How is calcium hydroxide prepared?

Which of the following hydrides are ionic?

Which compound is used in medicine as a barium meal for intestinal $X$-ray?

$A$ white solid '$A$' on heating gives off a gas which turns lime water milky. The residue is yellow when hot but turns white on cooling. This solid '$A$' is

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

$CO_2$ is passed through lime water. Initially,the solution turns milky and then becomes clear upon continued bubbling of $CO_2$. The clear solution is due to the formation of: