Epsom salt is

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:

Enzymes that utilize $ATP$ in phosphate transfer require an alkaline earth metal $(M)$ as the cofactor. $M$ is

The incorrect order of solubility in water is

The covalent alkaline earth metal halide $(X = Cl, Br, I)$ is:

Give a brief explanation of the ionisation enthalpy of alkaline earth metals (Group-$2$) elements.