Salt is produced by the reaction between an acid and a base. Identify the acid and the base from which ${\mathrm{Na}}_2{\mathrm{SO}}_4$ salts have been formed.

What happens when a solution of an acid is mixed with a solution of a base in a test tube? 

 (i) The temperature of the solution increases

 (Iii) The temperature of the solution decreases

 (iii) The temperature of the solution remains the same

(iv) Salt formation takes place