Explanation

Baking soda (NaHCO₃) decomposes on heating to release carbon dioxide gas (2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O). The CO₂ bubbles get trapped in the dough, making bread and cakes soft and spongy. The sodium salt by-product can taste bitter, which is why recipes often pair baking soda with an acid such as tartaric acid (as in baking powder).