Carbon dioxide is a simple molecule, but it plays important roles in your body, in our environment, and in our economy.

When you exhale, carbon dioxide is among the gases coming from your lungs because it is produced in the "burning" of  carbohydrates.

O₂(g) + CH₂O CO₂(g) + H₂O(l)

Some of the carbon dioxide from this "combustion" is not exhaled but is dissolved in body fluids.  It then reacts with water and can act as a buffer to control the pH of your blood and body tissues.

Carbon dioxide is also the end product of fermentation, the slow burning of carbohydrates.  When some beverages, such as beer and champagne, are made, the gas is allowed to dissolve in the liquid.  When a bottle of the beverage is opened, some of the dissolved CO₂ comes out of solution, and you see it as a foamy head on beer or bubbles in champagne.  The presence of dissolved carbon dioxide in many beverages is also part of the reason they are acidic.

Carbon dioxide bubbles in a carbonated beverage. (C. D. Winters)

More rapid burning of carbohydrates or of hydrocarbons, as in the burning of forests or the combustion of fossil fuels, is the source of an ever-increasing amount of carbon dioxide in our atmosphere.  The concentration of carbon dioxide has steadily increased since the beginning of the Industrial Revolution, and the increase is accelerating.  This added carbon dioxide has been implicated in the environmental phenomenon known as the greenhouse effect, and has been attributed to increasing the global temperature to potentially harmful levels. Carbon dioxide levels in Earth's atmosphere have increased to more than 350 parts per million today from 290 parts per million in 1890.

Carbon dioxide is in the top 20 chemicals made in the United States, where it is used as dry ice, in carbonation of beverages, and for refrigeration.

Kotz/Treichel:  Chemistry and Chemical Reactivity,  3/e,  pp. 666–667