Concluding and evaluating: notes and practice questions
- This topic covers the skills required to draw and justify conclusions from experimental data and to critically evaluate an investigation's methodology.
- Interpret processed data and analysis to draw and justify conclusions.
- Compare outcomes of an investigation to the accepted scientific context.
- Relate outcomes to the stated research question or hypothesis.
- Discuss the impact of uncertainties on the conclusions.
- Evaluate hypotheses and identify sources of random and systematic errors.
- Evaluate methodological weaknesses, limitations, and assumptions.
How it is examined
Paper 1B, usually the last parts of a question. May 2025 HL Paper 1B TZ1 asked for two reasons for a difference between two students' timings, capped at 2 marks with a long list of acceptable answers and an explicit list of rejects. Another part asked for an error plus how to reduce or eliminate it, marked as a paired Error/Reduce alternative worth 2 marks: the improvement mark is tied to the error named, so a vague error kills both marks. Command terms are `Suggest`, `Evaluate`, `Explain` and `Discuss`. Tariff 1 to 3 marks.
- Interpret processed data and analysis to draw and justify conclusions.
- Compare the outcomes of an investigation to the accepted scientific context.
- Relate the outcomes of an investigation to the stated research question or hypothesis.
- Discuss the impact of uncertainties on the conclusions.
No additional higher level content in Inquiry 3.
Guiding questions
- None given.
Linking questions
- Reactivity 1.1 Why do calorimetry experiments typically measure a smaller change in temperature than is expected from theoretical values?
- Reactivity 2.1 What errors may cause the experimental yield to be i) higher and ii) lower than the theoretical yield?
- Reactivity 2.2 How can graphs provide evidence of systematic and random error?
Practice questions
3 questions · 3 hardQuestion 1
HardPaper 1B · calculator12 marksA student investigates the decomposition of hydrogen peroxide, H₂O₂, in a commercial antiseptic solution when exposed to light over a period of 7 days. The reaction is:
The concentration of H₂O₂ is determined by titration with a standard solution of acidified potassium manganate(VII), KMnO₄. Several identical flasks are prepared. One flask is kept in darkness, while others are exposed to a light source for different durations each day.
(a) Identify two variables, other than the volume of the solution, that should be controlled in this experiment.
(b) (i) The antiseptic solution is diluted with deionized water before titration. Suggest why this is necessary.
(b) (ii) Identify a possible systematic error associated with the control sample kept in darkness.
(b) (iii) Suggest how the experimental setup could be improved to check if the systematic error identified in (b)(ii) is significant.
(c) The following data are collected for one of the titrations:
Final burette reading =
Initial burette reading =
Calculate the percentage uncertainty of the titre.
(d) The initial concentration of H₂O₂ in all flasks was . The concentration after 7 days was measured for each flask.
| Daily light exposure / hours | Final [H₂O₂] after 7 days / |
|---|---|
| 0 (darkness) | 0.85 |
| 1 | 0.76 |
| 2 | 0.61 |
| 4 | 0.34 |
(i) Calculate the average rate of decrease in hydrogen peroxide concentration, in , for the sample exposed to light for 4 hours daily over the 7-day period.
(d) (ii) The student's hypothesis is: "A greater decrease in hydrogen peroxide concentration will be observed in solutions exposed to light for longer durations, due to photochemical decomposition."
Discuss, with reference to the data, the extent to which the results support this hypothesis.
(d) (iii) State one implication of the results for the storage of hydrogen peroxide solutions.
(e) Suggest a relevant extension to this investigation that would provide further information on the stability of hydrogen peroxide solutions.
Think about the factors that could affect the rate of a chemical reaction. Which of these should be kept constant to ensure a fair test comparing the effect of light exposure?
Consider the practical aspects of a titration. What problems might arise if the solution being titrated (the analyte) is very concentrated?
A systematic error is a flaw in the experimental design that causes results to be consistently incorrect in one direction. How might the 'darkness' condition not be perfectly achieved?
Think about how you could make the 'darkness' condition more robust or how you could measure the baseline decomposition without any experimental handling.
First, calculate the titre by subtracting the initial reading from the final reading. Then, find the absolute uncertainty of the titre by adding the uncertainties of the two readings. Finally, use the formula for percentage uncertainty.
The average rate of decrease is the total change in concentration divided by the total time. Use the data for the 4-hour exposure.
A 'discuss' question requires you to consider both sides. Does the data show a trend that matches the first part of the hypothesis? Does the experiment provide conclusive evidence for the reason given in the second part of the hypothesis?
