(a) What is baking powder? How does it make the cake soft and spongy?
(b) In addition to sodium hydrogen carbonate, baking powders contain a substance $X$ . Name the substance $X$ . What is the role of substance $X$ in the baking powder?

Hint: Baking powder is a dry chemical leavening agent which is a mixture of a bicarbonate or carbonate and a weak acid. It is used to increase the volume and lighten the texture of the baked goods. It works by releasing carbon dioxide into a batter through an acid-base reaction, causing the bubble present in that wet mixture to expand and leave the mixture.

Complete answer:
(a) Baking powder is a mixture of tartaric acid and baking soda. When it is mixed with water, the sodium hydrogen carbonate reacts with tartaric acid and carbon dioxide gas is formed as a result of this reaction. This released carbon dioxide gas is trapped in the wet dough and bubbles out slowly making the cake soft and spongy.
(b) The substance $X$ is tartaric acid. It reacts with sodium carbonate formed and neutralizes it. Otherwise, breads and cakes will have a bitter taste.

Note:
Baking powder is used instead of yeast for end products where fermentation flavors are undesirable, where the batter lacks the elastic structure to hold the gas bubbles for more than a few minutes and to speed the production of the baked goods. As the carbon dioxide is released at a faster rate through the acid-base reaction than through the fermentation process, breads made by chemical leavening are called quick breads.