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Dihydrogen combines with a number of elements  to form binary compounds called hydrides. Their general formula being MHwhere M represents the element and x the number of hydrogen atoms. Depending upon the physical and chemical properties, the hydrides have been divided into the following three broad categories:

1. Ionic or salt-like or saline hydrides

2. Metallic or Interstitial hydrides

3. Molecules or Covalent hydrides

  • Saline Hydrides or Ionic hydrides

These are binary compounds of hydrogen and elements which are more electropositive than hydrogen such as alkali metals, alkaline earth metals (except Be), etc. Saline hydrides are formed by the transference of electron from metal to hydrogen. Some common examples of this category are: LiH, NaH, CaH2, CrH2 etc. The general characteristic of these hydrides are as follows:

(i) They are crystalline solids having white or greyish colour.

(ii) They have high melting and boiling points.

(iii) They have high density and high heat of formation.

(iv) They conduct electricity in molten state liberating dihydrogen gas at anode which confirm the presence of hydride (H-) in then.

CaH2 (melt) → Ca2+ + 2H-

At anode: 2H- → H2 + 2e-

 At cathode:   Ca2+ + 2e- → Ca

(v) They react vigorously with water and other protonic solvents such as ethanol and ammonia to liberate dihydrogen gas. Thus they act as strong bases.

NaH + H2O → NaOH + H2

LiH + H2O → NaOH + H2

LiH + CH3OH → LiOCH3 + H2

NaH + NH3 →  NaNH2 + H2

CaH+ 2H2O → Ca(OH)+2H2

  • Covalent Hydrides or Molecular Hydrides

These are binary compounds of hydrogen and elements of comparatively high electronegativity such as p- block elements. In these hydrides, H atoms are bonded to the other atoms by covalent bonds. Some examples of covalent hydrides are, HCl,H2O, PH3, NH3 etc. The general formula of covalent hydrides can be written as XH(8-n) where n is the number of outershell electrons of X atom. However, elements of group 13 are exception to this formula. The elements of group 
13 such as B, Ga form polynuclear hydrides which are electron deficient compounds. B2H6, Ga3H2, etc, are some examples. Some of the general characteristic of covalent hydrides are as follows:

(i) These hydrides consist of individual covalent molecules with relatively weak interparticle forces (Vander waal’s force of attraction). Hence they generally soft, with low melting and boiling points.

(ii) They are poor conductors of electricity.

(iii) Being covalent in nature, they are more soluble in organic solvents.

(iv) They undergo thermal decomposition into their respective elements.    

(v) They are covalent in nature, & are more soluble in organic solvents.

(vi) Some of them react with water to liberate H2.

    B2H6,+ 6H2O → 2B(OH)3+ 6H2 

(vii) Along any given row of periodic table, the covalent hydrides become increasing acidic in moving from left to right.


  • Interstitial Hydride or Metallic Hydrides 

These are binary compounds of hydrogen and transition elements.

These hydrides are generally formed by the

(a) transition metals of group 3, 4, 5 of d- block;

(b) Cr metal of group 6 and

(c) f – block elements.

It may be noted that elements of group 7, 8, 9 of d – block do not form hydrides at all. This inability of metal, of group 7, 8, 9 of periodic table to form hydrides is referred to as hydride gap of d – block.

In these compounds H atoms are supposed to occupy interstitial position in the metal lattices. Some scientists consider these compounds as simply solid solutions of hydrogen. The composition of these hydrides may not correspond to simple whole number ratio and therefore, they are also called non-stoichimotric hydrides. Their composition is also found to vary with the conditions of temperature and pressure. Some examples of interestial hydrides of elements of group 3 to 5 are ScH, YH,YH,LiH3, CrH, TiH2,ZrH2, HfH2  etc.

Some examples of non-stoichimetric hydrides are


Some general characteristics are as follows:

(i) They are generally powders or brittle solids having dark or metallic appearances.

(ii) They are good conductors of electricity. The conductivity, however, decreases with increase in temperature.

(iii) They have high thermal conductivity.

(iv) Most of these hydrides are harder than parent metals.

(v) They generally undergo reversible decomposition into H2 gas and metal.

