What is "baking powder"? How does it make the cake soft and spongy?
Baking powder is a sodium salt called sodium bicarbonate (NaHCO3). It is a mixture of baking soda (sodium hydrogencarbonate) and a mild edible acid such as tartaric acid. When baking soda mixes with water, the sodium hydrogencarbonate reacts with the tartaric acid to produce carbon dioxide gas. The carbon dioxide gas produced gets trapped in the wet dough made for baking a cake or bread, and bubbles out slowly, and the cake or bread becomes soft and spongy.