Gypsum $CaSO_4 \cdot 2H_2O$ on heating to about $120\ ^\circ C$ forms a compound which has the chemical composition represented by :-

Given below are two statements,one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A:$ Beryllium has a less negative value of reduction potential compared to the other alkaline earth metals.
Reason $R:$ Beryllium has large hydration energy due to the small size of $Be^{2+}$ but a relatively large value of atomization enthalpy.
In the light of the above statements,choose the most appropriate answer from the options given below.

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

Choose the correct statements from the following:
$(I)$ In vapour phase $BeCl_2$ exists as a chlorobridged dimer.
$(II)$ $BeSO_4$ is readily soluble in water.
$(III)$ $BeO$ is completely basic in nature.
$(IV)$ $BeCO_3$ being unstable, is kept in an atmosphere of $CO_2$.
$(V)$ $BeCO_3$ is the least soluble among all the carbonates of group $2$ elements.

The correct sequence of increasing melting points of $BeCl_2$,$MgCl_2$,$CaCl_2$,$SrCl_2$,and $BaCl_2$ is:

The pair of elements that form both oxides and nitrides when burnt in air are