Besides three main categories of hydrides some other types of hydrides are also known. Two of these are described as follows:

(a) Polymeric Hydrides

They are formed by the elements having electronegativity range between 1.4 and 2.0. They consist of molecules held together in two or three dimensions by hydrogen bridges. Some common examples are


They are amorphous solids and stable up to 525 K. Above this temperature they begin to evolve hydrogen gas.

(b) Complex Hydrides: These are the compounds which contain hydride ions (H-) co-ordinated to metal atom ions. Some common examples areLiAlH4 (lithium aliminium hydride),NaBH4 (sodium borohydride) etc. They are generally very good reducing agents.

  • Hydrogen Economy

Hydrogen as fuel

Hydrogen is another proposed alternative energy source. Some advantages of hydrogen are:

(i) Hydrogen is aboundantly available in the combined form as water.

(ii) Use of hydrogen as fuel provides pollution free atmosphere because its combination product is water.

(iii) Time required for regeneration of hydrogen is much shorter as is clear from the following diagram:

(v) Heat of combustion per gram of hydrogen is more than twice that of jet fuel.(iv) An automobile engine burning hydrogen is about 25 to 50% more efficient than an automobile engine burning gasoline.

(vi) Hydrogen – oxygen fuel cells provide other possibilities of powering motor vehicles.

(vii) Hydrogen is excellent reducing agent and can replace coal in many industrial processes involving reduction because it produces less atmospheric pollution than carbon.

The changes in out way of life by adopting widespread uses of hydrogen listed above refer to hydrogen economy.

Obstacles of Hydrogen Economy

Although hydrogen looks as very good future fuel but some of the tough problems must be solved before we adopt hydrogen economy. The problems are as described below:

(i) Availability.

Hydrogen is not available as much. It does not occur in free state is nature. Therefore, cheap production of Hydrogen is basic requirement of hydrogen economy. The most likely future source of hydrogen is water. Hydrogen might be generated at an appropriate site by using solar energy and then transporting it as fuel.

(ii) Storage and Transportation.

Hydrogen gas has explosive flammability which causes problem to its storage and transportation. Hydrogen can be stored in vacuum insulated cryogenic tanks (already in use for space programmes in USA) Liquid hydrogen can be transported by road and rail tankers. It can also be stored in underground tanks and transported pipelines. Another promising solution to this problem is the use of Fe-Ti alloy which act like a sponge to absorb hydrogen and results in the formation of the silvery power. Heating the power safely releases hydrogen gas. The other small storage units are alloys like etc. Such storage systems are safer than storage of hydrogen as gas or liquid.

(iii) Platinum Scarcity

In oxygen-hydrogen fuel cells, a lot of platinum is required as catalyst. In each succeeding year the demand of platinum exceeds the supply. This will cause problems for fuel cells which are highly energy source for automobiles.

Uses of Liquid Hydrogen As Fuel         

Liquid hydrogen has already been used as rocket fuel. The chemical reaction involved is:


Both reactions H2 and O2 are stored as liquid in separated tanks. The tank hold  of liquid hydrogen. The oxygen tank carries  of liquid oxygen. During the “lift off” operations, these properties power shuttle’s main engine for about 8.5 min. Here, liquid hydrogen is consumed at the rate of nearly 3000L/sec.

  • Water 

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 sp3 hybrid state. Each of the covalent bonds are formed by the axial overlap of 1s-orbital of H atom and  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 109.280to 104.50 as shown in the figure.

Since water molecule has bent shape, therefore bond moments of two

 Polar nature of H2O

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 

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.The extent of association, however, depends upon the conditions of temperature and pressure.In ice, a solid state of water, each H2O molecule is tetrahedrally surrounded by four neighboring H2Omolecules 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.

  • 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.



Molecular mass


Melting Point (K)


Boiling Point (K)


Temperature of maximum Density (K)


Maximum Density 




Heat of Vaporisation (373 K)(kJ/)


Heat of Fusion (kJ/)


Specific Heat 


Ionization Constant 

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.

Chemical Properties of Water

1. Action towards litmus

Pure water is neutral to litmus.

2.  Decomposition

Water is quite stable and does not dissociate at high temperature. The dissociation into its elements is only 0.02 % even at 1500K.


The small conductivity of pure water reveals its dissociation into H3O+ and-OH- ion.

