To confirm that the exhaled air contains Carbon dioxide.
Respiration
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 process of taking the air in is known as inhalation. The inhaled air contains more Oxygen that is used to create energy and less Carbon dioxide.
Whereas exhaling the air out is called exhalation. It is a continuous process that occurs all the time and throughout a organism's life. The exhaled air contains more Carbon dioxide and is produced as a waste to be expelled out and less Oxygen is produced.
Exhaled air contains Nitrogen, Carbon dioxide, Oxygen, as well as probably hundreds of other compounds. The only difference in the composition of the air when we exhale is a change in the proportions of Carbon dioxide and Oxygen.
As we breathe in, air enters our bodies. During this process air is taken in by the lungs. The removal of CO2 and other gases from the pulmonary capillaries to be exhaled and the exchange of oxygen into the pulmonary capillaries are made possible via diffusion in the alveoli.
Demonstration of exhaled air
The exhalation of air can be demonstrated in real lab using a lime water. Lime water is a solution of Calcium hydroxide (Ca(OH)2). When exhaled air is blown through Lime Water, the Carbon dioxide present in the exhaled air reacts with the Lime water and turns it into a milky solution, since the CO2 reacts with Ca(OH)2 and forms CaCO3 or Calcium Carbonate, which is insoluble and white in color.
CaOH2 + CO2 -------> CaCO3 + H2O
