What happens when a baking mixture containing baking powder is heated?

Hint: On heating the reaction mixture, the baking powder present in it undergoes reaction and produces gas. This gas makes the cake soft and spongy. Baking powder and soda are not the same components; they are different from each other.

Complete answer:
Baking powder is a mixture of acid reacting with alkali. Baking powders are made up of bases, acids, and some buffering materials which help in the prevention of early acid-base reactions. The commercially available baking powders contain baking soda or sodium bicarbonate along with one or many salts that can produce certain acidic reactions on dissolving in solvents.
In the baking mixture, the baking powder is used as a leavening agent. It enhances the texture of the baked food as well as leavens and increases the volume of the baked food. When in the mixture baking powder is added carbon dioxide is produced. This carbon dioxide makes the baked food spongy and fluffy.
 Baking powder and baking soda are slightly different from each other. Baking powder contains several ingredients mixed; the baking soda only contains one ingredient that is sodium bicarbonate. Baking powder does not react immediately when it is exposed to acids whereas baking soda reacts immediately with acids. The reaction duration of baking soda is very short and so when it is used in baking the products are not as fluffy as they should be. Whereas the reaction duration of baking powder is long because of which when it is used in baking the products turn out to be way fluffier and softer.

Note:
There are many more substances which can be used instead of baking powder for making the baked products fluffy. When baking soda is used the chemical reaction that occurs is: $ NaHC{O_3} + {H^ + } \to N{a^ + } + C{O_2} + {H_2}O $ . This carbon dioxide released is responsible for making baking foods fluffy.