Physical and Chemical Properties of Water

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.

Estimated World Water Supply:

Source

% of Total

Lakes

0.009

Oceans

97.33

Saline lakes and inland seas

0.008

Polar ice and glaciers

2.04

Ground water

0.61

Soil moisture

0.005

Atmospheric water vapour

0.001

Rivers

0.0001

Physical Properties of Water:

·        It is a colourless and tasteless liquid.

·        The unusual properties of water in the condensed phase (liquid and solid states) are due to the presence of extensive hydrogen bonding between water molecules. This leads to high freezing point, high boiling point, high heat of vaporisation and high heat of fusion.

·        In comparison to other liquids water has a higher specific heat, thermal conductivity, surface tension, dipole moment and dielectric constant, etc.

·        These properties allow water to play a key role in the biosphere.

Physical Properties of H2O:

Property

H2O

Molecular mass (g mol–1)

18.0151

Melting point/K

273.0

Boiling point/K

373.0

Enthalpy of formation/kJ mol–1

-285.9

Enthalpy of vaporisation (373K)/kJ mol–1

40.66

Enthalpy of fusion/kJ mol–1

6.01

Temp of max. density/K

276.98

Density (298K)/g cm-3

1.0000

Dielectric constant/C2 /Nm2

78.39

Viscosity/centipoise

0.8903

Electrical conductivity (293K/ohm–1 cm–1)

5.710–8

Structure of Water:

Water has a very simple atomic structure. This structure consists of two hydrogen atoms bonded to one oxygen atom. The nature of the atomic structure of water causes its molecules to have unique electrochemical properties. The hydrogen side of the water molecule has a slight positive charge. On the other side of the molecule a negative charge exists. The atomic structure of a water molecule consists of two hydrogen H atoms joined to one oxygen O atom.

Structure of Ice:

                Ice has a highly ordered three dimensional hydrogen bonded structure.

Chemical Properties of Water:

            Water reacts with a large number of substances. Some of the important reactions are given below:

1. Amphoteric 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 Bronsted sense it acts as an acid with NH3 and a base with H2S.

H2O (l) + NH3 (aq)

OH-(aq) +  (aq)

H2O (l) + H2S (aq)

H3O+ (aq) + HS- (aq)

The auto-protolysis (self-ionization) of water takes place as follows:

H2O (l) + H2O (l)  H3O+ (aq) + OH- (aq)

2. Redox Reactions Involving Water:

·        Water can be easily reduced to dihydrogen by highly electropositive metals.

2H2O (l) + 2Na (s)  2NaOH (aq) + H2 (g)

·        Thus, it is a great source of dihydrogen. Water is oxidised to O2 during photosynthesis.

6CO2 (g) + 2H2O (l)  4H- (aq) + 4F- (l) + O2 (g)

·        With fluorine also it is oxidised to O2.

2F2 (g) + 2H2O (l)  4H+ (aq) + 4F- (aq) + O2 (g)

3. Hydrolysis Reaction:

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

P4O10 (s) + 6H2O (l)  4H3PO4 (aq)

4. Hydrates Formation:

          From aqueous solutions many salts can be crystallised as hydrated salts. Such an association of water is of different types:

a)    Coordinated water

b)    Interstitial water

c)     Hydrogen-bonded water.