Identify the ore of magnesium from the following.

Which of the following gives a green color in the flame test?

Alloys of $......$ metal are light and strong and so are used in the manufacture of aeroplane parts.

What is the order of basic strength for the hydroxides of the following alkaline earth metals?

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 ......

Chemical $A$ is used for water softening to remove temporary hardness. $A$ reacts with $Na_2CO_3$ to generate caustic soda. When $CO_2$ is bubbled through $A$,it turns cloudy. What is the chemical formula of $A$?