Biology Extended Essay Exemplar: Do different concentrations (0%,… | RevisionDojo

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IB Biology SL Extended Essay Example

Do different concentrations (0%, 0.025%, 0.05%, 0.075%, 0.1%, 0.15%, 0.2%, and 0.25%) of three weak acid food preservatives - Sodium Benzoate, Potassium Sorbate and Calcium Propionate affect the growth of gram-positive bacteria - Bacillus subtilis, as measured by the diameter of zone of inhibition in mm over a period of 24 hours?SL

Official IB Result

23/34

Criteria A: Focus and Method

4/6

Criteria StrandsPro

A.1Topic communication and explanation

Good

A.2Research question formulation and focus

Good

A.3Methodology appropriateness and completeness

Moderate

Criteria Feedback

The student communicates a relevant biological investigation clearly and gives the reader enough context to understand the preservatives, organism, and general experimental purpose.
The student's research question is explicitly stated and operationalized with variables, organism, measurement type, and incubation time.
The student shows some awareness of methodological reliability through dilution tables, controls, repeated trials, and uncertainty/standard deviation considerations.

The framing is not yet fully comprehensive because pH, which is central to weak-acid preservatives, is not measured or controlled.
Some methodological details needed for replication are missing, such as exact well volume, inoculum standardization, and how replicates were organized.
The investigation scope is fairly wide because it compares three preservatives across many concentrations without sufficiently isolating key confounding factors.

1.1·Weakness

Page 4• Click to view

This explanation overgeneralizes Gram-positive susceptibility. The student should avoid presenting it as a universal rule and instead qualify the claim with the specific preservative and conditions used, since weak-acid activity also depends strongly on pH and membrane transport.

1.2·Strength

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The research question is clearly operationalized: the student specifies the preservative range, organism, dependent variable, and time period. This gives the investigation a workable laboratory focus and makes the later method and data tables easy to interpret.

1.3·Weakness

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This sentence is left incomplete, and the key factor of pH is not fully developed in the investigation framing. The student should complete the thought and explain how pH affects weak-acid preservatives, because that is central to interpreting the results fairly.

1.4·Suggestion

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This is a strong point to build on. The student should mirror this same pH-aware framing for potassium sorbate and calcium propionate, and then state in the research question or method how pH will be measured or controlled.

1.5·Suggestion

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The procedure would be stronger if the student stated the exact volume placed in each well and how the bacterial inoculum was standardized. Those details are essential for reproducibility and for judging whether the diffusion results are comparable across trials.

1.6·Weakness

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This is conceptually too absolute for a diffusion assay. A zone of inhibition is an assay-dependent proxy, not a direct measure of total bacterial growth, so the student should be careful not to treat it as equivalent to intrinsic preservative potency.

1.7·Question

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How were the three wells allocated among the different preservatives and controls on each plate? Clarifying this would help the reader judge whether the design used true independent replicates or only technical repeats within the same plate.

1.8·Weakness

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This description is methodologically unclear because nutrient broth powder is not the same as ready-to-pour agar medium. The student should identify the actual medium used and explain the preparation accurately, otherwise the method is difficult to replicate.

1.9·Weakness

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This stock-preparation sentence is inconsistent with the later dilution table, because the listed volumes do not clearly match the stated concentrations. The student should show the calculations for one worked example so the concentration logic is transparent and verifiable.

1.10·Suggestion

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This is good evidence of attention to reliability. The student could strengthen the method further by stating whether each repeat was an independent plate and how many total plates were used per concentration, since that determines how convincing the replication really is.

Criteria B: Knowledge and Understanding

3/6

Criteria StrandsPro

B.1Application of source material

Moderate

B.2Use of terminology and concepts

Moderate

Criteria Feedback

The student uses a relevant range of microbiology and food-preservation sources to support the background of the investigation.
The student applies several scientific terms correctly, including weak-acid preservative, inoculum, agar diffusion, zone of inhibition, mean, and standard deviation.
The student's background knowledge generally helps explain why the chosen organism and preservatives are appropriate for the study.

Several scientific claims are too broad or not consistently supported by the sources the student cites.
The student sometimes uses terminology imprecisely, especially when interpreting inhibition zones and statistical language.
The discussion does not always connect background knowledge directly to the specific experimental conditions, especially where pH and diffusion effects matter.

2.1·Strength

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The student grounds the investigation in relevant microbiological knowledge and explains why this organism is appropriate. This helps connect the experimental choice to the biological context rather than listing a species without justification.

