Question

The gas that is used for water purification.

Answer 1

As we know that, in periodic tables there are gases which are very reactive, when such a type of gas dissolves in water, it purifies water by killing microorganisms at low ${\rm{pH}}$. There are other processes by which water can be purified such as sedimentation, boiling, distillation, etc.

Complete answer
In periodic table, there are halogen gases which are very reactive, fluorine gas is very explosive due to its oxidizing property, it is not useful for water purification. Chlorine is the second element, it is also electronegative but less than fluorine so when it is dissolved in water, it forms hypochlorous acid which further breaks into ions such as chloride ion and hydrogen ions. Due to increase in hydrogen ion concentration, ${\rm{pH}}$ of water becomes low. At low ${\rm{pH}}$, microorganisms are not stable and hence killed.
${{\rm{H}}_{\rm{2}}}{\rm{O}} + {\rm{C}}{{\rm{l}}_{\rm{2}}}{\rm{(g)}} \to {\rm{HOCl}} + {\rm{HCl}}$
Further, this hypochlorous acid breaks into,
${\rm{HOCl}} \to {\rm{C}}{{\rm{l}}^ - } + {{\rm{O}}_{\rm{2}}}{\rm{(g)}} + {\rm{2}}{{\rm{H}}^{\rm{ + }}}$

**Therefore, our answer is chlorine gas which is used for water purification.

Additional information: **
Water also can be purified by many other processes such as sedimentation, boiling, distillation, electrolysis, etc.
In the process of sedimentation, the solid particles in water are suspended and can be removed. This is also known as suspension, these particles are large and visible so that we can see normally. Sedimentation is preferred over Boiling and distillation.

Note:
Chlorine gas is not effective at higher concentration because due to the presence of chloride ions, the ${\rm{pH}}$ of water becomes neutral. So, an overdose of chlorine gas is not useful for purification of water.