SURFACE CHEMISTRY

1
SURFACE CHEMISTRY
Adsorption
Phenomenon of attracting and retaining molecules of a substance on surface of liquid or solid
resulting in a higher concentration at a surface of solid than in the bulk is called adsorption. The
solid substance on the surface of which, adsorption takes place is called adsorbent. While the substance
which gets adsorbed on the solid surface due to molecular attractions is called adsorbate. Adsorbent may
be solid or liquid but adsorbate may be a gas or a solute in some solution. It is fast in beginnings but
become slow after some time. It is exothermic e.g. adsorption of water on silica gel.
Adsorption is due to the presence of unbalanced forces, believed to have developed either during
crystallisation of solids or due to presence of unpaired, electrons or free valencies in solids having d–orbital.
In liquids it is due to surface tension.
Desorption
It is the reverse of adsorption i.e. removal of adsorbed substance from the surface of adsorbent. This
phenomenon is most common in gases adsorbed on solid.
Absorption
When molecule of a substance are uniformly distributed throughout the body of another substance at
uniform rate is called absorption. e.g. absorption of water by CaCl2, NH3 in water.
It is assimilation of molecules into a solid or a liquid substance with the formation of a solution or a
compound.
Sorption
Both adsorption and absorption take place simultaneously.
e.g. dyes get adsorbed as well as absorbed on the cotton fibers.
Difference between adsorption and absorption
Sr. No. Adsorption Absorption
1 Adsorption is a surface phenomenon. The adsorbing
substance is called adsorbate and is only concentrated
on the surface of adsorbent.
It is a bulk phenomenon. In
absorption the substance
penetrates into the bulk of the
other substance.
2 The rate of adsorption is rapid to start with and its rate
slowly decreases.
Absorption occurs at a uniform
rate
Heat (Enthalpy) of Adsorption
Adsorption is a surface phenomenon and adsorbate molecules are held to the surface of adsorbent due to
attractive interactions. Since energy is always released during attractive interactions, adsorption is an
exothermic reaction.
The amount of heat evolved when one mole of an adsorbate gets adsorbed on the surface of an adsorbate is
called molar heat (enthalpy) of adsorption.
Adsorption in Terms of Gibb’s Helmholtz Equation
Adsorption is an exothermic reaction. Therefore, adsorption is accompanied by release of energy or H is
always negative and favours the process. Also absorbate molecules get lesser opportunity to move about
on the surface of adsorbent. Thus, entropy factor opposes the process. According to Gibb’s Helmholtz
equation,
G = H – T S
Since adsorption does actually take place, H is greater than T  S  G is negative. As the adsorption
continues, the difference between the two opposing tendencies becomes lesser and lesser till they are
equal i.e., H = TS or G = 0. At this stage, equilibrium called adsorption equilibrium gets established
and there is no net adsorption taking place at this stage.
Types of Adsorption
There are two types of adsorption.
a) Physical adsorption (or Vander waal’s adsorption) or physiosorption.
b) Chemisorption
Characteristics of Physical and Chemisorption
Physio–sorption Chemisorption
1. Adsorption by weak vander waal’s forces. By chemical force (covalent or ionic bond)
2. Multimolecular layer may be formed Unimolecular layer will be formed
3. Low heat of adsorption viz. about 20–40
kJ/mol
High heat of adsorption viz. about (200–400
kJ/mol)
4. Easily reversed Not reversed
5. Molecular state of adsorbate on
adsorbent is same. No surface
compounds are found.
Molecular state may be different. Surface
compounds are found.
6. Usually occurs rapidly at low
temperature and decreases with increase
in temperature.
It occurs at high temperature initially but then
decreases.

