$D_2O$ is used in

$A$ water sample contains $30 \, ppm \, MgSO_4$ and $19 \, ppm \, MgCl_2$. Calculate the hardness of water in $ppm$ in terms of $CaCO_3$ equivalent.

Molecular weight of heavy water is

The hardness-producing salt whose solubility in water decreases with a rise in temperature is

Match the following:

List-$I$ (Reactions)	List-$II$ (Methods)
$A$. $Mg(HCO_3)_2 \rightarrow Mg(OH)_2 \downarrow + 2 CO_2 \uparrow$	$I$. Clark's method
$B$. $M^{2+} + Na_4P_6O_{18}^{2-} \rightarrow [Na_2MP_6O_{18}]^{2-} + 2 Na^{+}$	$II$. Ion exchange method
$C$. $Ca(HCO_3)_2 + Ca(OH)_2 \rightarrow 2 CaCO_3 + 2 H_2O$	$III$. Boiling
$D$. $2 NaZ + Ca^{2+}_{(aq)} \rightarrow 2 Na^{+} + CaZ_2$ ($Z$ = Zeolite)	$IV$. Calgon's method

Which compound produces $H_2$ gas in an aqueous solution?