(a) A baker used baking soda instead of baking powder for baking the cake. Will this affect the taste of the cake? If yes how?
(b) Explain the conversion of baking soda into baking powder.
(c) What causes the cake to be spongy and soft?

If the baker uses baking soda instead of baking powder, the taste will be bitter as baking soda is bitter. This happens because of the formation of sodium carbonate.

$2NaHC{O}_3+Heat\to N{a}_{2}C{O}_3+C{O}_2+H_{2}O$

We can convert baking soda into baking powder by adding some tartaric acid. Baking soda is much stronger than baking powder. The $C{O}_2$ evolving baking powder tries to escape out of the dough. This $C{O}_2$ is responsible for making the cake spongy and soft. When baking powder mixes with water, sodium hydrogen carbonate reacts with tartaric acid to evolve $C{O}_2$ gas.

$NaHC{O}_3\left(aq\right)+H_2+\left(aq\right)\to Na+\left(aq\right)+C{O}_2\left(g\right)+H_{2}O\left(I\right)$

The $C{O}_2$ gas produced gets trapped in the wet dough and bubbles out slowly, causing the cake (or bread) to rise and become soft and spongy.