Carbon Dioxide in Breath

Materials: ★★☆ Available in most school laboratories or specialist stores
Difficulty: ★★☆ Can be done by science teachers
Safety: ★★☆ Some safety precautions required to perform safely

Categories: Body Systems, Respiration and Photosynthesis, Sports Science

Alternative titles: Comparing Carbon Dioxide in Inhaled and Exhaled Air

Summary

This experiment compares the carbon dioxide content of inhaled and exhaled air by passing a person’s breath through tubes into test solutions. Limewater is used to detect carbon dioxide, showing the difference between inhaled and exhaled air.

Procedure

Set up two boiling tubes, each containing an equal amount of limewater.
Insert a delivery tube into each boiling tube so the ends dip below the liquid surface.
Connect both tubes to a T-piece mouthpiece so one tube is for inhaling and the other for exhaling.
Check that all bungs are secure and the delivery tubes are at the same depth in the liquid.
Breathe in gently through the apparatus, drawing air through one tube.
Without removing the mouthpiece, breathe out through the other tube, bubbling air through the second liquid.
Repeat inhaling and exhaling so that equal amounts of air pass through each liquid.
Observe and record color or turbidity changes in the two tubes.

Links

Carbon dioxide content of inhaled and exhaled air - Caroline Osborne:

To Investigate the Carbon Dioxide Levels of Inhaled and Exhaled Air - Limewater Test - Simple Science and Maths:

📄 Experiments to compare carbon dioxide content of inhaled and exhaled air - Bio Topics: https://www.biotopics.co.uk/humans/inhaledexhaled.html

Variations

Collect exhaled air in a bag and pass it into the liquid instead of breathing directly.
Compare results for resting breathing versus after exercise.

Safety Precautions

Do not inhale directly from containers with chemicals—only breathe through the mouthpiece setup.
Use fresh, clean mouthpieces for each participant to avoid sharing germs.
Limewater (calcium hydroxide solution) can irritate skin and eyes—handle carefully and wash spills immediately.

Questions to Consider

What changes are observed in limewater with exhaled air? (It turns cloudy, showing CO2 is present.)
Why must the same volume of air pass through each tube? (For a fair comparison of CO2 levels in inhaled vs. exhaled air.)
What is the approximate CO2 content of inhaled vs. exhaled air? (Inhaled air ~0.04%, exhaled air ~4%.)
How does this experiment demonstrate the role of respiration in humans? (It shows oxygen is taken in and carbon dioxide is released during breathing.)