Hardness of water, Chemistry

(a) What is hardness of water? What do you understand by temporary and permanent hardness?


Explain temporary and permanent hardness of water. Discuss various chemical methods for disinfection of water.

Hardness in water is that characteristics which "prevents the lathering of soap". This is due to the soluble bicarbonates of calcium and magnesium. When water is boiled, the bicarbonate ions present in it are decomposed to carbonate and carbon dioxide.

Ca (HCO3)2 = CaCO3 + H2O + CO2

Mg (HCO3) = Mg (OH) 2 + 2CO2

The Ca+2 and Mg+2 ions readily combine with the carbonate ions to form insoluble CaCO3 and MgCO3. So the temporary hardness can be largely removed by easy means like boiling etc.

Permanent hardness (Non-carbonate):

It is due to dissolved chlorides, sulphates and nitrites of calcium and magnesium. The difference of total hardness and temporary hardness is also called non-carbonates hardness and temporary hardness is also called non-carbonates hardness. The hardness caused due to the presence of chlorides and sulphates of Ca, Mg, Fe and other heavy metals, cannot be removed by boiling. Hardness of water is objectionable for following reasons:

(a)          Large amount of soap is wasted to get the desired results in laundry industry, since soap has no cleaning action unless all the Ca, Mg or heavy metal ions are precipitated.

(b)         In industries, these salts of water during course of circulation get deposited at different parts of boilers are scales, damaging the system by corrosion, heat resistance, chocking etc.

The disinfection of water can be carried out by the following methods:

(1)         Chlorination method:

The chlorine is the most commonly used disinfectant. It may be added directly as gas or in form of concentrated solution in water. Correct does and sufficient time and of contact should be ensured for effective disinfection.

H2O + Cl2   =     HOCl +  HCl

HOCl + bacteria = Bacteria's are killed

(2)         By bleaching powder method:

Chlorine may be added in the form of bleaching powder. The chemical reaction of bleaching powder in water is given below:

                                                  CaCl2 + H2O = Ca (OH2) + Cl2 

                                                   Cl2 +H2O = HCl + HOCl (unstable hypochlorous acid)

                                                HOCl = HCl + [H]0 (necessary oxygen)

The nascent oxygen destroys pathogens by oxidation.

(3). Ozonization Method:

The ozone is highly unstable, harmless gas and an excellent disinfectant. It breakdown librating nascent oxygen and kills pathogens.

                                   O3 = O2 + [O] (nascent oxygen)

Boiling method:

Water becomes free from disease causing bacteria's if water is boiled for 10-15 minutes at 60 degree centigrade to 70 degree centigrade.

Chloramines' (CiNH2) method:

                                                 It is an excellent bacterial.

                                             CiNH2 + H2O = HOCl + NH3 (disinfectant)

                                             HOCl = HCl + [O]

Posted Date: 7/21/2012 6:45:52 AM | Location : United States

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