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7. It increases with pressure Change in pressure will have less effect on
chemisorptions
8. Not specific Highly specific
9. Extent of adsorption depends on ease of
adsorption depends on ease of
liquefaction i.e. surface area, critical
temperature, inversion temperature
Vanderwaal’s constant ‘a’, e.g. Adsorption
of gases on charcoal
There is no relative between extent of
adsorption and ease of liquefaction of gas.
e.g. Adsorption of H2 on Pt; decomposition of
NH3 in presence of tungsten, decomposition of
Hl on gold (zero order)
Positive and Negative Adsorption
When the concentration of the adsorbate is more on the surface of the adsorbent than in the bulk, it is
called positive adsorption. If the concentration of the adsorbate is less at surface relative to its
concentration in the bulk, it is called negative adsorption.
e.g. when a concentrated solution of KCl is shaken with blood charcoal, it shows positive adsorption but
with a dilute solution of KCl, it shows negative adsorption.
Factor Affecting Adsorption of Gases on Solids
Nature of Adsorbent
Greater the surface area of adsorbent, greater is the volume of gas adsorbed thus silica gel, aluminium
oxide and clay are best adsorbents. Transition metals act as good adsorbents for gases due to vacant or
half–filled d–orbitals and high charge – size ratio. Activated charcoal is a better absorbent.
Specific Area of Adsorbent
Specific area of an adsorbent is the surface area available for adsorption per gram of the adsorbent.
Highly porous substances like silica gel, charcoal are very good adsorbents since they have larger surface
area. Finely divided substances have large adsorption power.
Nature of Adsorbate
Easily liquefiable gases (with higher critical temperature) like NH3, HCl, CO2 etc. are adsorbed to a much
greater extent than permanent gases like N2, O2, H2 etc. Because easily liqufiable gases have stronger
intermolecular forces. e.g. 1 gm activated charcoal adsorbs more SO2 (critical temperature 630 K) then
CH4 (critical temperatures, 190 K)
Pressure
At constant temperature, if pressure is increased, adsorption increases. The increase in much greater if
temperature is low. Freundlich gave the relationship Between extent of adsorption and pressure.
Temperature
Adsorption is an exothermic process having an equilibrium: According to Le–Chatelier’s principle, the
magnitude of adsorption should increase with decrease in temperature.
Gas (Adsorbate) + Solid (Adsorbent) Gas adsorbed + heat
The chemisorptions first increases with temperature then decreases. The initial increase shows that like
chemical reaction, chemisorptions needs activation energy.
Activation of Solid Adsorbent
It means increasing the adsorbing power of an absorbent. This is usually done by increasing the surface
area of the adsorbent which can be achieved in any of the following ways:
(a) By making the surface of the absorbent rough.
(b) By subdividing the adsorbent into smaller pieces or grains.
(c) By removing the gases already adsorbed
Adsorption Isotherms
The amount of a gas adsorbed by a given amount of the adsorbent depends upon both temperature and
pressure. The variation of adsorption with pressure at a constant temperature is generally expressed
graphically. The curves obtained by plotting the amount of gas adsorbed (a = x/m) against gas pressure at
a constant temperature are called adsorption isotherms.

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Adsorption isotherm is a graph between quantities adsorbed under a constant gas pressure at different
temperatures.
Adsorption isostere is the plot of temperature versus pressure for a given amount of adsorption.
Freundlich Adsorption Isotherm for Physisorption
Amount of the gas adsorbed per unit mass of adsorbent increase linearly with pressure in the initial
stages and then much more slowly attaining a limiting value as the surface becomes fully covered by gas
molecules. The relationship between the magnitude of adsorption and pressure can be expressed
mathematically by an equation: )1n(kp
m
x n/1

The above relationship is commonly known as Freundlich adsorption isotherm. In this relation, x/m is
the amount of gas adsorbed per gram of the adsorbent at the pressure p, and ‘k’ and ‘n’ are the constants
depending upon the nature of the gas and adsorbent.
The quantity 1/n is generally less than one. This indicates that the amount of the gas adsorbed increases
less rapidly than the pressure.
At the lower values of pressure, the graph is nearly a straight line. 1
P
m
x

