At which value of $pH$ of the inner side of the mouth does the decay of teeth take place?

Write balanced chemical equations for the following:
$(i)$ Carbon dioxide gas is passed through lime water.
$(ii)$ $NaOH$ solution is heated with zinc granules.

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

According to the Arrhenius theory,an acid is a hydrogen-containing compound that produces $..........$ in its aqueous solution.

Write the chemical equations for the reactions of zinc granules with:
$(a)$ Sulphuric acid
$(b)$ Hydrochloric acid
$(c)$ Aluminium chloride
$(d)$ Sodium hydroxide
$(e)$ Nitric acid

Dry $HCl$ gas does not change the colour of dry blue litmus. Give reason.