Why cake or bread swell on adding baking soda?

Baking soda is made up of sodium bicarbonate (NaHCO₃) and when it  mix  with an acidic ingredient (lemon juice, buttermilk, etc.) of recipe, the addition of baking soda to food items before cooking leads to the production of carbon dioxide (CO₂), and helps cookies, cakes, and other baked goods to rise and swell.. This complete process of rising of baked items is called chemical leavening.