Which of the following is used as a moderator in a nuclear reactor?

Which of the following is the best scientific method to test the presence of water in a liquid?

When the first element of the periodic table is treated with dioxygen,it gives a compound whose solid state floats on its liquid state. This compound has an ability to act as an acid as well as a base. What products will be formed when this compound undergoes autoionisation?

Which of the following can be used to remove the permanent hardness of water?

Explain the structure of ice.

Match the following methods for treating hardness of water with their corresponding reagents used.

List-$I$	List-$II$
$A$. Treatment with washing soda	$I$. $Ca(OH)_2$
$B$. Calgon's method	$II$. $Na_2CO_3$
$C$. Clark's method	$III$. $NaAlSiO_4$
$D$. Zeolite/permutit process	$IV$. $Na_6P_6O_{18}$

The correct match is: