$A$ compound $X$ on heating gives a colourless gas. The residue is dissolved in water to obtain $Y$. Excess $CO_2$ is bubbled through the aqueous solution of $Y$ and $Z$ is formed. $Z$ on gentle heating gives back $X$. The compound $X$ is:

The correct order of density of $Be, Mg, Ca, Sr$ is

Which of the following contains both calcium and magnesium?

The commercial name for calcium oxide is

Water is periodically sprinkled on cement plasters during curing. What does this help in?

When water is added to compound $(A)$ of calcium,a solution of compound $(B)$ is formed. When carbon dioxide is passed into the solution,it turns milky due to the formation of compound $(C)$. If excess of carbon dioxide is passed into the solution,the milkiness disappears due to the formation of compound $(D)$. Identify the compounds $A$,$B$,$C$,and $D$. Explain why the milkiness disappears in the last step.