How is the concentration of a solution expressed in molarity?

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.

How is an aqueous solution with $pH = 2$ compared to an aqueous solution with $pH = 4$?

Calcium phosphate is present in tooth enamel. Its nature is

$A$ solution of a substance '$X$' is used for testing carbon dioxide. Identify '$X$'. What will be its reaction with carbon dioxide? Write a balanced equation for the reaction.

Equal volumes of hydrochloric acid and sodium hydroxide solutions of the same concentration are mixed. The $pH$ of the resulting solution is checked with a $pH$ paper. What would be the colour obtained? (You may use the colour guide given in the figure.)