Question

Addition of baking powder to cakes makes it soft and spongy why?

Answer 1

Because adding Baking powder into water or heated makes it to release Carbon Dioxide which makes the cake soft and spongy.
Eqn: NaHCO3 + H+ ---> CO2 + H2O

Answer 2

Baking Powder: Baking powder is a mixture of baking soda and a mild edible acid.

Chemical reaction: When baking powder is heated or mixed in water,CO2 is produced.This Carbon dioxide causes bread or cake to rise making soft and spongy.