The $pH$ scale is applicable to which of the following?

$(a)$ 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 of the following statements is correct about an aqueous solution of an acid and of a base?
$(i)$ Higher the $pH$,stronger the acid
$(ii)$ Higher the $pH$,weaker the acid
$(iii)$ Lower the $pH$,stronger the base
$(iv)$ Lower the $pH$,weaker the base

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

$(i)$ Name the products formed when sodium hydrogen carbonate is heated.
$(ii)$ Write the chemical equation for the reaction involved in the above.

In a basic aqueous solution,if the concentration of $[OH^-]$ decreases,what happens to the $pOH$?