Water

Hydrogen of Class 11

Water (H₂O) is an important hydride of oxygen, which is principal (about 75%) constituent of earth’s surface. It is most abundant, omnipresent and easily obtainable of all chemical compounds. It is a significant component of animal and vegetable matter and play a vital role in their process. It ranks next to oxygen in importance of out existence. It constitutes about 65% of human body and about 95% by weight of some plants. It can be easily transformed from liquid to solid or from liquid to gaseous states and vice versa.

Distribution of Water

The distribution of water over the earth’s surface is not uniform

The desert regions have no permanent surface water while the oceans cover about 78% of the earth’s surface. They contain 97% of the available water. Out of the total surface water only 2.7% is fresh water and rest is locked in polar ice caps, glaciers or under the ground and is not readily available.

Structure and Aggregation of Water Molecules

In water molecule, the two H atoms are bonded to O atoms by two covalent bonds. The oxygen atom assumes sp³hybrid state. Each of the covalent bonds are formed by the axial overlap of 1s-orbital of H atom and sp³ hybrid orbital of O atom. The two bond pairs and two lone pairs of electrons around oxygen atom assume tetrahedral arrangement. Consequently, the H₂O molecules has a bent structure. Due to relativity greater repulsive interactions of lone pairs the bond angle around the O atom decreases from 109.28′ to 104.5′ as shown in the figure.

Polar nature of H₂O

Since water molecule has bent shape, therefore bond moments of two
O – H bond causes the molecule to behave as permanent electrical dipole. The dipole moment of H₂O molecule has been found to be 1.84 D which confirms its polar nature, as shown in the figure.

Aggregation of Water Molecules

In gaseous state, the individual covalent molecules H₂Oexist as such. However in liquid state, large aggregates of H₂Ounits are formed because of their association through intermolecular hydrogen bonds as shown below

Water

The extent of association, however, depends upon the conditions of temperature and pressure.

The intermolecular hydrogen bonding is responsible for the abnormally high freezing point, boiling point, heat of fusion and heat of vaporization as compared to the hydrides of the other elements of oxygen family.

In ice, a solid state of water, each H₂O molecule is tetrahedrally surrounded by four neighboring H₂O molecules with their oxygen atoms occupying the corners of tetrahedron. There are four H atoms around each O atom. Two of the four H atoms are bonded by covalent bonds (bond length 100 pm) whereas the other two are linked through hydrogen bonds (176 pm) as shown in the figure. This gives highly three dimensional structure having large vacant spaces, which may be compared to open cage.

Due to open cage like structure, ice has a relatively larger volume for a given mass of liquid water. Consequently, density of ice is less than water and it floats over water.

As the temperature is raised beyond 273 K, open cage like structure dismentalling due to cleavage of some H-bonds and ice melts. The breaking of H-bond causes aggregation of H₂O molecules to have closer resulting in the decrease in volume and thereby increase in density. This continues till 277 K. When the density becomes maximum. Beyond 277 more and H-bonds cleaves and expansion of liquid water starts occurring due to increased K.E. of molecules and the density again starts decreasing however, it remains higher than ice. Hence density of water is maximum at 277 K.

This property of maximum density at 277 K is a boon for the survival of aquatic animals during winter months because when the upper layer of sea water freezes.

The frozen water does not sink to the bottom but keeps floating at the surface due to its lesser density. This provides thermal insulation to the water below it.

It is very interesting to note that nine different crystalline forms of ice have been found to be exist under different pressure conditions. Each one has different melting point. At one bar pressure ice has normal hexagonal form. However at low temperature it adopts cubic form.

Water

Properties of Water Physical properties

Water has some unique features which arise due to intermolecular H-bonding. Some important physical constants of water are given in table.

Property	Constant
Molecular mass	18.015
Melting Point (K)	273.2
Boiling Point (K)	373.2
Temperature of maximum Density (K)	277.1
Maximum Density (g cm^-3)	1.000
Density (g cm^-3)	0.997
Heat of Vaporisation (373 K)(kJ/mol^-1)	40.66
Heat of Fusion (kJ/mol^-1)	6.01
Specific Heat (Jg^-1K^-1)	4.177
Ionization Constant [H⁺][OH^-](mol²L^-2)	1.008 x 10^-14
ΔH^°_f(kJmol^-1)	-285.9

Some of the important physical properties are discussed below:

(i) The freezing point, boiling point, heat of fusion and heat of vapourization water are abnormally higher than those of the hydrides of the other elements of the same group (16) such as H₂S,H₂Se,H₂Te etc. This is due to the presence of intermolecular hydrogen bonding in H₂O molecules which is, however absent among the molecule of H₂S,H₂Se,H₂Te etc.

(ii) Water has a higher specific heat, thermal conductivity and surface tension than most other liquids. These properties allow water to play a vital role in the biosphere. For example, the high heat of vapourization and the high heat capacity of water are responsible for moderation of the climate and body temperature of living organisms.

(iii) Water because of its high dielectric constant (78.39) has the ability to dissolve most of the inorganic (ionic) compounds and its, therefore, regarded as a universal solvent. Whereas solubility of ionic compounds takes place due to ion-dipole interactions (i.e. solvation of ions), the solubility of covalent compounds such as alcohols, amines, urea, glucose, sugar etc, takes place due to the tendency of these molecules to form hydrogen bonds with water.