Which of the following properties is less for $D_2O$ than $H_2O$?

Permanent hardness in water is due to the presence of which of the following ions?

Plumbosolvency is a health hazard in the transportation of

Why is water used as a medium for chemical reactions?

Match List $I$ with List $II$ and select the correct answer using the codes given below the lists:

$1.$ Heavy water	$a.$ Bicarbonates of $Mg$ and $Ca$ in water
$2.$ Temporary hard water	$b.$ No foreign ions in water
$3.$ Soft water	$c.$ $D_2O$
$4.$ Permanent hard water	$d.$ Sulphates and chlorides of $Mg$ and $Ca$ in water

In the following reactions,the total number of oxygen atoms in $X$ and $Y$ is $........$.
$Na_2O + H_2O \rightarrow 2 X$
$Cl_2O_7 + H_2O \rightarrow 2 Y$