Which gas is produced when a base reacts with a metal?

$(i)$ State the purpose of developing $pH$ scale.
$(ii)$ Mention the $pH$ range for acids.
$(iii)$ What is the $pH$ of acid rain and how it affects the aquatic life?

Which enzyme becomes active in the stomach during a low $pH$ value?

Sodium carbonate is a basic salt because it is a salt of :

Write true or false for the following statement:
The separation of $H^+$ ions from $HCl$ molecules can occur in the absence of water.

$(a)$ $A$ salt is produced by the reaction between an acid and a base. Identify the acid and the base from which the following salts have been formed:
$(i)$ $Na_{2}SO_{4}$
$(ii)$ $NH_{4}Cl$
$(iii)$ $KNO_{3}$
$(iv)$ $NaCl$