The solubility of an ionic compound is compared in heavy water $(D_2O)$ and simple water $(H_2O)$. It is:

How many hydrogen atoms are surrounding each oxygen atom in the structure of ice?

How is heavy water prepared? Compare its physical properties with those of ordinary water.

When temporary hard water containing $Mg(HCO_3)_2$ is boiled,the precipitate formed is of:

Given below are two statements: One is labelled as Assertion $A$ and the other labelled as Reason $R$.
Assertion $A$: During the boiling of water having temporary hardness,$Mg(HCO_3)_2$ is converted to $MgCO_3$.
Reason $R$: The solubility product of $Mg(OH)_2$ is greater than that of $MgCO_3$.
In the light of the above statements,choose the most appropriate answer from the options given below.

Which of the following pairs of ions causes hardness in water?