SSC CGL 16 Aug 2015 Shift 1 Paper

Natural and wastewater contain small particulates that are suspended in water forming a colloid. These particles carry the same charges, and repulsion prevents them from combining into larger particulates to settle. Historically, dirty water is cleaned by treating with alum, $\mathit{A}{\mathit{l}}_2{\left(\mathit{S}{\mathit{O}}_4\right)}_3$ . $12{\mathit{H}}_2\mathit{O}$, and lime, $\mathit{C}\mathit{a}{\left(\mathit{O}\mathit{H}\right)}_2$. The phenomenon is known as coagulation:
$\mathit{A}{\mathit{l}}_2{\left(\mathit{S}{\mathit{O}}_4\right)}_3$ . $12{\mathit{H}}_2\mathit{O}$ Al ${\left(\mathrm{aq}\right)}^{3+}$ + $3\mathit{S}{\mathit{O}}_4{\left(\mathrm{aq}\right)}^{2-}$ + $12{\mathit{H}}_2\mathit{O}$$\mathit{S}{\mathit{O}}_4{\left(\mathrm{aq}\right)}^{2-}$ + ${\mathit{H}}_2\mathit{O}\mathit{H}\mathit{S}{\mathit{O}}_4$ (aq)- +$\mathit{O}{\mathit{H}}^{-}$ (causing pH change)
$\mathit{C}\mathit{a}{\left(\mathit{O}\mathit{H}\right)}_2\mathit{C}\mathit{a}{\left(\mathrm{aq}\right)}^{2+}$ + 2 $\mathit{O}{\mathit{H}}^{-}$
(causing pH change)
The slightly basic water causes $\mathit{A}\mathit{l}{\left(\mathit{O}\mathit{H}\right)}_3,\mathit{F}\mathit{e}\left(\mathit{O}\mathit{H}\right)3$ and $\mathit{F}\mathit{e}{\left(\mathit{O}\mathit{H}\right)}_2$ to precipitate, bringing the small particulates with them and the water becomes clear.