$BaSO_4 + C \rightarrow A + CO$
In the above reaction,$A$ is:

Which compound produces a paramagnetic acidic gas upon heating?

Chemical $A$ is used for softening temporary hard water. $A$ reacts with sodium carbonate to produce caustic soda. When $CO_2$ is passed through the solution of $A$,it turns milky. What is the chemical formula of $A$?

The alkaline earth metal nitrate that does not crystallise with water molecules is

Give the uses of cement.

Fill in the blanks:
$(1)$ Calcium oxide is commonly known as .......
$(2)$ $A$ suspension of calcium hydroxide in water is known as ....
$(3)$ The common name of calcium hydroxide is .......
$(4)$ The general name of calcium sulphate hemihydrate is ......