Which of the following is not soluble in water?

Sorel's cement is:

All the elements of group $2$ react with water to form metal hydroxide and hydrogen,except the element

$A$ compound $(A)$ is used in the preparation of washing soda to recover ammonia in the Solvay process. When $CO_2$ is bubbled through an aqueous solution of $(A)$,the solution turns milky. It is used in whitewashing due to its disinfectant nature. What is the chemical formula of $(A)$?

Magnesium does not decompose which of the following?

Plaster of Paris when mixed with the correct amount of water sets into a solid mass due to the formation of