why water is a dipole

8/6/2019 Why Water is a Dipole

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BY M.R.VIJAYA KUMAR,ASST PROFESSOR

DEPT OF CIVIL ENGINEERING, RYMEC.BELLARY.KARNATAKA


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Water Agentle introduction to water and

its structureWater has long been known to exhibit many physical

properties that distinguish it from other small molecules of comparable

mass.Chemists refer to these as the anomalous properties of water,

but they are by no means mysterious; all are entirely predictable

consequences of the way the sire and nuclear charge of the oxygen

atom conspire to distort the electronic charge clouds of the atoms ofother elements when these are chemically bonded to the oxygen.

Acovalent chemical bond consists of two atoms that share

a pair of electrons between them. In the water molecule H20, the single

electron of each H is shared with one of the six outer-shell electrons of

the oxygen, leaving four electrons which are organized into two non-

bonding pairs.Thus the oxygen atom is surrounded by four electron pairs

that would ordinarily tend to arrange themselves as far from each

iiI5lIn order to minimize repulsions between these clouds of negative

charge This would ordinary result In a tetrahedral geometry in which

the angle


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angle between electron pairs (and therefore the H-O-H bond angle) is

109.

However, because the two non-bonding pairs remain closer tothe oxygen atom, these exert a stronger repulsion against the two covalent

bonding pairs, effectively pushing the two hydrogen atoms closer

together.The result is a distorted tetrahedral arrangement in which the

HO--H angle is 104.5.

These two computer- generated images of the H2 O molecule come from

calculations that model the electron distribution in molecules.T

he outerenvelopes show the effective surface of the molecule.


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The average electron density around theoxygen atom in a water molecule is about

10 times that around the hydrogen atoms.

This non-uniform distribution of positive

and negative charges leads to the

substances unusual behavior.


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In liquid and solid water, hydrogen (blue) and oxygen (red)

molecules form so-called hydrogen bonds between the positive

hydrogen atoms (d+) and negative oxygen atoms (d -).This

bonding results in a relatively strong tetrahedral structure.


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The liquid air interface of water

has an abundance of molecules

whose hydrogen atoms areoriented into the air.The

phenomenon, known as surface

relaxation, may increase the

reactivity of molecules in this

region of water.


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The H2 O molecule is electrically neutral, but the positive and

negative charges are not distributed uniformly.This is shown

clearly in the two images above, and in the schematic diagram at

the left.The electronic (negative) charge is concentrated at theoxygen end of the molecule, owing partly to the nonbonding

electrons (solid blue circles), and to oxygens high nuclear charge.

This charge displacement constitutes an electric dipole,

represented by the arrow at the bottom you can think of this dipole

as the electrical image of a water molecule.


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As we all learned in school, opposite charges attract, so the

partially-positive hydrogen atom on one water molecule is

electro statically attracted to the partially- negative oxygen on a

neighboring mo1ecule This process is called (somewhat

misleadingly) hydrogen bonding.Notice that the hydrogen

bond (shown by the dashed blue line) is somewhat longer (117

pm) than the covalent OH bond (99 pm).This means that it is

considerably weaker; it is so weak, in fact, that a givenhydrogen bond cannot survive for more than a tiny fraction of a

second.

Pico meter: a metric unit of length equal to one ten billionth of a meter

10-12 (or 0.0001 micron); used to specify wavelengths of electromagnetic

radiation

1 pm = 1 x 10-12 metre

1 pm = 1000 femtometre

100 pm = 1 ngstrm

1000 pm = 1 nanometre


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ATOMS ANDATOMIC BONDSAtoms

Molecules of minerals are composed of atoms of chemical elementsand these are the basic building blocks of all matter.The atoms of an

element are formed of basic particles, viz., protons, neutrons and electrons.