2.2·Strength

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This is a well-applied biological concept because it links weak-acid behavior to the specific preservative being studied. The student shows that the chemistry of the preservative matters, not just the nominal concentration.

2.3·Weakness

Page 7• Click to view

This claim is too sweeping. The student should distinguish between a general trend observed in the assay and a universal causal statement, because zone size is also influenced by diffusion rate and medium conditions.

2.4·Weakness

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The hypothesis is biologically plausible, but the wording is repetitive and imprecise. The student should state the predicted change in the measurable outcome more cleanly, for example by predicting a larger zone of inhibition as concentration increases.

2.5·Weakness

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The source material here is used descriptively, but the student does not yet connect these differences to the actual experimental conditions. Since pH is not controlled, the organism-specific and preservative-specific claims should be used more cautiously in the interpretation.

2.6·Weakness

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This reverses the usual interpretation of the test output and confuses tabulated and calculated p-values. The student should state the decision rule correctly and consistently, because statistical meaning depends on that distinction.

2.7·Weakness

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A high

R^{2}

shows the curve fits the data well, but it does not by itself prove a strong causal relationship. The student should interpret it as evidence of fit, then support causation with experimental design and biological reasoning.

2.8·Weakness

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This sentence contains a logical contradiction: a larger inhibition zone means less growth, not more. The student should correct this immediately, because it undermines the scientific accuracy of the discussion.

2.9·Weakness

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The conclusion overstates what the assay proves. Because diffusion and pH were not controlled, the student should frame the result as a comparison within this method rather than as definitive proof of overall preservative effectiveness.

Criteria C: Critical Thinking

10/12

Criteria StrandsPro

C.1Research analysis

Poor

C.2Discussion and evaluation

Poor

C.3Argument development

Poor

Criteria Feedback

The student builds a recognizable overall argument from background to conclusion, so the report is easy to follow.
The student processes the data into means, standard deviations, graphs, and comparisons, which gives their discussion a quantitative basis.
The student does attempt biological explanation rather than only describing the numbers, and they acknowledge at least some limitations in the assay.

The student's analysis is weakened by inconsistent interpretation of zone size, p-values, and trendlines.
The evaluation of the data is limited because important constraints such as pH, diffusion differences, and multiple-comparison issues are not handled rigorously.
Several conclusions are stated too strongly for the method used, so the discussion sometimes overreaches what the evidence can actually support.

3.1·Strength

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The raw data are presented in a way that allows the reader to trace how the later means were derived. This is a strong foundation for analysis because it makes the processing steps auditable.

3.2·Weakness

Page 18• Click to view

This explanation is acceptable in broad terms, but the calculations should match the stated formula. The student should check whether the sample or population formula was intended, because the reported values must be consistent with the method used.

3.3·Weakness

Page 19• Click to view

The student needs to distinguish between statistical significance and hypothesis acceptance more carefully. A p-value alone does not prove the alternative hypothesis; it only supports rejecting the null at the chosen threshold.

3.4·Strength

Page 19• Click to view

The student processes the data into means and standard deviations, which is appropriate for comparing trends across concentrations. This makes the discussion more quantitative than descriptive.

3.5·Weakness

Page 20• Click to view

This reasoning is too strong. Small error bars suggest limited spread, but they do not automatically prove reliability, and a t-test table does not on its own validate the entire dataset or design.

3.6·Weakness

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This mixes up what the statistical tests show. The student should explain why one comparison is non-significant while others are significant, rather than collapsing both outcomes into one blanket claim.

3.7·Strength

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This is a good example of biological explanation being used to interpret the data rather than merely restating it. The student is making a mechanistic link between chemical form and observed inhibition, which is exactly the kind of reasoning this criterion rewards.

3.8·Weakness

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This comparison may be plausible, but it is asserted too confidently without direct evidence from the specific experiment. The student should either cite a source that addresses propionate directly or phrase the point more cautiously as a possible explanation.

3.9·Question

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If pH-dependent activity is central to the explanation, why was pH not measured or controlled in the method? Reflecting on that mismatch would help the student judge how far the comparison can really go.

3.10·Weakness

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The student should not overclaim from pairwise tests alone, especially because no correction for multiple comparisons is shown. The analysis would be stronger if it acknowledged that repeated testing inflates the chance of false positives.

3.11·Weakness

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The trendline shown is polynomial rather than linear, so the student should not call it a linear relationship. Using the correct statistical language will make the argument more credible.

