The compound $A$ on heating gives a colourless gas and a residue that is dissolved in water to obtain $B$. Excess of $CO_2$ is bubbled through aqueous solution of $B$,$C$ is formed which is recovered in the solid form. Solid $C$ on gentle heating gives back $A$. The compound $A$ is:

The compound formed when gypsum is dissolved in aqueous ammonium sulphate solution is:

Which of the following elements belongs to the $second$ group and $fifth$ period of the periodic table?

Given below are two statements: One is labelled as Assertion $A$ and the other is labelled as Reason $R$ :
Assertion $(A)$ : $BeCl_2$ and $MgCl_2$ produce characteristic flame.
Reason $(R)$ : The excitation energy is high in $BeCl_2$ and $MgCl_2$.
In the light of the above statements,choose the correct answer from the option given below :

How many of the following oxides are amphoteric?
$BeO, ZnO, Sb_2O_3, CO, CaO, SO_2, SO_3$

The most stable carbonate is