Heavy Water, Hard and Soft Water | Chemistry for JAMB PDF Download

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-

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.

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

Removal of Permanent Hardness:

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

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

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

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.

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 Mg^2+..

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.

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.

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.