Leavening agents are used to make bread and other foods rise, trapping many little air bubbles in the food. This gives the final product a less dense, lighter consistency than it would have if leavening had not taken place. The various methods for leavening include biological and chemical reactions, as well as mechanically trapping air in a substance.
Chemical leavening 
These leavening agents work by releasing carbon dioxide gas as they react with water, heat, or acid in the food.
Biological leavening 
These leavening agents work by fermenting sugars in the food to produce carbon dioxide. The fermentation typically produces additional flavour compounds, such as the sour flavours in sourdough bread.
Mechanical leavening 
These leavening agents work either by releasing gas already trapped in the product, or by incorporating air into the product by beating / stirring. If a gas is used, it is usually carbon dioxide, although sometimes nitrous oxide is used (as in the case of canned whipped cream).