Compare inhaled and exhaled air
Compare inhaled and exhaled air using limewater and cobalt chloride paper.
On this page
Breathing · about 25–35 min
What you need to understand
Limewater → carbon dioxide. Blue cobalt chloride paper → water vapour.
Definitions
- limewater
a solution used to test for carbon dioxide
For example: Carbon dioxide turns limewater milky.- water vapour
water in the gas state
For example: Exhaled air contains more water vapour than inhaled air.
Key idea
- LookExhaled air is bubbled through limewater and it turns milky.
- ThinkLimewater → carbon dioxide. Blue cobalt chloride paper → water vapour.
- DoCompare the air samples and match each test to the substance it detects.
Review the full breathing topic picture
Explanation
Compared with inhaled air in the usual resting comparison, exhaled air contains less oxygen and more carbon dioxide. Some oxygen has passed into the blood and been used by cells in respiration. Carbon dioxide produced by cells has been carried in the blood to the lungs and passed into the air.
Air gains water vapour as it passes over moist respiratory surfaces. Exhaled air is therefore usually more moist than inhaled room air. The starting humidity and sample temperature affect this comparison. Both samples still contain oxygen, and nitrogen remains their largest component.
Carbon dioxide: clear limewater turns milky. Water vapour: dry blue cobalt chloride paper turns pink. These positive results identify the substances; neither test measures oxygen.
A positive result alone does not prove that air was exhaled: room air also contains carbon dioxide and can contain water vapour. To compare samples, use equal air volumes, equal fresh reagent amounts and matched sample conditions. If comparing the time to a change, use the same airflow rate. Observe the initial limewater cloudiness; passing a large excess of carbon dioxide can make it clear again.
Pause and say it: Limewater → carbon dioxide. Blue cobalt chloride paper → water vapour.
Common mistake
Tempting wrong idea: Exhaled air contains more oxygen than inhaled air.
Why it fails: In the lungs, some oxygen passes from the air into the blood, so less oxygen is breathed out than was breathed in. Exhaled air still contains some oxygen, because the body does not remove it all in one breath.
Use this instead: Some oxygen passes from inhaled air into the blood. Exhaled air therefore has less oxygen under this comparison, although some remains.
Practical work
What to show: Compare the air samples and match each test to the substance it detects.
Before you finish: Keep reagent amount and observation time the same too.
Practical: Compare inhaled and exhaled air
What evidence distinguishes inhaled and exhaled air?
For G1, know the test material and positive result. The apparatus and method below illustrate a fair comparison; detailed test procedures are not required by the syllabus.
Safety: This is a teacher-supervised investigation using school-approved apparatus. Do not share mouthpieces, inhale through reagents or touch cobalt chloride paper. The teacher should handle the chemicals and arrange their disposal.
- Change
- inhaled or exhaled sample
- Measure
- carbon-dioxide and water-vapour test result
- Keep the same
- same air volume, same reagent amount, same observation time
View the apparatus, method and sample results
Apparatus
teacher-approved breathing apparatus with one-way valves, limewater, cobalt chloride paper or safe humidity sensor, timer, supplied composition data.
Method
Use separate, hygienic mouthpieces and one-way valves; never inhale through reagents.
Pass equal volumes of room and exhaled air through separate limewater samples using approved apparatus.
Expose fresh cobalt chloride paper or a humidity sensor to each sample without skin contact.
Record time and degree of change using the same scale.
Compare with supplied oxygen-composition data rather than attempting unsafe oxygen tests.
| Sample | Oxygen / % | Carbon dioxide / % | Water-vapour indication |
|---|---|---|---|
| inhaled | 21 | 0.04 | lower |
| exhaled | 16 | 4 | higher |
Limitation: These are supplied approximate values, not measurements from the colour tests. Breathing depth, activity and sampling conditions affect real samples; room humidity also varies.
Improvement: Use calibrated gas sensors and standardise sampled volume after normal resting breaths.
6. Worked examples
Modelled example 1
Identify exhaled carbon dioxide
Problem
Exhaled air turns fresh limewater milky. What substance does this result identify?
Study the worked solution
Reason from the evidence
Method
Match the named test material and its visible change to the substance it detects.
Reason
Each prescribed test has a different positive result: limewater tests for carbon dioxide, while cobalt chloride paper tests for water.
Working
- Limewater is the prescribed carbon-dioxide test.
- A milky change is the positive result.
Compare with inhaled or room air under controlled flow if judging amount.
State the conclusion
Working
The observation supports carbon dioxide in exhaled air.
Guided practice 2
Choose the correct test and observation
Problem
A student must test one sample of exhaled air for carbon dioxide and another for water vapour. For each substance, name the test material and the positive observation.
