To make a cake, you add baking powder to the cake mixture. Baking powder is a mixture of bicarbonate of soda, which is alkaline, and tartaric acid. When the baking powder dissolves in the runny cake mix, the chemicals in it react. As a result, carbon dioxide is released as a gas. The gas expands, forming bubbles in the cake mixture. This gives the cake extra volume as it bakes in the oven: it rises. The resulting texture is (or should be) light and fluffy. The chemical equation for this reaction is this: 2NaHCO₃ → Na2CO₃ + CO₂ + H₂O

Bicarbonate of soda (sodium bicarbonate, NaHCO₃)—also known as baking soda—is a salt composed of sodium ions and bicarbonate ions. Described as a leavening agent (a substance used in doughs or batters to lighten and soften them), it is a fine, white powder.