What is baking powder? How does it make the cake soft and spongy?

Five solutions $\mathrm{A}$, $\mathrm{B}$, $\mathrm{C}$, $\mathrm{D}$, and $\mathrm{E}$ when tested with universal indicator showed pH as $4$, $1$, $11$, $7$ and $9$ respectively, classify the solutions as given below.

Neutral

Five solutions $\mathrm{A}$, $\mathrm{B}$, $\mathrm{C}$, $\mathrm{D}$, and $\mathrm{E}$ when tested with universal indicator showed pH as $4$, $1$, $11$, $7$ and $9$ respectively, classify the solutions as given below. 

Strongly alkaline

Five solutions $\mathrm{A}$, $\mathrm{B}$, $\mathrm{C}$, $\mathrm{D}$, and $\mathrm{E}$ when tested with universal indicator showed pH as $4$, $1$, $11$, $7$, and $9$ respectively, classify the solutions as given below. 

Strongly acidic

Five solutions $\mathrm{A}$, $\mathrm{B}$, $\mathrm{C}$, $\mathrm{D}$ and $\mathrm{E}$ when tested with universal indicator showed pH as $4$, $1$, $11$, $7$, and $9$ respectively, classify the solutions as given below. 

(d) Weakly acidic

Five solutions $\mathrm{A}$, $\mathrm{B}$, $\mathrm{C}$, $\mathrm{D}$, and $\mathrm{E}$ when tested with universal indicator showed pH as $4$, $1$, $11$, $7$, and  $9$ respectively, classify the solutions as given below.

Weakly alkaline

A milkman adds a very small amount of baking soda to fresh milk. Why does he shift the pH of the fresh milk from slightly alkaline?