Correct Option: D

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, Al₂(SO₄)₃.12 H₂O, and lime, Ca(OH)₂. The phenomenon is known as coagulation: Al₂(SO₄ )₃ .12 H₂O Al(aq)₃+ + 3SO₄(aq)_2- + 12H₂O SO₄(aq)_2- + H₂O HSO₄(aq)- + OH- (causing pH change) Ca(OH)₂ Ca(aq)₂₊ + 2 OH- (causing pH change) The slightly basic water causes Al(OH)₃, Fe(OH)₃ and Fe(OH)₂ to precipitate, bringing the small particulates with them and the water becomes clear.