Use limewater to test for carbon dioxide
Recognise cloudiness as evidence from a chemical test rather than a measure of oxygen or water vapour.
Section B11.1 · Gas exchange in humans
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall the difference between observation and inference.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which gas is carbon dioxide?
What you will learn
- Understand the observed test
- State what the test cannot show
Use limewater to test for carbon dioxide
Understand the observed test
Clear limewater becomes cloudy when carbon dioxide is passed through it under the usual classroom test conditions. The cloudiness is a suspended solid formed by reaction with the dissolved calcium hydroxide. At this level, connect the observed change with carbon dioxide instead of calling the cloudiness a bubble of oxygen.
Inspired air contains some carbon dioxide, so it can eventually cloud limewater too. Expired air usually contains more carbon dioxide and clouds comparable limewater faster when equal gas delivery and other conditions are used. Neither observation establishes that expired air is pure carbon dioxide or has no oxygen.
State what the test cannot show
Cloudiness is not a direct test for water vapour, oxygen percentage or breathing depth. A comparison needs the same limewater volume and concentration, a consistent cloudiness endpoint and matched gas delivery. A faster result can support a comparative carbon-dioxide conclusion; a percentage concentration requires an appropriately calibrated method.
Use teacher-provided observations or an approved supervised apparatus. Limewater must not be inhaled or drawn into the mouth, and mouthpieces must not be shared. This written lesson interprets the test; it does not certify practical competence or ask learners to improvise a chemical breathing apparatus at home.
Sources for this lesson
Pause and explain
What does limewater cloudiness indicate in this standard test?
Worked example
A supplied gas sample turns initially clear limewater cloudy. What gas does the standard test indicate?
Show the worked solution
- Name the tested gas.
- Keep oxygen and water-vapour tests separate.
Answer Carbon dioxide is indicated under the stated test conditions.
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.
State the initial appearance, positive change and gas detected in a limewater test.
Give me a hint
Give the observation before the inference.
Compare my reasoning
- Initially the limewater is clear.
- Carbon dioxide makes it cloudy.
Clear → cloudy; carbon dioxide is indicated.
Look for these in your work
- State the observation
- Name the gas
Why can inspired air eventually cloud limewater in a controlled test?
Give me a hint
Inspired air is not carbon-dioxide-free.
Compare my reasoning
- It contains a small carbon-dioxide fraction.
- That gas can react with limewater.
Inspired air contains some carbon dioxide, so sufficient delivery can produce cloudiness.
Look for these in your work
- Acknowledge some CO₂
- Use the test mechanism
A cloudy tube is described as proof that all oxygen has been removed. Evaluate that inference.
Give me a hint
Identify what the test measures.
Compare my reasoning
- Cloudiness indicates carbon dioxide under these conditions.
- It does not measure the remaining oxygen.
The inference is unsupported; the test gives carbon-dioxide evidence, not an oxygen percentage.
Look for these in your work
- State the valid evidence
- Reject the unsupported gas claim
Common mistakes
- Calling cloudiness an oxygen test.
- Claiming inspired air has no carbon dioxide.
What can you explain now?
What does limewater cloudiness indicate in this standard test?
Compare with the explanation
Carbon dioxide
It reacts to form cloudiness.
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Make it stick
Describe the positive test and one conclusion it cannot establish.
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.