What you need to know
- Unit 3Comment on the relationship, discuss uncertainty qualitatively and quantitatively, and suggest realistic modifications.
Do not invent a critical value or a probability. Statistical tests are not required by these Unit 3 bullets. Core Practical conclusions stay tied to the data in front of you. No treatment advice follows from a school practical.
By the end, you should be able to…
- Write a conclusion that matches the relationship and no further.
- Discuss uncertainty in words and, when numbers are given, as a percentage.
- Suggest a modification that would actually improve the experiment.
Build your understanding
The conclusion stops at the evidence
Start from the relationship: as substrateThe molecule that binds to an enzyme’s active site and is changed in the reaction. concentration increased, the rate increased and then levelled off. Add a comparison if you have two conditions. Then stop, unless you are asked for a biological explanation. An explanation uses specification knowledge and must be labelled as an explanation, not as extra data.
Do not write “significant” unless a test and a critical value were supplied and you have compared them. Overlapping ranges, or a small difference between two means, are not that test. “Proves” is almost always too strong for one illustrated investigation.
Qualitative and quantitative uncertainty
Qualitative uncertainty is a description: the end-point was judged by eye, bubbles may have different volumes, or tissue slices differed in thickness. Name the measurement that is uncertain and the direction in which it could affect the result.
Quantitative uncertainty uses the numbers you were given. Percentage uncertainty = absolute uncertainty ÷ measurement × 100. Illustrative length 50 mm, read to ±1 mm: percentage uncertainty = 1 ÷ 50 × 100 = 2%. If a difference is calculated from two readings and each has an absolute uncertainty, add those absolute uncertainties before dividing by the difference. Do not invent an uncertainty the question did not state or a rule it did not give you.
A modification has to be realistic
The specification asks for realistic modifications to reduce error or improve the experiment. Name the limitation, then the change, then the improvement. “Use more accurate equipment” is not enough. “Use a gas syringe instead of counting bubbles, so volume is measured rather than estimated from bubble number” is a modification.
A better range, intermediate values around a possible optimum, a blank, a control tube, or a stated number of repeats are realistic school improvements. A modification that changes the independent variable as well has not improved the original test. Say what you would keep the same.
Make your reasoning visible
Claim, evidence, limit
One sentence for the pattern, one for a value or comparison, one for what the data cannot show. That shape answers most conclusion questions.
Percentage, not a feeling
If an absolute uncertainty is printed, calculate the percentage. Words such as “quite uncertain” do not replace the calculation.
Avoid this misconception
The claim: A clear trend means the result is statistically significant.
The correction: A trend is a description of the data. Significance needs a named test and a comparison with a critical value, which these Unit 3 tasks do not ask you to invent.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
An illustrative graph shows the rate of an enzyme increasing from 20 °C to 40 °C and falling at 50 °C and 60 °C. Write a conclusion that does not overclaim.
Show suggested answer
The rate increased as temperature increased from 20 °C to 40 °C, then decreased at the two higher temperatures tested. The data do not identify the exact optimum between 40 °C and 50 °C, and they are not a significance test.
Practice question
Retrieval and explanation · suggested answer
A burette reading is 12.50 cm³ ± 0.05 cm³. Calculate the percentage uncertainty to one decimal place.
Show suggested answer
0.05 ÷ 12.50 × 100 = 0.4%. The percentage uncertainty is 0.4% to one decimal place.
Practice question
Retrieval and explanation · suggested answer
Bubble counts were used for a photosynthesis rate. Suggest one realistic modification and the error it reduces.
Show suggested answer
Collect the gas in a gas syringe over the same stated time and record the volume. That measures volume instead of assuming every bubble has the same volume. Light, temperature and the plant should stay controlled so the modification does not create a new unfair test.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
An unfamiliar plant extract produced a zone of inhibition 8 mm wider than a water control on one plate. The question gives no repeat and no identification of the chemical. What conclusion is justified, what modification would you want, and what must you not claim?
Show a hint
One plate is a comparison, not a medicine.
Explore the reasoning
The extract inhibited the test bacterium more than water under these conditions, by 8 mm on this plate. That is one unreplicated result, so a repeat on further plates is the realistic next measurement. You must not claim that the extract is a safe or effective medicine, name a chemical that was not identified, or call the difference statistically significant. The assumption is that the control disc was water and that the organism and conditions were those stated in the question.
Recall this lesson
Lesson recall questions
Question 1 of 5: State the rule for significant figures when a mean rate is calculated from fictional times 12.2 s, 12.4 s and 12.1 s for 5.0 cm³ of oxygen.
Unit 3 processing results
Practice question
Retrieval and explanation · suggested answer
State the rule for significant figures when a mean rate is calculated from fictional times 12.2 s, 12.4 s and 12.1 s for 5.0 cm³ of oxygen.
Lesson: R1: Tables, units and processed numbers
Try this from memory first.
Beyond the classroomOne plate is not a treatmentEvidence · public health · Enrichment, not required recall
Antimicrobial resistance is a documented public-health problem. School disc-diffusion work can show a zone compared with a control. It cannot identify a new medicine or decide a clinical dose. Evaluation means staying inside the evidence.
Back to the spec: Unit 3 processing: the conclusion must match the relationship you were shown, and a modification should improve that experiment rather than change the claim into a bigger one.
World Health Organization: antimicrobial resistance fact sheetQuestion
Why is “this plant cures infection” not a valid conclusion from one illustrative zone of inhibition?
Explore a possible response
The zone shows inhibition of a test microbe on one plate under stated conditions. It does not show activity in a person, safety, dose, or activity against other microbes. Those claims need evidence the plate does not contain.