3.   Acid-base reactions

Water is amphoteric substance because it can act as acid as well as base as shown -

However, the pH of water at  is 7 and its is neutral towards litmus.

4.   Hydrolytic reactions

Water can hydrolyse many non-metallic oxides, halides and also some metallic phosphides, carbides and nitrates.




  • Heavy Water

Heavy water is the oxide of heavy hydrogen (deuterium) and is also called deuterium oxide. It is represented by the formula  D2O or 2H2O. The credit of discovery of heavy water goes to Urey who first proved that 6000 parts of ordinary water contains about 1 part of heavy water. Lewis and Donald (1933) were able to isolate a few ml of heavy water by the prolonged electrolysis of alkaline water.

Preparation of Heavy Water

The main source of heavy water is ordinary water. From which it is prepared either by prolonged electrolysis or by fractional distillation.

Prolonged Electrolysis of Ordinary Water

This method involves multistage electrolysis of ordinary water containing NaOH. The cell used for electrolysis was designed by Brown, Daggat and Urey. It is a cylindrical vessel made of steel which acts as cathode. The anode is a perforated cylindrical sheet. The electrolysis is carried out in different stage as described below and in actual practice large number of electrolysis cells are used.

First Stage

In this stage thirty electrolytic cell are used. Each cell is filled with about 3% solution of NaOH. The electrolysis is carried out for about 72 hrs, using a current of 110 volts. The volume reduces to about 1/6th of the original volume taken. The gases evolved  are discard the volume left contain about 2.5% of heavy water.

Second Stage

This stage involves the electrolysis of residue left from the first stage using 6 electrolytic cells. The gaseous evolved are burnt and water formed is returned to the first stage cell. The residual liquid contains about 12% of heavy water.

Third Stage

This involves the electrolysis of residue of second stage. The content of heavy water is raised to about 60%. The gases evolved are burnt to get water that is fed to 2nd stage cells.

Fourth Stage

This stage involves the electrolysis of residue of third stage and here, nearly 99% of heavy water is obtained. The gases evolved are burnt as usual, and sent to third stage cells.

Fifth stage

The 99% heavy water from fourth stage is made free from alkali and other impurities by distillation and distillate is electrolysis. Here, the gases evolved are D2 and O2 which are burnt to get 100% pure water. A flow sheet diagram of the process is shown below in the figure.

By Fractional Distillation of Ordinary Water

This method involves partial separation of heavy water from ordinary water. Advantage is taken of the small difference in the boiling points of protium oxide (373.2K) and deuterium oxide (374.3K). Since the difference in boiling points is very small, a long fractionating column (about 13 m) is used for distillation and the process is repeated several times. The lighter fraction ( H2O) is distilled first while heavier fraction ( D2O) is left behind. The heavier fraction becomes rich in( D2O).

Properties of Heavy Water

(i)   Heavy water is colourless, tasteless and odourless liquid.

(ii)   All physical constants of heavy water are higher than the corresponding values of ordinary water. Some of the physical constants of heavy water are given below in table.



Molecular mass



Melting Point (K)



Boiling Point (K)



Temperature of maximum Density (K)



Maximum Density 






Heat of Vaporisation (373 K)(kJ/)



Heat of Fusion (kJ/)



Specific Heat 



Ionization Constant 

Chemical Properties of Heavy Water

Heavy water is chemically similar to ordinary water. The chemical reactions of heavy water are slower than those of ordinary water. Some of the important reactions of heavy water are listed below:

1.   Reaction with metals

      Alkali metals and alkali earth metals reacts with heavy water to form heavy hydrogen(D2).




2.   Reaction with metal oxides

      reacts slowly with metal oxides to form corresponding deutroxides



3.   Reaction with non-metallic oxides

      Non-metallic oxides react with  to form corresponding deutro acid,




4.   Electrolysis

      A solution of heavy water containing, when electrolysed evolve heavy hydrogen at cathode


5.   Exchange reactions

      When compounds having mobile hydrogen react with heavy water, hydrogen is exchanged by deuterium partially or completely.




6.   Deutero hydrates

Heavy water like ordinary water may be associated with salts as water of crystallization, giving deutro hydrates, e.g.,.

