State whether the following statement is true or false:
When the $pH$ of rainwater is more than $5.6,$ it is called acid rain.

Which of the following is acidic in nature?

In the following schematic diagram for the preparation of hydrogen gas as shown in the Figure,what would happen if the following changes are made?
$(a)$ In place of zinc granules,the same amount of zinc dust is taken in the test tube.
$(b)$ Instead of dilute sulphuric acid,dilute hydrochloric acid is taken.
$(c)$ In place of zinc,copper turnings are taken.
$(d)$ Sodium hydroxide is taken in place of dilute sulphuric acid and the tube is heated.

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

$(a)$ Tooth enamel is one of the hardest substances in our body. Explain the changes in $pH$ of the mouth that indicate tooth decay. How does toothpaste help in preventing it?
$(b)$ What is the nature of the salt if the $pH$ of its solution is greater than $7$? Name the acid and base that would be used to prepare the following salts:
$(i)$ Potassium sulphate.
$(ii)$ Ammonium chloride.

Fill in the missing data in the following table:

Name of the salt	Formula	Base	Acid
$(i)$ Ammonium chloride	$NH_{4}Cl$	$NH_{4}OH$	-
$(ii)$ Copper sulphate	-	-	$H_{2}SO_{4}$
$(iii)$ Sodium chloride	$NaCl$	$NaOH$	-
$(iv)$ Magnesium nitrate	$Mg(NO_{3})_{2}$	-	$HNO_{3}$
$(v)$ Potassium sulphate	$K_{2}SO_{4}$	-	-
$(vi)$ Calcium nitrate	$Ca(NO_{3})_{2}$	$Ca(OH)_{2}$	-