Which of the following compounds liberates an acidic gas during its hydrolysis?

The metal salts formed during the softening of hard water using $Clark's$ method are:

Which salt does not cause hardness in water?

Which relation is incorrect?

Heavy water $(D_2O)$ freezes at which temperature (in $^oC$)?

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