Final answer:
The bubbles that make a cake light and tender come from the decomposition of carbonic acid (H2CO3) into water and carbon dioxide. The carbon dioxide gas produced creates bubbles inside the cake, making it fluffy.
Step-by-step explanation:
In baking, the bubbles that make a cake light and tender come from the decomposition of carbonic acid (H2CO3) in the cake batter. When an acidic ingredient in the cake reacts with sodium bicarbonate, carbonic acid is formed. This weak acid can then decompose into water and carbon dioxide, especially at high temperatures during baking. The carbon dioxide gas produced creates bubbles inside the cake, making it fluffy.