Respiration and carbon dioxide test

The solution turns milky when exhaled air is bubbled through it.

Scientific name: Lime water reacts with carbon dioxide producing insoluble calcium carbonate.


It's a Gas 12 - Respiration

by Royal Society Of Chemistry

Science Experiment : Carbon Dioxide (Co₂) & Limewater (Chemical Reaction)

by Kane Homeschool

CO2 and Limewater.wmv

by wiscolt

Science Experiment : Carbon Dioxide and Limewater (Breath Test)

by Kane Homeschool

Testing for gas Carbon Dioxide

by Ong SK 1984

Testing for CO2 gas

by stephenstephenlow


Wear eye protection goggles and gloves.

Always follow general safety recommendations. Please note that conducting chemistry experiments you must comply with the relevant legal procedures in your country.

Reaction formula

3C + 2O2 → 2CO + CO2

CaCO3 + 2HCl → CaCl2 + CO2 + H2O

2CH3COOH + Na2CO3 → 2CH3COONa + CO2 + H2O

C6H12O6 + 6O2 → 6CO2 + 6H2O

Ca(OH)2 + CO2 → CaCO3 + H2O

Step-by-step instruction

  1. This experiment consists of two parts. Both of them are based on the reaction between lime water and carbon dioxide. To perform this demonstration you need to prepare lime-water solution, two cylinders, two watch-glasses, an oxygen source, two carbon roods, a gas-burner and tongs.
  2. Also you need to set up a special device for the second part of the experiment. It consists of a respirator connected by two tubes and flap with bottles filled by half with lime water. As you breath in the air is coming through the first bottle and as you breathe out it comes out of the second bottle.
  3. The first part of the experiment demonstrates the formation of carbon dioxide in a direct reaction between carbon and oxygen.
  4. Fill the glass cylinders with oxygen and cover them with the watch glass.
  5. Take the carbon rod with tongs and heat it on the gas burner.
  6. When it turns red-hot put it into the glass jar. The rod starts glowing bright.
  7. For the comparison put the second unheated carbon rod into the other gas jar. Nothing happens. 8.We suppose that the reaction between oxygen and carbon proceeds only in the first cylinder. To make sure of it add the solution of lime water to the first cylinders.
  8. In the first cylinder the solution turns cloudy. In the second cylinder it stays clear.
  9. In order to determine whether carbon dioxide is released during respiration put on the apparatus described above and breathe for some time.
  10. Observe the color change in the bottle connected with an exhaled air. It becomes milky pointing to the presence of carbon dioxide.

Scientific background

In the first part of the experiment the reaction between oxygen and carbon is demonstrated. Carbon is quite an inert material. It does not react with oxygen at normal conditions. But at high temperature it becomes possible to start the reaction. Heated carbon rod reacts with oxygen producing carbon dioxide. Glowing is the sign of the proceeding reaction. The presence of carbon dioxide could be detected by its action on lime water. Lime water is the name of a water solution of calcium hydroxide. Calcium hydroxide reacts with carbon dioxide forming insoluble calcium carbonate.

Similarly carbon dioxide produced during respiration could be determined using lime water and a special device. The air we exhale contains about 4-5% of carbon dioxide. So when it is bubbled through the lime water it turns the solution milky. At the same time the amount of carbon dioxide in the surrounding atmosphere is not enough for visible changes. The solution in the “inhaling” bottle stays clear.


Published on 18 January 2016

  • Fire
  • Heating with fire
  • Explosion
  • Poisoned gas
  • Organic
  • Electricity
  • Solution
  • Oxidation reduction
  • Color change
  • Precipitate
  • Gassing
  • Catalyst