3.12·Weakness

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This sentence would benefit from a clearer explanation of what the comparisons mean biologically. The student should state whether the differences are large enough to matter in practice, not just statistically significant.

3.13·Weakness

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This is too definitive. The student should treat

R^{2}

as a measure of fit, not proof that concentration is the only or even the main determinant, because the design does not isolate pH or diffusion effects.

Criteria D: Presentation

2/4

Criteria StrandsPro

D.1Structure appropriateness

Excellent

D.2Layout and formatting

Good

Criteria Feedback

The student's report has a clear overall structure, with the main sections appearing in a sensible academic order.
Headings, tables, graphs, and page numbering help the reader move through the investigation.
Most of the document is readable and organized well enough to follow the argument.

Some table references and numbering become inaccurate, which makes cross-checking harder for the reader.
Formatting is not always polished consistently, with occasional spacing, font, and cross-referencing issues.
A few presentation choices reduce clarity, especially where graphs or captions could explain the data more explicitly.

4.1·Suggestion

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The title communicates the topic well, but it could be tightened to sound more investigative and specific. A sharper title would help signal the exact variable relationship being tested.

4.2·Strength

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The report follows a logical academic structure, moving from introduction to methodology, data analysis, discussion, and conclusion. This organization helps the reader navigate the investigation and understand how the evidence builds toward the final claim.

4.3·Strength

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Section headings are clear and the overall sequencing is easy to follow. This supports readability because the reader can identify where data processing ends and interpretation begins.

4.4·Strength

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The use of a labeled table for dilution planning is effective because it makes the concentration series easy to trace. This is good presentation practice for an experimental report.

4.5·Weakness

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Calling this a sample of raw data can be fine, but the student should make the relationship between the sample table and the full appendix clearer. Otherwise, the reader may not know whether this table is representative or complete.

4.6·Weakness

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The figure is useful, but the accompanying explanation should state more explicitly what the error bars represent and how to read them. That would make the graph easier to interpret for a reader who is not already familiar with the analysis.

4.7·Weakness

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The numbering and cross-referencing become difficult to follow here, especially because the conclusion later refers to a different table number. The student should check every table reference so the reader can move through the report without confusion.

4.8·Weakness

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The cross-reference here is inaccurate and the sentence is awkwardly phrased. The student should correct the table number and rewrite the sentence in a clearer, more formal style.

4.9·Suggestion

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This is a good place to be more precise in style and terminology. The student should use consistent statistical notation and avoid turning a result into a claim of reliability without explaining what aspect of the data it supports.

Loading scores...

Official IB Result

23/34

Criteria A: Focus and Method

4/6

Criteria StrandsPro

A.1Topic communication and explanation

Good

A.2Research question formulation and focus

Good

A.3Methodology appropriateness and completeness

Moderate

Criteria Feedback

The student communicates a relevant biological investigation clearly and gives the reader enough context to understand the preservatives, organism, and general experimental purpose.
The student's research question is explicitly stated and operationalized with variables, organism, measurement type, and incubation time.
The student shows some awareness of methodological reliability through dilution tables, controls, repeated trials, and uncertainty/standard deviation considerations.

The framing is not yet fully comprehensive because pH, which is central to weak-acid preservatives, is not measured or controlled.
Some methodological details needed for replication are missing, such as exact well volume, inoculum standardization, and how replicates were organized.
The investigation scope is fairly wide because it compares three preservatives across many concentrations without sufficiently isolating key confounding factors.

1.1·Weakness

Page 4• Click to view

1.2·Strength

Page 4• Click to view

1.3·Weakness

Page 5• Click to view

1.4·Suggestion

Page 6• Click to view

1.5·Suggestion

Page 7• Click to view

1.6·Weakness

Page 7• Click to view

1.7·Question

Page 12• Click to view

1.8·Weakness

Page 12• Click to view

1.9·Weakness

Page 13• Click to view

1.10·Suggestion

Page 14• Click to view

Criteria B: Knowledge and Understanding

3/6

Criteria StrandsPro

B.1Application of source material

Moderate

B.2Use of terminology and concepts

Moderate

Criteria Feedback

The student uses a relevant range of microbiology and food-preservation sources to support the background of the investigation.
The student applies several scientific terms correctly, including weak-acid preservative, inoculum, agar diffusion, zone of inhibition, mean, and standard deviation.
The student's background knowledge generally helps explain why the chosen organism and preservatives are appropriate for the study.