Give two complete test → result pairs
Hints
Hint 1: carbon dioxide
Think of the clear solution that becomes cloudy.
Hint 2: water vapour
State both the starting and final colour of the indicator paper.
View solution step by step
Test for carbon dioxide
Method
Pass the sample through limewater.Reason
Limewater gives a visible positive result for carbon dioxide.
Working
Positive result: clear limewater turns milky.
Test for water vapour
Method
Expose dry blue cobalt chloride paper to the sample.
Reason
The colour change indicates water.Working
Positive result: blue cobalt chloride paper turns pink.
Common misconception 3
Exhaled air still contains oxygen
Learner response
“All inhaled oxygen is used, so exhaled air contains no oxygen and is mostly carbon dioxide.” Correct the comparison.
Use more, less and still contains
View solution step by step
Compare, do not erase
Method
Describe how the amounts change rather than claiming complete removal.
Reason
The body uses some inhaled oxygen and adds carbon dioxide and water vapour, but exhaled air is not pure carbon dioxide.
Working
Compared with inhaled air, exhaled air contains less oxygen, more carbon dioxide and more water vapour. It still contains oxygen and is not mostly carbon dioxide.
Examiner practice 4
Compare carbon dioxide fairly
Practice question
In a teacher-supervised investigation, fresh limewater is used to compare room air and exhaled air. Describe a fair comparison and predict the initial result. [4 marks]
Write a safe comparison, not direct mouth contact
View solution step by step
Set up the comparison
2 marksMethod
Use equal fresh volumes of limewater and pass separate air samples through them.
Reason
Separate apparatus avoids mixing samples; suitable tubing prevents direct contact with limewater.
Working
Prepare two labelled containers with equal limewater volumes and deliver room air to one and exhaled air to the other through separate tubing.
Control and predict
2 marksMethod
Pass equal air volumes at the same rate, then compare the initial cloudiness. Alternatively, use the same airflow rate and measure the time to the same initial cloudiness.
Reason
A controlled difference can be linked to the air sample rather than sample size.
Working
Exhaled air should turn the limewater milky faster or produce more milkiness because it contains more carbon dioxide than room air.
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Credit a workable, fair and safe comparison.
Challenge 5
Use two tests to identify an air sample
Problem
One sample is room air and one is exhaled air from a resting person. Equal volumes are tested at the same flow rate and temperature, with equal fresh reagent amounts. Sample X turns limewater milky and dry blue cobalt chloride paper pink faster than Y. Which is more likely to be exhaled air? Explain the two results and state what these tests do not measure.
Interpret each observation separately
Hints
Hint 1: two substances
Match each test to carbon dioxide or water vapour.
Hint 2: limit
Neither test directly measures oxygen.
View solution step by step
Combine the evidence
Method
Choose X and connect both changes to the expected composition.
Reason
Exhaled air normally contains more carbon dioxide and water vapour than inhaled or room air.
Working
X is more likely to be exhaled air. Its quick milky limewater result supports more carbon dioxide, and its cobalt chloride change supports water vapour. These tests do not directly show how much oxygen is present.
Guided practice
Try it with support
Compare inhaled and exhaled air for oxygen, carbon dioxide and water vapour, then name the two prescribed tests.
Use less or more rather than none or all.
Check the guided answer
Answer: Exhaled air has less oxygen and more carbon dioxide and water vapour. Carbon dioxide turns limewater milky; water vapour turns dry blue cobalt chloride paper pink.
Check: Exhaled air still contains oxygen and much nitrogen.
Practise and continue
Practise this
Why must equal air volumes be used when comparing limewater changes?
Need a hint?
Volume changes the amount of carbon dioxide delivered.
Check your answer
Answer: Equal volumes make the comparison fair because a larger sample could produce a stronger change even at the same carbon-dioxide concentration.
Check: Keep reagent amount and observation time the same too.
Think like a scientist
Why should supplied oxygen data be used instead of improvising an oxygen test with exhaled air?
Check the reasoning
Supplied or sensor data allow a safe comparison without flames or improvised breathing apparatus, which could create contamination or inhalation risks.
Remember: Use teacher-approved one-way apparatus for any breathing demonstration.
One-minute check
Hide the page. Compare the oxygen and carbon dioxide in inhaled and exhaled air.
Name the two gas tests and their positive observations. State which gas neither test measures.
Correct this common mistake: “Exhaled air contains more oxygen than inhaled air.”
Syllabus and review details
- SEC G1 Science 2027 · 2027
Content structure and syllabus content, PDF pages 8–17
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