(i) Dilution of a strong concentrated acid is an exothermic reaction. Hence, we must add acid to water dropwise to control the heat evolved in the reaction. If you add water to acid, enormous amount of heat will be released causing the acid to splash out and cause burns.
(ii)The salt formed is sodium sulphate. Its pH is 7 as it is a neutral salt formed by the neutralisation of a strong acid and a strong base.
(iii) Only the ions are responsible for the pH of a substance. Hydrogen ions in HCl are produced in the presence of water. The separation of H+ion from HCl molecules cannot occur in the absence of water. Hence, dry HCl does not give characteristics of an acid as H+ions are not present in it.