7.   Deuterolysis

Water brings hydrolysis of certain inorganic salts. gives similar reactions which are termed deuterolysis.

AlCl3 + 3D2O → Al(OD)3 + 3DCl

8.   Biological and Physiological effects

It has been established that heavy water of high concentration retards the growth of plants and animals. It has been confirmed by Lewis that tobacco do not grow in heavy water. Pure heavy water kill small fishes, tadpoles and mice, when fed on with it.

Taylor has shown that heavy water has germicide and bactericide properties. Water containing small quantity of  D2O acts as a tonic and stimulates vegetable growth. Certain moulds have been found to develop better in heavy water in comparison to ordinary water.

Uses of Heavy Water

The following are the important uses of heavy water:

1.   As a neutron moderator.

Fission in uranium – 235 is brought in by slow speed neutrons. The substances which are used for slowing down the speed of neutrons are called moderators. Heavy water is used for this purpose in nuclear reactors.

2.   For the preparation of deuterium.

Heavy water on electrolysis by its decomposition with metals produces deuterium.

3.   As a tracer compound.

Heavy water is commonly used as a tracer compound for studying various reactions mechanisms. It has also used for studying the structure of some oxyacids of phosphorus such as H3PO2 and H3PO3 as to determine the number of ionisable hydrogen atoms.

Production in India

Various units have been set in India to manufacture heavy water. These are situated at Nanital, Trombay, Rourkel, Namrup and Neyveli.

  • Hard & Soft Water

Water is classified into categories depending upon its behaviour towards soap solution. These are: soft water and hard water.

(a) Soft water

Water which produces lather with soap solution readily is called soft water. Distilled water and rain water are common examples of soft water.

(b) Hard Water

Water which does not produce lather with soap solution easily is called hard water. Sea water, tap water are common examples of hard water.

Cause of Hardness of water

Hardness of water is due to the dissolved impurities of the salts like bicarbonates, chlorides and sulphates of calcium and magnesium. Water gets contaminated by these salts when it passes through the ground and rocks. Hard water does not produce lather with soap solution readily because the cations (Ca2+ & Mg2+) present in hard water react with soap (which is a mixture of sodium salts of higher fatty acids like stearic acid, palmitic acid, oleic acid, etc) to form a precipitate of calcium and magnesium salts of fatty acids.


      (M = Ca or Mg)

Thus, no lather is produced until all the calcium and magnesium ions have been precipitated. This leads to the consumption and hence, wastage of lot of soap. Hard water is, therefore, not fit for washing purposes

Types of Hardness

The hardness of water is of two types: temporary hardness and permanent hardness.

(i) Temporary hardness.

It is due to the presence of soluble bicarbonates of calcium and magnesium. Such water is also said to posses carbonate hardness. The term temporary indicates that most of the hardness can be removed by simply boiling the water. The bicarbonates of calcium and magnesium are formed in water by dissolution of carbonates of calcium and magnesium in the presence of atmospheric carbon dioxide.

(ii) Permanent hardness.

It is due to the presence of chlorides and sulphates of calcium and magnesium. Such water is also said to posses non-carbonate hardness. The term permanent indicates that type of hardness can not be removed by boiling of water.

Softening of Hard Water

The process of removal of metallic ions (Ca2+ & Mg2+) responsible for hardness of water is known as softening of water. A number of methods are available to soften water depending upon the nature of dissolved mineral salts as described below:

Removal of Temporary Hardness 

Temporary hardness can be removed by the following methods:

(i) Boiling

Temporary hard water is taken in large boilers and boiled for about fifteen minutes. Consequently, the bicarbonate of calcium and magnesium present in the water decompose into their insoluble carbonates which settle at the bottom of the tank as precipitate which are removed by filtration or decantation.


(ii) Calcium hydroxide (or Clarke’s) method

Calculated quantity of lime (calcium hydroxide) is added to temporary hard water. The soluble bicarbonates are converted into insoluble carbonates which settle at the bottom of the tank and are removed by filtration.



Removal of Permanent Hardness

Permanent hardness of water can be removed by the following methods:

1. By chemical additives

(a) Addition of washing soda.

