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Heavy Water, Hard and Soft Water Notes | Study Chemistry for JEE - JEE

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Heavy Water [D2O]:

It was discovered by Urey in 1932. It can be prepared by exhaustive electrolysis of ordinary water using nickel electrodes. It is colourless, odourless, tasteless liquid.

Chemical Reactions of Heavy Water-

Heavy Water, Hard and Soft Water Notes | Study Chemistry for JEE - JEE


Uses of Heavy Water:

It is used:

  1. in nuclear reactors to slow down the speed of neutrons and are called moderators.
  2. as a tracer compound to study the mechanisms of many reactions.

Soft and Hard Water-

The water which produces large amount of lather with soap is known as soft water and which forms a scum with soap is known as hard water.

Types of Hardness of Water-

  • Temporary hardness: It is due to the presence of bicarbonates of calcium and magnesium.
  • Permanent hardness: It is due to the presence of chlorides and sulphates of calcium and magnesium.

Removal of Temporary Hardness:

It can be achieved:

(a) By boiling- The soluble bicarbonates are converted into insoluble carbonates.

Heavy Water, Hard and Soft Water Notes | Study Chemistry for JEE - JEE

(b) By Clark’s process- By adding lime water or milk of lime.

Heavy Water, Hard and Soft Water Notes | Study Chemistry for JEE - JEE


Removal of Permanent Hardness:

(i) By adding washing soda: The calcium or magnesium salts are precipitated as carbonates.

Heavy Water, Hard and Soft Water Notes | Study Chemistry for JEE - JEE

(ii) By adding caustic soda: The temporary and permanent hardness can be removed by adding caustic soda.

Heavy Water, Hard and Soft Water Notes | Study Chemistry for JEE - JEE

(iii) By adding sodium phosphate(Na3PO4): The phosphates of calcium and magnesium are precipitated.

Heavy Water, Hard and Soft Water Notes | Study Chemistry for JEE - JEE

Similarly, magnesium also precipitates out in the form of magnesium phosphate, Mg3(PO4)2.

(iv) Calgon’s process: Calgon is sodium hexa metaphosphate (Na6P6O18). This calgon when added to hard water forms a soluble complex.

Heavy Water, Hard and Soft Water Notes | Study Chemistry for JEE - JEE

Similarly. Mg2+ can also precipitate as Na2[Mg2(PO3)6] and water becomes free from Ca2+and Mg2+ Ions.

(v) Permutit process: Permutit is hydrated sodium aluminium silicate Na2Al2Si2O8.xH2O. It exchanges its sodium ions for divalent ions Ca2+ and Mg2+..

Heavy Water, Hard and Soft Water Notes | Study Chemistry for JEE - JEE

Permutit when fully exhausted can be regenerated by treating with 10% solution of sodium chloride. It is the most efficient method to gel water with zero degree of hardness.

(vi) By synthetic resins:

These are of two types:

(a) Cation exchange resins are big molecules containing sulphonic acid group (-SO3H). It is first changed into sodium salt with the general formula RNa. The hard water is passed through it so Ca2+ and M2+ are exchanged and removed.

Heavy Water, Hard and Soft Water Notes | Study Chemistry for JEE - JEE

The resins like permutit can be regenerated with a solution of NaCl.

(b) Anion exchange resins are also big molecules and can exchange anions. They contain an amino group.

Heavy Water, Hard and Soft Water Notes | Study Chemistry for JEE - JEE

The water is first passed through cation resins and then through anion resins and pure distilled water is obtained.


Measurement of Degree of Hardness:

Degree of hardness is defined as the number of parts of calcium carbonate or equivalent to various calcium and magnesium salts present in one million parts of water by mass. It is expressed in ppm.

Degree of hardness (in ppm) = (wt. of CaCO3 (g)/ wt. of hard water (g)) x 106

The molecular wt. of Ca(HCO3)2, Mg(HCO3)2, CaCl2, MgCl2, CaSO4 and MgSO4 is 162, 146, 111, 95, 136 and 120 respectively. The mol. wt. of CaCO3 is 100.

Thus, 162 g Ca(HCO3)2, 146 g Mg(HCO3)2, 111 gCaCl2, 95 g MgCl2 136 g CaSO4 and 120 g MgSO4 are equivalent to 100 g CaCO3.

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