At the equilibrium pressure or the saturation pressure (ps), x/m reaches its maximum value, i.e., no more
adsorption takes place even if the pressure is further increased 0
P
m
x
 .
At intermediate pressure n
1
P
m
x

n
1
Pk
m
x

By taking logarithm, the above equation becomes
Plog
n
1
klog
m
x
log 
Thus, if we plot a graph between log (x/m) and log p, a straight line is
obtained thus the Frenudlich isotherm is valid. The slope of the line is
equal to 1/n and ntercept on log (x/m) axis will correspond to log k.
Freundlich isotherm explains the behaviour of adsorption in an approximate manner. The factor 1/n can
have taken value between 0 and 1 (probable range 0.1 to 0.5). Thus, the above equation holds good over a
limited range of pressure.
When 
m
x
,0
n
1
constant, the adsorption is independent of pressure.
When ,P
m
x
.e.i,Pk
m
x
,1
n
1
 the adsorption varies directly with pressure.
Both the conditions are supported by experimental results. The experimental isotherms always seem to
approach saturation at high pressure. This cannot be explained by Freundlich isotherm. Thus, it fails at
high pressure.
Adsorption from Solution Phase
Solids can adsorb solutes from solutions also. When a solution of acetic acid in water is shaken with
charcoal, a part of the acid is adsorbed by the charcoal and the concentration of the acid decreases in the
solution.
It has been observed that the extent of adsorption:
i) Decreases with an increase in temperature
ii) Increases with an increase of surface area of the adsorbent.
iii) Depends on the concentration of the solute in solution.
iv) Depends on the nature of the adsorbent and the adsorbate.
The precise mechanism of adsorption from solution is not known. Freundlich’s equation approximately
describes the behaviour of adsorption from solution with a difference that instead of pressure,
concentration of the solution is taken into account, i.e., n/1
Ck
m
x

Adsorption Isobar

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The chemical adsorption isobar shows an initial increase with temperature because the heat supplied act
as activation energy required in chemisorption.
Application of Adsorption
1. In gas mask:
Finally divided coconut charcoal is used as gas masks for absorbing toxic gases like CH4, CO, COCl2.
It is usually used for breathing in coal mines to adsorb poisonous gases.
2. In preserving Vacuum:
In Dewar flasks, activated charcoal is placed between the walls of the flask so that any gas which
enters into annular space either due to glass imperfection of diffusion through glass is adsorbed.
3. As dehumidizer e.g silica gel
4. Fe(OH)3 antidote adsorbent in Arsenic poisoning.
5. In clarification of sugar: Sugar decolorized by treating sugar solution with charcoal powder. The
latter adsorbs the undesirable colours present.
6. Heterogeneous catalysis: Adsorption of reactants on the solid surface of the catalysts increases
the rate of reaction. There are many gaseous reactions of industrial importance involving solid
catalysts. Manufacture of ammonia using iron as a catalyst, manufacture of H2SO4 by contact
process and use of finely divided nickel in the hydrogenation of oils are excellent examples of
heterogeneous catalysis.
7. In Chromatography: The different chromatographic techniques such as adsorption, paper or
column chromatography which are used for the purification and the separation of the substances
available in small amounts, are based upon the theory of selective adsorption.
Catalysis
Catalyst is a substance which changes the speed of a reaction, and usually, can be recovered completely
unchanged at the end of a reaction. However it may take part in a reaction consumed in one step and
regenerated in another. This phenomenon is known as Catalysis.
A substance is termed a positive catalyst or simply as catalyst if it accelerates the rate of chemical
reaction. On the other hand, the added substance is termed as negative catalyst if it retards the rate of a
chemical reaction.
Example of Positive Catalysts:
(1) Lead chamber process of H2SO4 using catalyst No.
e.g. 2SO2(g) + O2(g) NO
2SO3(g)
(2) 2KClO3   2MnO
2KCl + 3O2
(3) 2CO + O2 NO
2CO2
(4) H2O2   )s(Pt
2H2O + O2
(5) Hydrolysis of ester in acidic medium.
Activity of a catalyst refers to the ability of a catalyst to accelerate chemical reaction. e.g. Pt acts as a
catalyst in the reaction.
H2(g) + 1/2O2(g)  platinum
H2O(l)
Contact process of H2SO4, Where V2O5(s) is used to convert SO2 to SO3.
Example of Negative Catalysts:
i) H3PO4 or acetanilide in the decomposition of H2O2.
ii) Alcohol in the oxidation of chloroform leading to the formation of phosgene.
iii) Tertraethyl lead or nickel carbonyl acting as antiknock material in internal combustion engines.
iv) Antifreezes like glycerol which retard the rusting of the machines.
Catalytic Promoters
There are certain substances which when added only in small quantity to a catalyst enhance its activity.
This substance itself may not be catalyst. Such substances which enhance the activity of a catalyst are
called catalytic promoters. E.g. Molybdenum is used as Promoter for Fe catalyst in Haber’s process.
Catalytic Inhibitors or poisons
The rates of some reactions are reduced considerably by the presence of small amounts of other
substances called inhibitors. For example, the oxidation of sodium sulphide by oxygen gas is inhibited
by small amounts of alcohol, aniline and benzaldehyde. Thus, inhibitor is a substance which when added
during the preparation of a catalyst in small amounts, reduces the catalytic activity to a considerable
extent.

