Which one of the alkaline earth metal carbonates is thermally the most stable?

Milk of lime is

Discuss the general characteristics and gradation in properties of alkaline earth metals.

$A$ compound $X$ on heating gives a colourless gas. The residue is dissolved in water to obtain $Y$. Excess $CO_2$ is bubbled through the aqueous solution of $Y$ and $Z$ is formed. $Z$ on gentle heating gives back $X$. The compound $X$ is:

An element reacts with hydrogen to form a compound $A$,which on treatment with water liberates hydrogen gas. The element can be

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