An atom consists of a nucleus with one or more protons, each carrying a

positive electromagnetic charge, and may or may not contain one or more

neutrons carrying no charge.The number of satellite electrons revolving

about the nucleus is the same as the protons but carry the opposite charge,that is, the negative charge.The individual elements are determined by the

atomic number which is equal to the number of protons in the nucleus or

the number of electrons attached to the nucleus of their atom.Each atom of

every element is electrically balanced since the number of protons and

electrons are equal in number and carry opposite charges.


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Ionic Bond or Electrovalent Bond.When two atoms , one of which

can lose one or more electrons to attain a noble gas configuration and

the other can receive these electrons and thereby acquire a noble gas

configuration, they are said to be bonded by an ionic band.Since the

loss and gain of electrons by atoms results in the formation of ions ,

ionic bond is formed when two ions interact with each other and are

thus held together by electrostatic attraction.Let us consider the

formation of potassium chloride (KCI), and aluminium fluoride (AlF3to illustrate ionic bonds.


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Covalent Bond.The covalent bond Is formed when two atoms achieve

stability by the sharing of an electron pair, each contributing one

electron to the electron pair. in many cases the atoms can be regarded ashaving acquired a noble gas configuration.The arrangement of electrons

in a covalent molecule is often shown by a Lewis structure In which only

valence shells (outer shells) are depicted. For clarity sake the electrons

are denoted by dots and crosses. For example,


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Polarity ofBonds.Acovalent bond is set up by sharing u electrons

between two atoms. It is further classified as polar or non-polar depending

upon the fact whether the electron pair is shared unequally between theatoms or shared equally. For example, the covalent bond in H2 and Cl2 are

called non-polar as the electron pair is equally shared between the two

atoms.


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In the case of hydrogen fluoride the bond is polar as the electron pair is

unequally shared. Fluorine has a greater attraction for electrons (or has

higher electro negativity) than does hydrogen and the shared pair ofelectrons is nearer the fluorine atom than hydrogen.The hydrogen end of

the molecule, therefore, appears positive with respect to fluorine.

Arrangement of some atoms In the decreasing order of eletronegativity is

Bond polarities affect both physical and chemical properties of compounds

containing polar bond.The polarity of a bond determines the kind of reaction

that can take place at that bond and even affects the reactivity at nearby

bonds.The polarity of bonds can lead to polarity of molecules and affect

melting point, boiling point and solubility.

A


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Apolar molecule like that of HF having two centres of charge is called

affect a dipole.The degree of polarity in a polar compound is specified by

its dipole moment (a vector quantity) which is equal to the charge times

the distance between the positive and negative centres.Mathematically,


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In general a polar bond is established between two atoms of different

radii and different electro negativities while positive centres (nuclei) of

different magnitudes combine to share an electron pair.Greater the value of

the dipole moment, greater is the polarity of the bond.

The following points may be borne in mind regarding dipole inn(I) In case a molecule contains two or more polar bonds, its dipole moment is

obtained by the vectorial addition of the dipole moments of the constituent

bonds.

(II) Asymmetrical molecule is non-polar even though It confa1us polar

bonds.

For example, carbon dioxide, methane and carbon tetrachloride. beingsymmetrical molecules, have zero dipole moments.

Dipole moment of methyl chloride is a vectorial addition of dipole moments

of three CH bonds and one CCl bond.


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The Hydrogen Bond.Compounds containing OH or NH groups often

exhibit unexpected properties, e.g., relatively high boiling points.To account

for these abnormalities and on the basis of available evidence of many kinds,

it is proposed that when a hydrogen atom lies between two atoms havingstrong electro negativities, it shows a unique property of forming a bond or

bridge between them, holding one by covalent bond and the other by purely

electrostatic forces.The electrostatic bond (hydrogen bond) has a strength of

about 208 kJ mol-1 (compared with 200-400 kJ mol-1 for most covalent

bonds).Liquids whose molecules are held together by hydrogen bonds are

said to he associated and their high boiling points are doe to greater energyrequired to break the hydrogen bonds.Hydrogen bonding between two

atoms is generally indicated by dotted lines. For example, in the following

compounds hydrogen bonding is represented below:


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ELEROSTATIC ORCES

Covalent BOND


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Hydrogen bonding is only a kind of specially strong dipole-dipole attraction.

