Which ions are responsible for the chemical properties of bases in their aqueous solution?

Identify $A, B$ and $C$ in the following reactions and write the chemical name of $A, B$ and $C$.
$(i)$ $HNO_{3} + NaOH \rightarrow \underline{A} + H_{2}O$
$(ii)$ $\underline{B} + Ca(OH)_{2} \rightarrow CaSO_{4} + 2H_{2}O$
$(iii)$ $HCl + \underline{C} \rightarrow KCl + H_{2}O$

$(a)$ Why should curd and sour substances not be kept in brass and copper vessels?

What is the $pH$ value of a basic aqueous solution?

Which one of the following can be used as an acid-base indicator by a visually impaired student?

Name the gas which is liberated when an acid reacts with a metal. Illustrate with an example. How will you test the presence of this gas?