Which statement is incorrect?

$(a)$ 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.

Which ions are responsible for the chemical properties of bases in their aqueous solution?

If a few drops of a concentrated acid accidentally spill over the hand of a student,what should be done?

What happens when $CO_2$ is passed through lime water in limited amount?

Complete and balance the following chemical equations:
$(a)$ $Ca(OH)_{2} + Cl_{2} \longrightarrow$
$(b)$ $CaSO_{4} \cdot 1/2 H_{2}O + H_{2}O \longrightarrow$
$(c)$ $Na_{2}CO_{3} \cdot 10H_{2}O \xrightarrow{\text{air}} \dots$