(a) When an acid is added to a base, the acid nullifies the effect of the base, and conversely, the effect of an acid is nullified by a base to give a salt and water. Such a type of reaction between an acid and a base to give a salt and water is known as a neutralisation reaction.
The reaction can be represented as follows.
Acid + Base → Salt + Water
For example, when sodium hydroxide is treated with hydrochloric acid, the reaction can be written as follows.
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
(b) Dry HCl gas does not undergo dissociation to form ions, because of the absence of an aqueous medium, whereas the colour of litmus paper changes only in the presence of ions. Hence, dry HCl gas does not change the colour of dry litmus paper.
(c) A phenolphthalein indicator turns pink when added to an alkali (such as sodium hydroxide).