Magnesium wire is heated in the atmosphere of $CO_2$ because

Which alkali metal carbonate decomposes on heating to liberate $CO_2$ gas?

Which of the following compounds is completely water soluble?

Which elements of group $1$ and $2$ differ from the other elements of the same group? Give a brief explanation of the diagonal relationship.

Identify the correct statements from the following :
$I$. Tendency to form halide hydrates gradually increases from $Be$ to $Ba$ down the group.
$II$. Tendency to form stable superoxides increases from $Li$ to $Cs$ down the group.
$III$. Low solubility of $LiF$ is due to its high lattice energy.
$IV$. Solubility of carbonates of group-$2$ elements increases down the group.

Which one of the following options is correct pertaining to the below transformations?
$Na + NH_{3(l)} \longrightarrow$ Coloured solution $\xrightarrow{\text{on standing}} X + Y$