In this method, Ca2+ & Mg2+ions can be precipitated by the addition of calculated amount of washing soda 

(b) Addition of sodium polymetaphosphate (calgon process)

In this method Ca2+ & Mg2+ ions are rendered ineffective by the addition of sodium polymetaphosphate. The trade name of which is calgon (meaning calcium gone). The general formula of sodium polymetaphosphate is, where the value of n is sometime as large as 1000. However, commonly, calgon is represented by sodium hexametaphosphate. Ca2+ & Mg2+of hard water reacts with calgon to form soluble complexes.



The complexes of calcium and magnesium so formed remain dissolved in water but they do not cause hindrance in the formation of lather. This is because calcium and magnesium ions are not free to react with soap but these have been tied up in stable complexes. This is also known as sequestraction of Ca2+ & Mg2+ions.

2. Ion Exchange method

This is a modern method of softening of water. In this method, the ions present in the hard water are exchanged for less damaging ions from the exchangers. There are two main types of ion-exchanger as:

(a) Inorganic cation exchangers (Permutit Method)

These are complex inorganic salts like hydrated sodium-aluminum silicate Na2Al2Si2O3 . xH2O  which have interesting property of exchanging cations such as calcium and magnesium ions in hard water for sodium ions. These complex salts are known as zeolites which can be either a naturally occurring or an artificially synthesized substances. Their technical name is permutit. For artificial synthesis of permutit, a mixture of soda ash(Na2CO3), sand (SiO3) and alumina (Al2O3) is fused together. The product is washed with water to remove soluble impurities leaving behind a porous mass of permutit.

The permutit is loosely packed in a big tank over a layer of coarse sand. Hard water is introduced into the tank from the top. Water reaches the bottom of the tank and then slowly rises through the layer of permutit in the tank. The cations present in hard water are exchanged for sodium ions.

        (M = Ca or Mg)

                                                            where Z = 

As the process is continued, zeolites gets exhausted because of its conversion into calcium and magnesium zeolite. The exhausted resin is regenerated allowing about 10% solution of sodium chloride to percolate through it when the following reaction occurs.

                   (M = Ca or Mg)

The regenerated or reactivated resin can be used again and again for a quite longer period.

(b) Organic ion exchangers

These are complex organic molecules having giant hydrocarbon frame work with either acidic group (-SO3 or -COOH) or basic group (OH- & NH2-)attached to them. The resins with acidic group are capable of exchanging the  ions for the cations and are called cation exchangers. They are represented as resin. The resins with basic group are capable of exchanging their OH- or  NH2- ions for other anions and are called anion exchangers. They are represented as resin.

Method of Removal of Hardness By Organic Exchangers

First of all, the hard water is passed through a bed of cation exchange resin. The cations present in hard water are exchanged with of resin as:



The water which comes out of the bottom of first tank is richer in  ions.

This water is then passed through a bed of anion exchange resin where anions contained in water are exchanged with OH-  ions as

Cl- + HO-- resin → Cl-resin + OH-

 SO42- + 2HO--resin →SO42- -resin + 2OH-

Therefore, the  ions (formed in the first tank) combine with the OH- ions (formed in the second tank) to produce water.

Thus, water obtained by this method is free from all types of cations as well as anions. It is known as deionised or demineralised water.

This method is particularly suitable for obtaining pure water for laboratory purposes.

Regeneration of resins

The exhausted resin in the first tank is regenerated by treatment with moderately concentrated hydrochloric or sulphuric acid.

Ca(resin)2 + 2HCl → CaCl2 + 2H--resin

Similarly, the exhausted resin in the second tank is regenerated by treatment with moderately concentrated solution of sodium hydroxide.

Cl – resin + NaOH → NaCl + HO--resin

  • Degree of Hardness of Water

Degree of hardness of water is defined as number of parts of mass of CaCO3 (Calcium carbonate), equivalent to various calcium and magnesium salts present in one million parts by mass of water. It is expressed in ppm (parts per million).

It may be noted degree of hardness up to 100 – 150 ppm in water required for our daily needs such as cooking, bathing, washing of clothes, etc, is tolerable. But if degree of hardness exceeds this limit, then water is not suitable for domestic use.

You can also refer to  

JEE Organic Chemistry Syllabus 

 Reference books of Organic Chemistry

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