Water

The student will be able to explain1. Water — physical and chemical properties

2. Hardness of water and methods to remove it.

3. Heavy water

A major part of all living organisms is made up of water. Human body has about 65% and some plants have as much as 95% water. It is a crucial compound for the survival of all life forms. It is a solvent of great importance. The distribution of water over the earth’s surface is not uniform.

Introduction

Structure of waterO

H H104.5°

95.7 pm

O

H H H

HH H H

O O

Gaseous state

Solid state

Structure of iceH

O

H H

H

H

H

H

H

H

H

H

H

O

O

OO

H

H

O

O

O

H

O

Vacant space

H

H

H

Chemical PropertiesAmphoteric Nature: It has the ability to act as an acid as

well as a base i.e., it behaves as an amphoteric substance. In the Brönsted sense it acts as an acid with NH3 and a base with H2S.

Redox Reactions Involving Water:Water can be easily reduced to dihydrogen by highly electropositive metals.

Water is oxidised to O2 during photosynthesis.

6CO2(g) + 12H2O(l) → C6H12O6(aq) +6H2O(l) + 6O2(g)

Hydrolysis Reaction: Due to high dielectric constant, it has a very strong hydrating tendency. It dissolves many ionic compounds. However, certain covalent andsome ionic compounds are hydrolysed in water.

Hydrates Formation

From aqueous solutions many salts can be crystallised ashydrated salts. Such an association of water is of different types.Example- CuSO4.5H2O

Hard waterContains dissolved salts of bicarbonates, sulphates and chlorides of calcium and magnesium.

Hard water does not produce lather with soap solution.

Soft water is free from bicarbonates, sulphates and chlorides of calcium and magnisum. It produce lather with soap solution easily.

Soft water

e.g., distilled water, rain water

Types of hardness

• due to presence of soluble bicarbonates of calcium and magnesium.

• can be removed by simple boiling.

The hardness of water is of two types

(i)Temporary hardness

• due to presence of chlorides and sulphates of calcium and magnesium.

• requires treatment of water to remove this type hardness.

(ii)Permanent hardness

• Temporary hardness is also called carbonate hardness

• Permanent hardness is also called non-carbonate hardness

Softening of waterThe process of removal of Ca2+ and Mg2+ ions from water is called softening of water.

3 3 2 22inso lub leSo lub le

(i) M HCO MCO H O CO (M = Ca or Mg)

Boiling: Removal of temporary hardness:-

Clark’s method or calcium hydroxide method

3 2 3 22insolubleSoluble

Ca HCO Ca(OH) 2CaCO 2H O

3 2 3 222insoluble insolubleSoluble

Mg HCO 2Ca(OH) 2CaCO Mg OH 2H

Removal of permanent hardness

2 2 3 3

insolubleCaCl Na CO CaCO 2NaCl

4 2 3 3 2 4

insolubleMgSO Na CO MgCO Na SO

(i) By washing soda (Na2CO3.10H2O) treatment

(ii) Calgon’s methodSodium hexametaphosphate (Na6P6O18), commercially called ‘calgon’, when added to hard water, thefollowing reactions take place.

Removal of permanent hardnessiii)By using inorganic cation exchanger

(permutit method or Zeolite method):

2 2 2 8 2 2 2 8 2Zeolite settles atbottom

Na Al Si O CaCl Ca Al Si O 2NaCl

The zeolite can be regenerated by treatment with sodium chloride solution.

2 2 8 2 2 2 8 22Ca Al Si O 2NaCl Na Al Si O CaCl

Removal of permanent hardness(iV) By organic ion exchanger:

2

2In hard water Cation exchanger

Mg 2H re sin Mg (resin) 2H

2

2Hard water Cation exchanger

Ca 2H re sin Ca (resin) 2H

Cation exchanger

Anion exchanger

2

4 4 2Anion exchangerHard water

SO 2HO resin SO resin 2OH

Hard water Anion exchangerCl HO resin Cl re sin OH

22 Re generated resinExchausted resin

Ca resin 2HCl 2H re sin CaCl

Exchausted resin Re generated resinCl re sin NaOH HO re sin NaCl

Heavy Water (D2O)•Was discovered by Urey in 1932.

•Ordinary water contains one part of heavy water in 600 parts of it.

•Used as moderator in nuclear reactions.

•Used as tracer in study of mechanism of physiological process.

•Used in preparation of deuterium compounds.

Hydrogen economy (Hydrogen as fuel)With advancement of science and technology we realise in order to make our lives comfortable fossil fuels are depleating at an alarming rate and will be exahausted soon. The electricity cannot be stored to run automobiles. It is not possible to store and transport nuclear energy. Hydrogen is another alternative source of energy and hence called as ‘hydrogen economy’. Hydrogen has some advantages as fuel

Hydrogen economy (Hydrogen as fuel)

• Available in abundance in combined form as water.

• On combustion produces H2O. Hence pollution free.

• H2-O2 fuel cell give more power.

• Excellent reducing agent. Therefore can be used as substitute of carbon in reduction for processes in industry.