What Do We Know?
The air that you breathe contains a variety of gases. An inhaled breath of dry air is composed of approximately 78.0% nitrogen (N2), 21% oxygen (O2), 0.9% argon (Ar), 0.04% carbon dioxide (CO2) and minute amounts of other trace gases.
Fun Fact: Did you know that the air you exhale is different from the air you inhale? Since an exhaled breath has more carbon dioxide and water vapour that dry air, the percent contents are shifted. An exhaled breath is about 75.0% nitrogen, 16.0% oxygen, 0.9% argon, 4.0% carbon dioxide and 4.0% water vapor.
Why does the chemical composition of the air you breathe change while it is in your body? The change in air composition occurs because of the way that your body uses the oxygen that you breathe. Oxygen directly supports life because it is used to produce energy from the food that you eat through the process of cellular respiration. Therefore, the concentration of oxygen is lower in an exhaled breath than in an inhaled breath because you use some of the oxygen from the inhaled breath to keep your body working. Since cellular metabolism produces carbon dioxide that is carried to the lungs, the concentration of carbon dioxide in an exhaled breath is higher than in an inhaled breath. Also, the exhaled breath has picked up water vapor from the warm, moist linings of the lungs, trachea, and nose, so it usually contains more water vapor than the inhaled breath.
The chemical composition of the atmosphere is determined using chromatographic and spectroscopic techniques. For example, gas chromatography can be used to separate a sample of air into its various compounds and then each of the compounds can be identified using mass spectrometry.