally realistic.
3.3·Strength
Page 16
The raw-data table is clearly organised by trial and salinity, and the anomaly highlighting shows the student is actively screening the data rather than simply copying values into a table. That makes the processing traceable and easy to follow.
3.4·Weakness
Page 16
The anomaly removal is described only as “anomalie”/highlighted values, but no statistical rule is given. The student should define a criterion, such as a threshold in standard deviations or an interquartile rule, so readers can judge whether excluded values were removed objectively.
3.5·Strength
Page 19
The worked average calculation is a strength because it shows the exact arithmetic used to move from raw titres to processed values. This transparency helps the reader verify the calculations and reproduce the method.
3.6·Weakness
Page 20
The moles-of-oxygen calculation is not presented consistently with the preceding stoichiometric steps and later tables. The student should check that each intermediate quantity is carried through correctly and that the same value is used everywhere, because internal calculation consistency is essential for trustworthy processing.
3.7·Weakness
Page 22
There is an inconsistency in the molarity section: the calculation from gives one value, but the next line uses . The student should correct this chain of calculations and recheck the ppm conversion, since a single power-of-ten error can shift every processed result.
3.8·Strength
Page 24
The student goes beyond basic processing by calculating both correlation statistics and standard deviation. This is a strong analytical move because it shows attention to both trend strength and spread, rather than relying only on the graph.
3.9·Weakness
Page 24
The justification for pairing a curved fit with linear correlation statistics is still not fully convincing. If the student wants to use Pearson’s , they should explain why the relationship is sufficiently linear to warrant it, or alternatively choose a statistical tool that matches the visible pattern more closely.
3.10·Weakness
Page 28
The uncertainty table shows attention to apparatus precision, but the method appears to combine percentage uncertainties by simple addition rather than quadrature. The student should recalculate independent random uncertainties using quadrature and then carry the uncertainty through to the final DO values, not just the intermediate apparatus list.
3.11·Suggestion
Page 29
It would be stronger to show how the uncertainty in the gradient was derived from the extreme lines on the graph and then connect that back to the measured values. That would make the uncertainty analysis easier to follow and more clearly tied to the evidence.
3.12·Strength
Page 30
The strengths section appropriately identifies the Winkler method as a reliable technique for dissolved oxygen analysis. This is a sound evaluative point because it reflects understanding of why the method supports the investigation’s aims.
3.13·Suggestion
Page 31
The student should explain the expected gain from each proposed improvement, such as reduced oxygen ingress, lower endpoint uncertainty, or smaller temperature drift. Stating the likely measurable benefit would make the evaluation more convincing and practical.
3.14·Strength
Page 31
The evaluation identifies several concrete methodological limitations, including oxygen ingress, endpoint subjectivity, incomplete salt dissolution, and temperature/pressure effects. This specificity is helpful because it shows the student is evaluating the actual investigation rather than listing generic weaknesses.
3.15·Weakness
Page 31
The limitations are identified well, but their relative impact is still not fully developed. The student should rank which error source most affected validity or reliability, and estimate its likely direction and magnitude, so the evaluation goes beyond description into judgement.
3.16·Strength
Page 31
The suggested improvement of using BOD bottles is well matched to the stated oxygen-contamination problem. That is a strong example of an improvement targeted at a specific weakness rather than a generic fix.
3.17·Weakness
Page 32
The issue of incomplete dissolution is important, but the evaluation would be stronger if it explained why this is especially problematic for the 50 g trial rather than for all trials. The student should connect the limitation to its effect on the independent variable itself, since the intended salinity was not fully achieved.
3.18·Suggestion
Page 32
Consider adding a short feasibility note for each improvement, such as whether the school lab can realistically provide a colorimeter, water bath, or magnetic stirrer. That would show the student has thought about implementation, not just ideal methods.
Criteria A: Research Design
5/6
0
3
6
Criteria Strands
A.1Research question context
Excellent
A.2Methodological considerations
Good
A.3Methodology description
Excellent
Criteria Feedback
- You place the investigation in a clear and relevant environmental and chemical context, linking salinity, dissolved oxygen, and the chemistry of the Winkler method.
- Your methodology is detailed enough that another student could largely follow and repeat the procedure.
- You show strong awareness of important control variables and reliability issues such as temperature, reagent volumes, and repeated trials.
- Your hypothesis framing is scientifically inconsistent with the background chemistry, which weakens the coherence of the investigation.
- Some methodological decisions are not fully justified, such as the salinity range and the omission of pH control.
- A few procedural details that affect reproducibility, such as glassware conditioning and headspace control, are still missing.
1.1·Strength
Page 3
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.
1.2·Weakness
Page 3
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.
1.3·Strength
Page 6
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.
1.4·Weakness
Page 7
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.
1.5·Suggestion
Page 8
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.
1.6·Suggestion
Page 8
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.
Criteria B: Data Analysis
5/6
0
3
6
Criteria Strands
B.1Communication of data recording and processing
Excellent
B.2Consideration of uncertainties
Good
B.3Data processing quality
Good
Criteria Feedback
- Your data presentation is clear and well organized, with raw tables, processed values, and a graph that make the flow of analysis easy to follow.
- You show strong processing skills by converting titration data through stoichiometry into dissolved oxygen values and by including statistical treatment such as standard deviation and correlation.
- Your work demonstrates good awareness of uncertainty through apparatus uncertainties and an attempt to account for error in the analysis.
- Some processing steps are inconsistent, especially in the conversion from molarity to final dissolved oxygen values.
- Outlier handling is not based on a clearly defined rule, which affects the reliability of the processed means.
- Your uncertainty treatment is considered, but it is not fully propagated through to the final values in a rigorous way.
Criteria C: Conclusion
2/6
0
3
6
Criteria Strands
C.1Conclusion relevance and support
Poor
C.2Scientific context comparison
Good
Criteria Feedback
- You do make an attempt to connect your results to published scientific knowledge about salinity and dissolved oxygen.
- Your conclusion refers to the correlation results rather than ignoring the processed data entirely.
- Your final claim conflicts directly with the trend in your data, because the results show a decrease in dissolved oxygen as salinity rises.
- Your comparison with scientific context is qualitative rather than quantitative, so it does not strongly validate the plausibility of your results.
- You rely on the correlation coefficient and R² as proof rather than using them to support a cautious, evidence-based conclusion.
2.1·Suggestion
Page 33
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.
2.2·Weakness
Page 33
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.
2.3·Strength
Page 33
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.
2.4·Weakness
Page 33
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.
2.5·Weakness
Page 33
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.
Criteria D: Evaluation
5/6
0
3
6
Criteria Strands
D.1Methodological weaknesses
Good
D.2Suggested improvements
Good
Criteria Feedback
- You identify several specific limitations in the method, including oxygen ingress, incomplete salt dissolution, endpoint subjectivity, and temperature effects.
- Your suggested improvements are practical and directly linked to the weaknesses you identified.
- You show good understanding that changes to apparatus and procedure can improve reliability and validity.
- Your evaluation would be stronger if you explained which limitations had the biggest effect on the results.
- Some improvements are described well but not clearly linked to the size of the improvement they would produce.
- You could discuss feasibility and priority more explicitly so the evaluation becomes more analytical rather than mainly descriptive.