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The substance whose presence decreases or destroy the activity of a catalyst are called poison. CO or H2S
in hydrogen gas acts as a poison for Fe catalyst in Haber process. As2O3 acts as a poison for Pt–asbestos
in contact process for H2SO4.
Types of Catalysis
Broadly, two types of catalysis are known:
(a) Homogeneous catalysis (b) Heterogeneous catalysis
(a) Homogeneous catalysis
If the catalyst is present in the same phase as the reactants, it is called a homogeneous catalyst
and this type of catalysis is called homogeneous catalysis.
Ex:
2
Cl
3 O2OO 
)g(CO2)g(O)g(CO2 2
NO
2 
FructoseecosGluSucrose
61266126
SOH
2112212 )aq(OHC)aq(OHC)l(OH)aq(OHC 42
 
)g(SO2)g(O)g(SO2 3
)g(NO
22  
)aq(OHCH)aq(COOHCH)(OH)(COOCHCH 33
)(HCl
233   

b) Heterogeneous catalysis:
In this type of catalysis the catalyst is present in a difference phase than that of the reactants. In
heterogeneous catalysis, catalyst is generally a solid and the reactants are generally gases but
sometimes liquid reactants are also used. It is also known as surface catalysis.
Ex:
i) Synthesis of methyl alcohol (CH3OH) from CO and H2 using a mixture of copper, ZnO and Cr2O3 as
catalyst.
CO(g) + 2H2(g)    32OCrZnO,Cu
CH3OH(l)
ii) Manufacture of ammonia from N2 and H2 by Haber’s process using iron as catalyst.
N2(g) + 3H2(g)
Fe
2NH3(g)
Theories of Catalysis
Intermediate Compound Formation Theory
This theory explains homogeneous catalysis mainly.
According to this theory, the catalyst combines with one of the reactants to give an intermediate
compound. This compound intermediately reacts with the other reactants and given the product and
regenerates the catalyst in its original form.
Thus the reactants do not directly combine with each other, instead they react through the catalyst which
provides an alternative pathway which involves lesser energy of activation.
For example, the function of nitric oxide [NO] as a catalyst in the formation of SO3 is explained as follows:
2NO + O2 2NO2
Catalyst Reactant Intermediate
NO2 + SO2 SO3 + NO
Intermediate Reactant Product Catalyst
regenerated
Adsorption Theory:
This theory explains the heterogeneous catalysis. The role of a solid catalyst in enhancing the reaction
rate is explained on the basis of this theory in the following steps:
(i) The reactant molecules are adsorbed on the surface of the catalyst at adjacent point. Adsorption
leads to higher concentration of the adsorbed reactant on the surface of a catalyst.
(ii) As adsorption is an exothermic process, the heat of adsorption provides the necessary activation
energy for the chemical reaction to proceed and enhance rate of greater.
(iii) The product molecules rapidly leave the catalyst surface to make room for the other reactant
molecules to get adsorbed. Thus the chemical combination between reactant molecules occurs at
the surface of the catalyst at a must faster rate.
e.g. Hydrogenation of ethane in presence of Ni
H – H + 2M  2M – H
C2H4 + 2M – H  C2H6 + 2M
Modern Adsorption Theory of Heterogeneous Catalysis
The modern adsorption theory is the combination of intermediate compound formation theory and the
old adsorption theory. The catalytic activity is localized on the surface of the catalyst. The mechanism
involves five steps:

