Carbon dioxide (chemical formula CO2) is a colorless gas with a density about 60% higher than that of dry air. Carbon dioxide consists of a carbon atom covalently double bonded to two oxygen atoms. It occurs naturally in Earth's atmosphere as a trace gas. The current concentration is about 0.04% (410 ppm) by volume, having risen from pre-industrial levels of 280 ppm. (extracted from Wikipedia)

Carbon dioxide, along with ethanol, is the result of fermentation in breadmaking process. The yeast or bacteria feeds of the sugars present in the dough and releases CO2, causing the dough to rise and, subsequentially, ferment, while chemical leaveners such as baking powder and baking soda release carbon dioxide when heated or if exposed to acids.