Which of the following ions forms a hydroxide that is readily soluble in water?

The number of water molecules in gypsum,dead burnt plaster and plaster of paris,respectively are:

Heating a solid compound $X$ produces $CO_2$ gas and a residue. Adding water to this residue produces compound $Y$. Passing excess $CO_2$ gas through a solution of $Y$ produces a clear solution $Z$. Boiling $Z$ regenerates compound $X$. What is compound $X$?

Powdered Beryllium burns in air to give

An element $P$ has atomic number $56$. What will be the formula of its halide?

Lime reacts exothermally with water to give '$A$' which has low solubility in water. Aqueous solution of '$A$' is often used for the test of $CO_{2}$,a test in which insoluble $B$ is formed. If $B$ is further reacted with $CO_{2}$ then a soluble compound is formed. '$A$' is