The substance not likely to contain $CaCO_3$ is

Assertion $(A)$: $MgO$, $CaO$, $SrO$, and $BaO$ are insoluble in water.
Reason $(R)$: In aqueous medium, the basic strength of $MgO$, $CaO$, $SrO$, and $BaO$ increases with an increase in the atomic number of the metal.
The correct option among the following is:

The metal $X$ is prepared by the electrolysis of fused chloride. It reacts with hydrogen to form a colourless solid from which hydrogen gas is released on treatment with water. The metal is

What is the purpose of adding gypsum $(CaSO_4 \cdot 2H_2O)$ to cement?

The commercial name for calcium oxide is

Write a note on hydration enthalpy of group $2$ elements (Alkaline earth metals).