When hydrogen is attached to a strongly electronegative atom (e.g., F, o or N),the electron cloud is strongly distorted towards the electronegative atom,

exposing the hydrogen nucleus.

The strong positive charge of the thinly shielded hydrogen

nucleus is strongly attracted by be negative charge of the electronegative

atom of the second molecule.This force of attraction is much weaker than that

of the covalent bond holding it to the first electronegative atom.Theelectrostatic attraction is, however, much stronger than other dipole-dipole

attractions.


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T

here are six known major energy levels in which an electron or a groupof electrons revolve around the nucleus of an atom.Each energy level is

termed as a shell, only a limited number of electrons can exist in any one

energy level or shell.Table 2.5 gives the number of electrons that can

exist at each shell.The closest and the farthest shells to the nucleus are

termed as K and P respectively.


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ValenceValence of an element is the number of electrons that is in excess or

deficient at a shell level. If the electrons are in excess it is termed as

positive valence, and negative if deficient.For example, Magnesiumatom contains 12 electrons and protons with 10 of the electrons below

shell level M, and 2 at shell level M. It is an excess of two above the

shell level L or lacks 6 to complete the shell level M.The least of the

two is 2 which is taken as the valence of Mg and is positive.Table 2.6

gives the valence of some of the common elements that compose therocks of the earth crust.


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Atomic BondsAtoms unite together to form molecules and molecules to aggregates.There are

certain forces that bind them together.The nature of these bonding forces is not

completely understood.However, some of the important bonding forces thatare recognized as useful in the field of soil mechanics are:

1.Primary valence bond

2.Secondary valence bond

PrimaryValence Bond[Ionic Bond]Primary valence bond is a chemical combination of two or more elements

because of the lack of a complete complement of electrons in their outermostshells.One atom joins with another atom by adding electrons to its outer shell

or by losing them to arrive at a stable compound.The number of electrons an

atom gains or loses depends upon the valence of the element given in Table

2.6.Atoms which lose or gain electrons in this manner are called ions and the

forces binding them together are called Ionic Bonds. For example in the

formation of a molecule, two ions ofAl join with three ions of 0 to form A1203.This is possible because each Al ion has an excess of 3 electrons in its outer

ring and 0 lacks 2 electrons in its outer ring.Astable compound can be formed

since the two Al ions are in excess of 6 electrons and the three 0 ions lack 6

electrons.The ionic bonds are the strongest of the bonds that hold atoms

together.


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Secondary valence forces. Secondary valence forces are also

sometimes called residual valence forces,V

an der Waals forces,V

anderWaals-London forces, intermolecular forces of attraction, and

intermolecular cohesive forces.Lambe (1953) recognized two types

of secondary valence forces : the Van der Walls forces and the

hydrogen bonds.Secondary valence forces acting between

molecules are attributed, to the presence of electric moments in the

individual molecules.If, in an electrical s stem the centre of action ofpositive charge coincides wit the centre of action of negative

charges, the system has no dipole moment, and is termed non-polar

Fig.5.2 (a)].Although a molecule is electrically neutral, the centre of

gravity of the positive and negative...charges may not coincide.An

electric moment is thus developed, and the system is referred to asbeing polar.System of Fig.5.2 (b) has a dipole moment of e X L.The

atoms in the water molecule are held together by a heteropolar bond

and the resulting molecule is not electrically symmetrical; the

hydrogen oxygen bonds are at an angle of 105 [Fig.5.2 (c )] .The

water molecule is therefore a dipole. Similarly, the unsymmetrical

distribution


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of electrons in the silicate crystals (the most widespread and abundant

constituent of clay particles) makes them polar.