Several scientific claims are too broad or not consistently supported by the sources the student cites.
The student sometimes uses terminology imprecisely, especially when interpreting inhibition zones and statistical language.
The discussion does not always connect background knowledge directly to the specific experimental conditions, especially where pH and diffusion effects matter.

2.1·Strength

Page 4• Click to view

2.2·Strength

Page 6• Click to view

2.3·Weakness

Page 7• Click to view

2.4·Weakness

Page 9• Click to view

2.5·Weakness

Page 10• Click to view

2.6·Weakness

Page 19• Click to view

2.7·Weakness

Page 20• Click to view

A high

R^{2}

2.8·Weakness

Page 24• Click to view

2.9·Weakness

Page 29• Click to view

Criteria C: Critical Thinking

10/12

Criteria StrandsPro

C.1Research analysis

Poor

C.2Discussion and evaluation

Poor

C.3Argument development

Poor

Criteria Feedback

The student builds a recognizable overall argument from background to conclusion, so the report is easy to follow.
The student processes the data into means, standard deviations, graphs, and comparisons, which gives their discussion a quantitative basis.
The student does attempt biological explanation rather than only describing the numbers, and they acknowledge at least some limitations in the assay.

The student's analysis is weakened by inconsistent interpretation of zone size, p-values, and trendlines.
The evaluation of the data is limited because important constraints such as pH, diffusion differences, and multiple-comparison issues are not handled rigorously.
Several conclusions are stated too strongly for the method used, so the discussion sometimes overreaches what the evidence can actually support.

3.1·Strength

Page 16• Click to view

The raw data are presented in a way that allows the reader to trace how the later means were derived. This is a strong foundation for analysis because it makes the processing steps auditable.

3.2·Weakness

Page 18• Click to view

3.3·Weakness

Page 19• Click to view

3.4·Strength

Page 19• Click to view

The student processes the data into means and standard deviations, which is appropriate for comparing trends across concentrations. This makes the discussion more quantitative than descriptive.

3.5·Weakness

Page 20• Click to view

This reasoning is too strong. Small error bars suggest limited spread, but they do not automatically prove reliability, and a t-test table does not on its own validate the entire dataset or design.

3.6·Weakness

Page 24• Click to view

3.7·Strength

Page 29• Click to view

3.8·Weakness

Page 30• Click to view

3.9·Question

Page 31• Click to view

3.10·Weakness

Page 31• Click to view

3.11·Weakness

Page 32• Click to view

The trendline shown is polynomial rather than linear, so the student should not call it a linear relationship. Using the correct statistical language will make the argument more credible.

3.12·Weakness

Page 33• Click to view

3.13·Weakness

Page 33• Click to view

This is too definitive. The student should treat

R^{2}

as a measure of fit, not proof that concentration is the only or even the main determinant, because the design does not isolate pH or diffusion effects.

Criteria D: Presentation

2/4

Criteria StrandsPro

D.1Structure appropriateness

Excellent

D.2Layout and formatting

Good

Criteria Feedback

The student's report has a clear overall structure, with the main sections appearing in a sensible academic order.
Headings, tables, graphs, and page numbering help the reader move through the investigation.
Most of the document is readable and organized well enough to follow the argument.

Some table references and numbering become inaccurate, which makes cross-checking harder for the reader.
Formatting is not always polished consistently, with occasional spacing, font, and cross-referencing issues.
A few presentation choices reduce clarity, especially where graphs or captions could explain the data more explicitly.

4.1·Suggestion

Page 1• Click to view

The title communicates the topic well, but it could be tightened to sound more investigative and specific. A sharper title would help signal the exact variable relationship being tested.

4.2·Strength

Page 2• Click to view

4.3·Strength

Page 3• Click to view

Section headings are clear and the overall sequencing is easy to follow. This supports readability because the reader can identify where data processing ends and interpretation begins.

4.4·Strength

Page 13• Click to view

The use of a labeled table for dilution planning is effective because it makes the concentration series easy to trace. This is good presentation practice for an experimental report.

4.5·Weakness

Page 16• Click to view

4.6·Weakness

Page 20• Click to view

4.7·Weakness

Page 26• Click to view

4.8·Weakness

Page 33• Click to view

The cross-reference here is inaccurate and the sentence is awkwardly phrased. The student should correct the table number and rewrite the sentence in a clearer, more formal style.

4.9·Suggestion

Page 34• Click to view