Which of the following is used for the dissolution of gold?

$A$ metal compound $'A'$ reacts with dilute sulphuric acid to produce a gas which extinguishes a burning candle. Identify the compound $'A'$ and the gas produced. Write a balanced chemical equation for the reaction if one of the compounds formed in the reaction is sodium sulphate.

What range of $pH$ is considered perfect for a fertile land?

According to the Bronsted-Lowry acid-base theory,the substance which accepts a proton $(H^+)$ from another substance is called ...........

Which ions are produced when caustic soda undergoes ionization in water?

$A$ metal carbonate $X$ on reacting with an acid gives a gas which when passed through a solution $Y$ gives the carbonate back. On the other hand,a gas $G$ that is obtained at anode during electrolysis of brine is passed on dry $Y$,it gives a compound $Z$,used for disinfecting drinking water. Identify $X$,$Y$,$G$ and $Z$.