Based on your results, what is the best way to store a solution of hydrogen peroxide to minimize its decomposition?
Think of another variable, apart from light, that might affect the rate of decomposition of hydrogen peroxide. How would you investigate it?
Question 2
HardPaper 2 · calculator12 marksA student investigated the effectiveness of three different brands of antacid tablets (Brand A, Brand B, and Brand C) in neutralizing hydrochloric acid, which simulates stomach acid. The active ingredient in all three brands is calcium carbonate, .
The following procedure was used for each brand:
1. One tablet was crushed using a mortar and pestle.
2. The crushed tablet was transferred to a conical flask containing of HCl(aq). This is an excess of acid.
3. The mixture was stirred for a fixed time until the reaction appeared complete.
4. The resulting solution was titrated with NaOH(aq) using a suitable indicator.
5. The experiment was repeated three times for each brand.
(a) State the independent and dependent variables for this investigation.
(b) Suggest two experimental conditions, other than the volumes and concentrations of the acid and alkali, that must be controlled to ensure a fair comparison between the brands.
(c) Suggest a reason for crushing the tablet before adding it to the acid.
The average results for the titrations are shown in the table.
| Brand of Antacid | Average volume of NaOH added / |
|---|---|
| A | 28.55 |
| B | 21.10 |
| C | 29.20 |
(d) Based on the data, deduce which brand of antacid is the most effective, giving a reason.
(e) Calculate the amount, in mol, of HCl neutralized by one tablet of Brand B.
(f) The manufacturer of Brand A claims their tablet contains of calcium carbonate, . Determine if the student's results for Brand A support this claim. The equation for the reaction is: .
The independent variable is the one the experimenter chooses to change. The dependent variable is the one that is measured in response to this change.
Consider factors that could influence the rate or extent of the reaction between the solid tablet and the acid.
Think about the factors that affect the rate of a chemical reaction involving a solid reactant.
The most effective antacid neutralizes the most acid. This means less acid will be left over to react with the NaOH in the back-titration.
This is a back-titration calculation. First, find the initial moles of HCl. Then, find the moles of NaOH used to titrate the excess HCl. The difference will be the moles of HCl that reacted with the antacid.
Use the data for Brand A to find the moles of HCl it neutralized. Then use the reaction stoichiometry to find the corresponding moles, and then mass, of . Compare this experimental mass to the manufacturer's claim.
Question 3
HardPaper 2 · calculator6 marksCaffeine is a stimulant found in tea leaves. A student carried out an experiment to extract caffeine from tea. The procedure involved adding solid sodium carbonate () to boiling water with tea leaves, followed by a liquid-liquid extraction using dichloromethane () as the organic solvent. The crude caffeine obtained after evaporating the dichloromethane was then purified by recrystallization.
(a) State one key property of a solvent required for a liquid-liquid extraction and suggest why dichloromethane is a suitable choice for extracting caffeine from the aqueous solution.
(b) The aqueous solution from the tea leaves is alkaline due to the presence of sodium carbonate. Suggest how the addition of sodium carbonate improves the separation of caffeine from other acidic compounds, such as tannins, also present in tea.
(c) During the final recrystallization step, the student cooled the hot solution of crude caffeine rapidly in an ice bath. Outline how this would affect the purity of the caffeine obtained.
(d) Dichloromethane is a chlorinated hydrocarbon. State one reason, other than its toxicity, why the use of such solvents is being phased out.
For a liquid-liquid extraction, you need two distinct layers. What does this imply about the solvents? Also, consider where the desired substance (caffeine) should preferentially dissolve.
Consider the reaction between a base (sodium carbonate) and an acid (tannins). How does this reaction change the properties, particularly the solubility, of the tannins?
Think about how crystals form. What happens if they form too quickly? Where might impurities get trapped?
Chlorinated hydrocarbons are known to affect a specific protective layer in the Earth's atmosphere. What is this layer?
No question on this page matches those filters. Try another difficulty or paper.
Every Concluding and evaluating question, marked for you
Every answer is marked mark by mark, IB-style, and the AI tutor helps when you are stuck.
Where marks are lost
- Reaching for "human error" or "only one trial." A source of error has to be a specific step in the method, not a general apology for the result.
- Joining the dots instead of drawing a curve.
- Naming a chemical instead of the property that distinguishes it, or vice versa. Answering with the nearest fact that comes to mind rather than the fact the command term and stem jointly ask for is a recurring way to answer a question that was not, quite, the one asked.
