Hardness of Water
Water and soap are used for washing and bathing. Usually when the soap is dissolved in water it gives lather. But water drawn from some wells contain chlorides, sulphates and bicarbonates of calcium and magnesium. Water containing bicarbonates and sulphates of calcium dissolves soap but it does not give lather. Instead it forms a residue.
Based on this reaction of water with soap.
Water in classified into
1. Hard water.
2. Soft water.
Water which gives much lather with soap is called soft water. Water which does not give lather with soap and instead forms residue is called hard water. The salts of hard water and sodium stearite in soap, react with each other and forms a residue.
Types of Hardness of Water :
Hardness of water is of two types
1. Temporary hardness
2. Permanent hardness
Temporary hardness: Temporary hardness to water occurs due to the presence of bicarbonates of calcium and magnesium in water.
Permanent hardness : Permanent hardness to-water occurs due to the presence of sulphates and chlorides of calcium and magnesium in water. We cannot remove this type of hardness by boiling water.
Removal of hardness :
1. By boiling :- Temporary hardness of water can be removed by boiling it. By boiling water the bicarbonates of calcium Ca (HCO3)2 and magnesium Mg (HCO3)2 are decomposed into carbonates which are not soluble in water, and carbon dioxide escapes in the form of gas. Then the water is filtered. It is pure soft water. In this way temporary hardness can be removed.
2. Clark's Method :- Temporary hardness can also be removed when hard water is mixed with slaked lime in solid or liquid form. The bicarbonates present in the water are changed into carbonates. These are not soluble in water. So they form a residue at the bottom. By this method temporary hardness of water can be removed and soft water is obtained.
3. By mixing with washing soda:- Washing soda is Sodium Carbonate. Temporary hardness of water can be removed by mixing washing soda to water.
Types of Modern Processes for removal of Permanent Hardness.
Hardness of water can be removed by the following two Processes.
1. Permutit Process
2. Exchange of Ions Process.
1. Permutit Process:- Sodium Permutit (Na2Al2 Si208 XH20) is filled in the cylindrical tube as shown in the figure. Allow the flow of hard water from, above the cylindrical tube. The chlorides and sulphides of Calcium and Magnesium are changed into permutites. These permutites remain in the cylindrical tube and soft water is obtained at the
bottom.
After some time the cylindrical tube contains permutites of Calcium and Magnesium. At this stage the rate of reaction in changing the hard water into soft water is decreased. In order to avoid this, solution of Sodium chloride is sent through a tube. Thus the reaction takes place in the normal stage.
2. By exchange of Ions :- Hard water contain Ions of Calcium (Ca2+), magnesium (Mg2+) (cations) Sulphate (SO4-2) (anions) and Chloride (CV). By displacement of Positive Ion H+ with Negative Ions OH- the hardness of water can be removed.
There ate two cylindrical tubes A and B. The tube A contains zero cation and tube B contains di-aectite. Tube A is known as cation exchanger and tube B is known as anion exchanger. When hard water is sent through tube 'A' the positive Ion present in the hard water are attracted by Hydrogen Ion (W). When water flows through the tube
'B negative Ions are attracted by OH- ions These H+ and OH Ions combine and form water.
After some time the capacity of H+ and OH- ions decreases in the A and B tubes. At this stage Hydrochloric acid and Sodium carbonate are added in the tubes.
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