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The hydrogen bond.The hydrogen bond occurs when an atom of hydrogen

is rather strongly attracted by two other atoms.The hydrogen cannot

decide to which atom to bond, and oscillates between them.The bestexample of the hydrogen bond is the bond between water molecules (Fig.

5.5).

The hydrogen bond is normally considered a secondary valence

bond. It is, however, stronger than the usual secondary valence bond, and

is somewhat similar in character to the

coordinate bond, a primary valence bond.

It can be considered a strong secondary or

a weak primary valence bond, or a unique

bond between the secondary and primary

bonds.

An indication of the relative strength of the

various bonds and the distance between

bonded atoms can be obtained from the

typical values given below (Lambe 1953)


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CLAYPARTICLE-WATERRELATIONSIntroduction

The property of a soil mass depends upon the behaviour of the

discrete particles composing the mass and the pattern of particlearrangement. In all these cases water plays an important part.The behaviour of

the soil mass is profoundly influenced by the inter-particle-water relationships,

the ability of the soil particles to adsorb exchangeable cations and the amount

of water present.

Adsorbed WaterWe have seen earlier the net charge on the surface of every particle isnegative.The intensity of the charge depends to a considerable extent on the

mineralogical character of the particle.The physical and chemical

manifestations of the surface charge constitute the surface activity of the

mineral.

Minerals are said to have high or low surface activity, depending

on the intensity of the surface charge.As pointed out earlier, the surfaceactivity depends not only on the specific surface but also on the chemical and

mineralogical composition of the solid particle.The surface activity of sand,

therefore, shall not acquire all the properties of a true clay, even if it is ground

to a fine powder. The presence of water does not alter its properties of coarser

fractions considerably excepting changing its unit weight.


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The adsorbed film of water on coarse particles is thin in

comparison with the diameter of the particles.In fine grained soils,

however, this layer of adsorbed water is relatively much thicker and

might even exceed the size of the grain.The forces associated with theadsorbed layers therefore play an important part in determining the

physical properties of the very fine-grained soils, but have little effect

on the coarser soils.

Soils in which the adsorbed film is thick compared to the grain

size have properties quite different from other soils having the samegrain sizes but smaller adsorbed films.The most pronounced

characteristic of the former is their ability to deform plastically without

cracking when mixed with varying amounts of water.

This is due to the grains moving across one another

supported by the viscous inter layers of the films.Such soils are called

cohesive soils, for they do not disintegrate with pressure but can berolled into threads with ease.Here the cohesion is not due to direct

molecular interaction between soil particles at the points of contact but

to the shearing strength of the adsorbed layers that separate the grains

at these points.


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Diffuse double layer.Experiments have shown that a soil colloidsuspended in water carries nearly always a net negative charge.The acidic

nature of soil minerals suggests that hydrogen atoms of the hydroxyl groupscome off in the presence of water and thereby give the minerals a net negative

charge.Since the net electrical charge of the entire Soil water suspension must

( be zero the charge on each colloid must be neutralized by ions from the

water which swarm around each colloid.These ions are called counter ions or

exchangeable ions, since they can be replaced.

If there were no thermal activityP

ossessed by these ions, and if therewere no attraction exerted on them by other ions and colloids, these counter

ions would all swarm to the surface of the particles to neutralize the surface

charge of the particle.Thus, their positions are compromises between the

particle charge which pulls them, and their thermal activities plus the attraction

by other bodies, which keeps them away.

The counter ions thus constitute the diffuse double layer (Fig.5.6) of

the colloid ; the surface charge of the colloid is the other layer of the double

layer.The force fields that develop between the charged soil particles, the

surrounding water, and the associated ions have a controlling influence on the

soil properties which can be varied within wide limits by changing such factors

as the types of ions, and their concentration, temperature, and the and amount

of pore fluid.


