$NH_{4}Cl$ is a salt of which type?

$(a)$ $A$ solution of a substance $'X'$ is used for testing carbon dioxide. Write the equation of the reaction of $'X'$ with carbon dioxide.
$(b)$ How is $'X'$ obtained? Write the chemical equation.

The marble statues are often slowly corroded when kept in the open for a long time. Give a suitable reason.

Which of the following substances will not give carbon dioxide on treatment with dilute acid?

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

Calcium phosphate is present in tooth enamel. Its nature is