$(a)$ 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.

(N/A) Tooth decay starts when the $pH$ of the mouth falls below $5.5$. This occurs when bacteria present in the mouth produce acids by degrading sugar and food particles remaining in the mouth after eating. Using toothpaste,which is generally basic,helps neutralize the excess acid and prevents tooth decay.
$(b)$ If the $pH$ of a salt solution is greater than $7$,the nature of the salt is basic.
$(i)$ For Potassium sulphate $(K_2SO_4)$: Base is $KOH$ (Potassium hydroxide) and Acid is $H_2SO_4$ (Sulphuric acid).
$(ii)$ For Ammonium chloride $(NH_4Cl)$: Base is $NH_4OH$ (Ammonium hydroxide) and Acid is $HCl$ (Hydrochloric acid).