Plan a fair inspired and expired air comparison
Control gas delivery and the chemical endpoint when comparing limewater observations.
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Recall the limewater positive result.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is a controlled variable?
What you will learn
- Match the two test conditions
- Plan safely and reduce uncertainty
Plan a fair inspired and expired air comparison
Match the two test conditions
For a teacher-approved comparison, use equal volumes of limewater at the same concentration and temperature in equivalent vessels. Compare inspired and expired air samples delivered at the same flow rate or equal measured gas volumes. Use the same stated cloudiness endpoint so the outcome reflects the air sample rather than a different amount of reagent.
A timing method records seconds to that endpoint at a fixed flow. A fixed-volume method compares cloudiness after equal gas volumes. These are different designs, so do not mix a time result from one with an uncontrolled gas-volume result from the other. Record the method, variables, observations and repeat results before drawing the conclusion.
Plan safely and reduce uncertainty
Use supplied data where the setup cannot be supervised. An approved practical must prevent limewater entering anyone’s mouth or lungs and must use appropriate separate equipment; never share mouthpieces. The teacher determines handling and hygiene. The written plan describes investigation reasoning without claiming that a learner has performed or passed a practical.
Repeat comparable trials, use a consistent endpoint and check that the starting limewater is clear. Different flow rates, reused cloudy reagent or subjective endpoints can bias timings. Keep the interpretation comparative unless calibration is supplied; equal apparatus does not automatically turn cloudiness into an exact carbon-dioxide percentage.
Sources for this lesson
Pause and explain
Which comparison best isolates air-sample type?
Worked example
One sample is delivered twice as fast as another. Is its shorter cloudiness time a fair concentration comparison?
Show the worked solution
- Delivery rate changes the amount entering per second.
- Keep rate equal before comparing timings.
Answer No; flow rate is a confounding variable.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →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.
Name the independent variable, dependent variable and two controls for a fixed-flow timing comparison.
Give me a hint
Distinguish sample, outcome and matched conditions.
Compare my reasoning
- Compare inspired with expired air.
- Measure time to a common cloudiness endpoint; match limewater volume/concentration and flow.
Independent: air-sample type. Dependent: time to cloudiness. Controls include limewater volume/concentration and gas flow rate.
Look for these in your work
- Name the variables
- Give meaningful controls
Explain why a fixed cloudiness endpoint and fresh clear reagent improve the comparison.
Give me a hint
Keep the start and finish comparable.
Compare my reasoning
- Fresh clear reagent gives comparable initial conditions.
- The same endpoint avoids timing different amounts of cloudiness.
They match the starting appearance and the observation criterion across trials.
Look for these in your work
- Explain both choices
- Link to fair measurement
An unsupervised learner proposes drawing limewater through a shared mouthpiece. Revise the investigation plan.
Give me a hint
Use supervised approved equipment or supplied observations.
Compare my reasoning
- Do not inhale chemicals or share mouthpieces.
- Use teacher-provided results or an approved setup that prevents liquid entry.
Use supplied observations, or a supervised approved apparatus with no chemical inhalation and separate appropriate equipment.
Look for these in your work
- Remove unsafe handling
- Retain a valid comparison
Common mistakes
- Changing both sample and flow rate.
- Reusing already cloudy limewater.
What can you explain now?
Which comparison best isolates air-sample type?
Compare with the explanation
Equal clear limewater and equal gas flow to one endpoint
The main other conditions are matched.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Design the comparison and identify one confounder.
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.