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O
O
O
O
O
O
Catalyst surface
having free valencies
+ A + B
Reacting molecuels
Adsorption of
reacting molecules O
O
O
O
O A
O B
O
O
O
O
O
O
Catalyst
+ A - B
Product
Desorption of product
molecules
O
O
O
O
O A
O B
|
Intermedaite
Adsorption of reacting molecules, formation of intermediate and desorption of products
i) Diffusion of reactants to the surface of the catalyst.
ii) Adsorption of reactant molecules on the surface of the catalyst.
iii) Occurrence of chemical reaction on the catalyst’s surface through formation of an intermediate.
iv) Desorption of reaction products from the catalyst surface, and thereby, making the surface
available again for more reaction to occur.
v) Diffusion of reaction products away from the catalyst’s surface. The surface of the catalyst unlike
the inner part of the bulk, has free valencies which provide the basis for chemical forces of
attraction. When a gas comes in contact with such a surface, its molecules are held up there due to
loose chemical combination. If different molecules are adsorbed side by side, they may react with
each other resulting in the formation of new molecules. Thus, formed molecules may evaporate
leaving the surface for the fresh reactant molecules.
This theory explain why the catalyst remains unchanged in mass and chemical composition at the
end of the reaction and is effective even in small quantities. It however, does not explain the action
of catalytic promoters and catalytic poisons.
General Characteristics of Catalytic Reactions
1. The catalyst is unchanged chemically at the end of the reaction:
The amount of catalyst remains chemically unaffected at the end of a reaction, though there may be
change in physical state such as the particle size or change in the colour of the catalyst.
2. Only a small quantity of the catalysts in generally needed:
Since the catalyst is not used up, a very small amount of catalyst is required.
3. The catalyst does not alter the position of equilibrium in a reversible reaction
Presence of a small amount of a catalyst does not affect the position of equilibrium.
4. The catalyst does not initiate the reaction
A catalyst simply accelerates or retards a reaction. It does not initiate a reaction. However, this is
not true in all the reactions. Many reactions are known to occur only in the presence oif a catalyst.
Important feature of solid catalysts
(a) Activity
It is ability of a catalyst to catalyse a process. The activity of a catalyst depends upon the strength of
chemisorption to a large extent. The reactants must get adsorbed reasonably strongly on to the
catalyst to become active. However, they must not get adsorbed so strongly that they are
immobilized and other reactants are left with no space on the catalyst’s surface for adsorption. It
has been found that for hydrogenation reaction, the catalytic activity increases from Group 5 to
Group 11 metals with maximum activity being shown by group 7–9 elements of the periodic table.
)(OH2)g(O)g(H2 2
Pt
22 
(b) Selectivity
The selectivity of a catalyst is its ability to direct a reaction to yield a particular product. For
example, starting with H2 and CO using different catalysts we get different products.
i) CO(g) + 2H2(g) Ni
CH4(g) + H2O(g)
ii) CO(g) + 2H2(g)   32OCrZnO/Cu
CH3OH(g)
iii) CO(g) + H2(g) Cu
HCHO (g)
Hydrogenation of alkyne with Ni + H2 or Lindlar catalyst give different product. Action of a catalyst
is highly specific (selective) in nature i.e., a given substance can act as a catalyst only a in a
particular reaction and not for all the reactions. It means a substance which acts as a catalyst in one
reaction may fail to catalyse other reaction i.e., a catalyst is highly selective in nature.
Shape – Selective Catalysis by Zeolites
The catalytic reaction that depends upon the pore structure of the catalyst and the size of the reactant
and product molecules is called shape selective catalysis. The size of the pores generally varies between
260 pm and 740 pm. Thus only those molecules can be adsorbed in these pores whose size is small
enough to enter these cavities and also leave easily. It will not adsorb those molecule which are too big to
enter. Thus they act as molecular sieves e.g.: Sodium aluminium silicate can adsorb straight chain
hydrocarbons and not branched chain or aromatic ones.

