Study the Exhaled air contains Carbon Dioxide


To confirm that the exhaled air contains carbon dioxide.  




Respiration is a metabolic process in which an organism's living cells use oxygen to obtain energy in the form of ATP and release carbon dioxide as a byproduct of the oxidation of complex organic molecules. The breathing air rich in oxygen into the body is called inhalation, and releasing air rich in carbon dioxide is known as exhalation. It is a continuous process that occurs continuously throughout an organism's life.  

Compositions of air inhalation & exhalation 

Inhale air composition 

  • When we inhale, we can take in the atmospheric mixture of gases from the atmosphere through the nose.  
  • The atmospheric air contains a mixture of 78% nitrogen, 21% oxygen, 1% argon, 0.04% carbon dioxide, and 0.5% water vapour.  
  • Also, the composition may vary depending on the surrounding environment. For example, the underwater water air contains more oxygen or helium than the other gases.  

Exhale air composition  

  • When we release the air out, the composition of air remains the same except for the changes in oxygen and carbon dioxide.  
  • The inhaled air contains 21% oxygen and 0.04% carbon dioxide, whereas the exhaled air contains 16.4% oxygen and 4.4% carbon dioxide.  
  • This is because the inhaled oxygen is utilized by cells, and for exchange, it gives out excess carbon dioxide as a by-product.  

                                                                       Fig.1 Demonstration of compositions of air (Inhalation & Exhalation) 


The process of inhalation and exhalation occurs in the lungs. In lungs, the alveolar membrane helps in the removal of carbon dioxide and other gases from the pulmonary capillaries and simultaneously, the oxygen enters the bloodstream for vital cellular functions. The inhaled oxygen is utilized by cells, giving out excess carbon dioxide as a by-product.  


                                                                                Fig.2 Demonstration of exhaled air containing carbon dioxide  

  • The exhalation of air containing more carbon dioxide can be demonstrated in a real lab using a lime water experiment.  
  • Lime water is a solution of calcium hydroxide (Ca (OH)2). When exhaled air is blown into a solution of lime water, it turns into milky white due to the reactions of carbon dioxide. 
  • The carbon dioxide in the exhaled air reacts with the calcium hydroxide to produce calcium carbonate, which shows a milky white appearance in the solution.  

                                                                         CaOH2 + CO2 -------> CaCO3 + H2O

                                                                  (Calcium hydroxide)      (Calcium carbonate) 


  • It is a diluted solution of calcium hydroxide (Ca (OH)2 ) at a saturated level.  
  • It is clear, colourless, with a slight earthy smell and has a bitter taste of calcium hydroxide. 
  • It is used to detect the presence of carbon dioxide gas (CO₂). 


Learning Outcomes

Students learn and understand:   

  • The process of respiration and compositions of inhaled air.  
  • The composition of exhaled air and the variations in both compositions. 
  • The experiment involved reactions through the demonstration.