Plan dilutions, controls and risk checks
Prepare a defensible concentration series, select a meaningful control and assess risks within a supervised practical plan.
Section Paper 5 · Planning, analysis and evaluation
Cambridge International Biology 9700, 2028–2030. Paper 5 assesses planning, analysis and evaluation. These original practice tasks are not Cambridge exam questions or mark schemes.
Before you begin
- Use biological vocabulary and basic calculations from the AS Biology course. Follow your school laboratory rules and teacher instructions for all practical work.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
In a dilution, what does the stated final volume mean?
What you will learn
- Describe serial or proportional dilution with volumes and concentrations.
- Choose a control that tests the role of the treatment.
- Link identified hazards to sensible precautions.
Make a concentration series
A proportional dilution mixes measured volumes to make a stated final concentration. A serial dilution transfers a fixed volume successively into fresh diluent, reducing concentration by a known factor at each step.
State the starting concentration, volumes, final volume and labelling method. Use clean equipment or suitable rinsing to avoid carrying one concentration into another.
Control and risk belong in the plan
A control omits or changes the factor being tested while keeping relevant conditions comparable. Explain what it establishes; a control is not simply another replicate.
Assess risk by considering hazard, likelihood and severity using the school’s process. State precautions and follow teacher guidance, product information and local rules.
Pause and explain
What should a control in a treatment investigation help establish?
Worked example
Prepare 10 cm³ of 0.20 mol dm⁻³ solution from 1.0 mol dm⁻³ stock by proportional dilution.
Show the worked solution
- Use C₁V₁ = C₂V₂.
- V₁ = (0.20 × 10) ÷ 1.0 = 2.0 cm³.
- Add diluent to a final total volume of 10 cm³.
Answer Measure 2.0 cm³ of the 1.0 mol dm⁻³ stock and add diluent to make 10.0 cm³ total. Label the final concentration and follow the teacher-approved procedure.
From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Use 4 cm³ of 0.5 mol dm⁻³ stock to make a 0.1 mol dm⁻³ dilution. Find the final volume.
Give me a hint
Use C₁V₁ = C₂V₂.
Compare my reasoning
- Final volume = (0.5 × 4) ÷ 0.1.
- Calculate 20 cm³.
The final volume is 20 cm³, so add diluent to the 4 cm³ stock until the total reaches 20 cm³.
Look for these in your work
- Equation applied
- Final total volume stated
Catalase is tested with and without hydrogen peroxide. Suggest a meaningful control.
Give me a hint
Change the presence of the tested substrate while keeping other parts comparable.
Compare my reasoning
- Prepare a comparable setup without hydrogen peroxide, or replace it with the same volume of water.
- Use the same enzyme volume, temperature and measurement interval.
- Explain that the control checks whether foam requires the substrate.
Use the same catalase volume and conditions but replace hydrogen peroxide with an equal volume of water; this tests whether the foam response depends on the substrate.
Look for these in your work
- Control condition specified
- Purpose explained
A plan uses a reagent whose hazard information indicates eye irritation. State a suitable risk-control approach.
Give me a hint
Use the school’s approved safety procedure and supervision.
Compare my reasoning
- Identify possible eye contact as the harm.
- Use the teacher-directed eye protection and handling procedure.
- Follow the school spill and disposal instructions.
Follow the school’s risk assessment: use the directed eye protection, handle the reagent as instructed and tell the teacher immediately about splashes or spills.
Look for these in your work
- Risk linked to hazard
- Precaution practical and supervised
Common mistakes
- Describing the added diluent as the final volume.
- Using the treated sample as the control when no comparison is made.
- Listing a precaution unrelated to the named hazard.
What can you explain now?
In a dilution, what does the stated final volume mean?
Compare with the explanation
The total volume after stock and diluent are combined
The stock volume is only part of the final mixture.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How do serial and proportional dilution differ, and what should a useful control demonstrate?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.