The baker had forgotten to add baking powder. Baking powder is a mixture of baking soda (sodium hydrogencarbonate) and a mild edible acid such as tartaric acid. When this baking powder is added with water, then sodium hydrogencarbonate (NaHCO3) reacts with tartaric acid to evolve carbon dioxide gas. This CO2 gas causes the cake to rise and become soft and spongy.