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. The estimated world water supply is given below.

Physical properties of water

It is a colorless 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 vaporization and high heat of fusion in comparison to H₂S and H₂Se. 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.

The high heat of vaporization and heat capacity are responsible for moderation of the climate and body temperature of living beings. It is an excellent solvent for transportation of ions and molecules required for plant and animal metabolism. Due to hydrogen bonding with polar molecules, even covalent compounds like alcohol and carbohydrates dissolve in water.

 Structure of water

In the gas phase water is bent molecule with a bond angle of 104.5^o, and O-H bond length of 95.7 pm as shown in below figure. It is a highly polar molecule. Its orbital overlap picture is also shown in the figure. In the liquid phase water molecules are associated together by hydrogen bonds.

The crystalline form of water is ice. At atmospheric pressure ice crystallizes in the hexagonal form, but at very low temperatures it condenses to cubic form. Density of ice is less than that of water. Therefore, an ice cube floats on water. In winter season ice formed on the surface of a lake provides thermal insulation which ensures the survival of the aquatic life. This fact is of great ecological significance.

Structure of Ice

Ice has a highly ordered three dimensional hydrogen bonded structure as shown in below figure. Examination of ice crystals with X-rays shows that each oxygen atom is surrounded tetrahedrally by four other oxygen atoms at a distance of 276 pm.

Hydrogen bonding gives ice a rather open type structure with wide holes. These holes can hold some other molecules of appropriate size interstitially.

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.

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

(2) Redox Reactions involving Water : Water can be easily reduced to dihydrogen by highly electropositive metals.

2H₂O (1)  +  2Na (s)   →  2NaOH (aq)  +  H₂ (g)

Thus, it is a great source of dihydrogen.

Water is oxidized to O₂ during photosynthesis.

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

With fluorine also it is oxidized to O2.

2F₂ (g)  +  2H₂O (I)   →   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 hydrolyzed in water.

P₄O₁₀ (s)  +  6H₂O (I)   →  4H₃PO₄ (aq)

SiCl₄ (I)  +  2H₂O (I)   →   SiO₂ (s)  +  4HCl (aq)

N^3- (s)  +  3H₂O (I)  →  NH₃ (g)  +  3OH^- (aq)

(4) Hydrates Formation : From aqueous solutions many salts can be crystallized as hydrated salts. Such an association of water is of different types viz.,