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Double layer

The double layer model is used to visualize the ionic environment in thevicinity of a charged colloid and explains how electrical repulsive forces occur

as shown in figure 2. Initially, attraction from the negative colloid causes some

of the positive ions to form a firmly attached layer around the surface of the

colloid; this layer of counter-ions is known as the Stern layer.Additional

positive ions are still attracted by the negative colloid, but now they are

repelled by the Stern layer as well as by other positive ions that are also trying

to approach the colloid.This dynamic equilibrium results in the formation of a

diffuse layer of counter ions.They have a high concentration near the surface

which gradually decreases with distance, until it reaches equilibrium with the

counter-ion concentration in the solution. In a similar, but opposite, fashion

there is a lack of negative ions in the neighbourhood of the surface,because

they are repelled by the negative colloid.Negative ions are called co-ions

because they have the same charge as the colloid.Their concentration will

gradually increase with distance, as the repulsive forces of the colloid are

screened out by the positive ions, until equilibrium is again reached.


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The diffuse layer can be visualized as a charged atmosphere surrounding the

colloid.The charge density at any distance from the surface is equal to the

difference in concentration of positive and negative ions at that point.Charge

density is greatest near the colloid and gradually diminishes toward zero as

the concentration of positive and negative ions merge together.The attached

counter-ions in the Stern layer and the charged atmosphere in the diffuse

layer are what is called the double layer.The thickness of this layer depends

upon the type and concentration of ions in solution.The point where the Stern

layer and the diffuse layer meet is called the slip plane.The Stern layer is

considered to be rigidly attached to the colloid, while the diffuse layer is not .

The electrical potential at the slip plane in a basic sense, is related to the

mobility of the particles and is called the zeta potential. (Zeta-

Meter,1997)(top)


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Isomorphous SubstitutionFrequently in a clay mineral lattice, metallic ions of one kind may be

substituted by other metallic ions of a lower valence, but of the same

physical size. Such a substitution is called isomorphous substitutionand may lead to different clay minerals with different physical

properties.For example, one silicon ion in a tetrahedral may be

substituted by an aluminium ion, which could happen when aluminium

ions are more readily able in water.As an aluminium ion has 3 positivecharges and a silicon 4 positive charges, there would be a net unit

charge deficiency of positive charge per substitution.This would mean

an increase in the net

negative residual charge.

Different clay minerals simply consist of the two basic sheets-the silicasheet and the octahedral sheet-stacked together in certain unique

fashions with certain metallic ions present in these sheets.The structures

of a few important clay minerals are described below.


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Base Exchange (CATIONEXCHANGE) IN SOILS :Electrolytes dissociate when dissolved in water into positively

charged cations and negatively charged anions.Acids break up into

cations of hydrogen and anions like Cl or SO4.Salts and bases split intometallic cations such as Na,K or Mg, and nonmetallic anions.Even water

itself is an electrolyte, because of very small fraction of its molecules

always

dissociate into hydrogen ions H+ and hydroxyl ions OH.- These

positively charged H ions migrate to the surface of the negatively

charged particles and form what is known as adsorbed layer.These H

ions can be replaced by the other cations like Na,K or Mg.These cations

enter the adsorbed layers and constitute what is termed as an

Adsorption complex.

The process of replacing cations of one kind by those of another in an

adsorption complex is known as base exchange. By base exchange ismeant the capacity of colloidal particles to change the the cations

adsorbed on their surface.


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Thus a hydrogen clay (colloid with adsorbed H cations)

can be changed to sodium clay (colloid with adsorbed Na cations)

by a constant percolation of water containing dissolvedN

a salts.

Such changes can be used to decrease the permeability of a soil.

Not all adsorbed cations are exchangeable.

The base exchange capacity is generally defined in terms of

the mass of a cation which may be held on the surface of 100 gm dry

mass of mineral. It is generally more convenient to employ a

definition of base exchange capacity in milli-equivalent(meq) per 100gm dry soil.

one meq is one milligram of hydrogen or the portion of any ion which

will combine with or displace 1 milligram of Hydrogen.This can be

explained with examples.


