96It is sometimes possible to take advantage of the difference in the effect of temperature to separate
mixtures of different soluble salts. As the chart in the following Figure shows, a solution originally
of equal concentration in KClO 3 and KNO 3 should upon heating and evaporation of water precipitate
KClO 3 because KNO 3 is by far the more soluble near the boiling point of water.
The solubility of solid salts in water, and in most other solvents, increases with temperature while
that of gases decreases. The heat or enthalpy change of the dissolution reaction for most solids is
positive so the dissolution reaction is endothermic. For some solids, such as NaCl, the heat of
solution is very small and so the effect of temperature is small also. For other salts, such as KNO 3 ,
the effect of temperature is much larger:
NaCl(c) <--> Na+(aq) + Cl-(aq); H0 = (-240.12-167.159) – (-411.153) = +3.87 kJ/mol
KNO 3 (c) <--> K+(aq) + NO 3 - (aq); H0 = (-252.38-205.0)-(-494.63) = +37.3kJ/mol
Chemical coagulation in the water/wastewater treatment is the process of bringing suspended
matter in untreated water together for the purpose of settling and for the preparation of the water
for filtration.
Coagulation involves three specific steps, which are:
Coagulation
Flocculation
Sedimentation
Primary Clarifier