Which of the following statements is true for acids?

Calcium phosphate is present in tooth enamel. Its nature is

The $.........$ of the molar concentration of $[H_3O^+]$ in an acidic aqueous solution is called the $pH$ of that solution.

Write balanced chemical equations for the following:
$(a)$ Calcium carbonate reacts with hydrochloric acid.
$(b)$ Calcium oxychloride reacts with hydrochloric acid.
$(c)$ Sodium hydroxide reacts with zinc.

$(i)$ State the purpose of developing $pH$ scale.
$(ii)$ Mention the $pH$ range for acids.
$(iii)$ What is the $pH$ of acid rain and how it affects the aquatic life?

Acid and base react with each other and form salt and water. What is this reaction called in scientific language?