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The cations surrounding the clay micelle are called exchangeable

cations because they can be replaced by other cations.For examplewhen ammonium sulphate is added to the soil, the ammonium ions

gradually replace other cations specially calcium as shown in the

following equation:


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(iii) Nature of clay:The cation exchange capacity of soils also depends on the nature

of clay minerals which are present it.The cation exchange capacities of different

clay minerals are as follows:

Hence the cation exchange capacity of a soil dominated by montmorillonite or

vermiculite is much higher than the cation exchange capacity of another soil

dominated by Illite or Kaolinite.

(iv) Soil reaction:When the pH (See Sec. 7.14) of soils increases, more.hydrogen ions dissociate from the hydroxyl groups located on the broken edge

of clay minerals especially kaolinite exposing more hydroxyl groups.Hence the

cation exchange capacity of soils dominated by Kaolinite increased when the

pH of the soil increased.


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one meq is

one

milligram of

hydrogen or

the portion ofany ion which

will combine

with or

displace 1

milligram of

Hydrogen.This can be

explained

with

examples.


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Base (Cation) Exchange CapacityThe surface atoms are

predominantly negative oxygen hydroxyl ions, whereas the

metallic (Al, Si) ions occupy positions within the gibbsite or

silica sheets Thus, although the particle as a whole may be

electrically neutral, its surface can be considered to strongly

negatively charged.Also, with the exception ofKaolin,

isomorphous substitution, mentioned earlier, is one of the

sources of negative charge at the surface of the clay crystal.

Positive ions or cation the water are, therefore,

attracted to these surfaces of the particles to render the crystal

electrical neutral.Different clays have different charge

deficiencies and have varying ability to adsorb exchangeable

cations.The ability of a clay particle to adsorb ions on itssurface or edges is called itsbase or cation exchange

capacity which is a function of the mineral structure of the

clay and the size of the particles.


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The cations are termed exchangeable because

one cation can easily be exchanged with one of these

valence or by two of the cations with one-half the valence of

the original cation.

The base exchange capacity is expressed in

terms of the weight of a cation which may be held on the

surface of 100 g of dry soil material.Conventionally, it ismeasured in milli equivalent (meq) per 100 gm of dry soil

where 1 meq is 1 mg of hydrogen or the portion of any ion

which will combine with or displace 1 mg of hydrogen.On

the basis of earlier discussions, it can be seen that

montmorillonite has a much greater base exchangecapacity than kaolinite, with Illite being intermediate in

position, as shown in Table 4.3 below:


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Depositional environment, weathering action, leaching, etc., will

govern the kind of ions present in a deposit.Ca and Mg are the majorexchangeable cations in soils.Marine clays have predominantly Na and

Mg cations.The valence of the cation is the basic factor in the process of

replacement or exchange.HigherValency cations can easily replace

lower valence cations.When the ions have the same valence, larger the

size ion, greater its replacement capacity.Potassium ion, even though

monovalent, has more replacement capacity than sodium (which is alsomonovalent) because of its ability to fit into the hexagonal holes in the

silica sheet. Cations can be placed in approximate order of their

replacement ability as follows:


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Li+ < Na+ < H+ < K+ < NH+ < Mg++ < Ca++ < Al+++

The principle of cation exchange can be used withadvantage in many practical situations, as for example, the

stabilization of sodium clay soil by using lime.Here,

calcium ions replace the sodium ions by virtue of their

superior replacing power and reduces the swelling ofsodium montmorillonite, because the adsorbed layer of

water would be thinner when the valence of the cation is

larger.

one meq is one milligram of hydrogen or the portion ofany ion which will combine with or displace 1 milligram

of Hydrogen.This can be explained with examples.


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why water is a dipole

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