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The catalysis that depends upon the pore–structure of the catalyst and molecular sizes of
reactants and product molecules is called shape selective catalysis. e.g. zeolites are shape selective
catalysts due to their honey–comb structure. ZSM–5 is used for converting methanol to gasoline.
Zeolites are micro–porous aluminosilicates of the general formula Mx/n [(AlO2)x (SiO2)y]. zH2O.
where n is the charge on the metal cation Mn+ which is usually Na+, K+ or Ca2+ and z is the number of
water molecules of hydration which are highly variable. They are three dimensional network silicates in
which some silicon atoms are replaced by Al giving Al – O – Si frame work.
Enzymes
All biological reactions are catalysed by special catalysts called enzymes. Thus, enzymes are defined as
biological catalysts. Chemically, all enzymes are globular proteins with high molar mass ranging from
15,000 to 1,000,000g mol–1 and form colloidal solution in water.
Properties of Enzymes: Some important properties of enzymes are:
(i) Specificity
Each enzyme catalyses only one chemical reaction. For example, the enzyme urease hydrolyses
urea to NH3 and CO2 but it does not catalyse the hydrolysis of N–methylurea which is similar in
constitution to urea.
(ii) Efficiency
Enzymes are very efficient catalysts. They speed up rate of a reaction by factors of upto 1020.
(iii) Small quantity
Only small amounts of enzymes can be highly efficient.
(iv) Optimum temperature and pH
Enzyme catalysed reactions is maximum at particular pH called optimum pH, which is between 7.4
and temperature of 298–310K under one atmospheric pressure. Under the these conditions, most
of the chemical reactions do not occur at appreciable rates if ordinary laboratory catalysts are
used. Human body temperature being 310 K is suited for enzyme catalysed reactions.
(v) Enzyme activators (Co–enzymes)
The activity of certain enzymes is increased in the presence of certain substances called co–
enzymes. e.g. if a protein contains a small amount of vitamin as the non–protein part, its activity is
enhanced. The activators are generally metal ions like Na+, Cu+2, Mn+2. Co+2. Amylase in presence of
sodium ions are catalytically very active.
(vi) Influence of Inhibitors and poisons:
Like ordinary catalysts, enzymes are also inhibited or poisoned by the presence of certain
substances. The inhibitors or poisons interact with active functional groups on the enzyme surface
and often reduce or completely destroy the catalytic activity of the enzymes. The use of many drugs
is related to their action as enzyme inhibitors in the body.
The action of enzymes is controlled by a number of mechanisms and are inhibited by certain
organic molecules called inhibitors.
Mechanism of Enzyme Catalysis
There are a number of cavities present on the surface of colloidal particles of enzymes. These cavities are
of characteristics shape and posses active groups such as – NH2, –COOH, –SH-, –OH, etc. These are actually
the active centres on the surface of enzyme particles. The molecules of the reactant (substrate), which
have complementary shape, fit into these cavities just like a key fits into a lock. On account of the presence
of active groups, an activated complex is formed which then decomposes to yield the products.
Thus, the enzyme–catalysed reactions may be considered to proceed in two steps.
Step 1: Binding of enzyme to substrate to form an activated complex.
E + S  ES*
Step 2: Decomposition of the activated complex to form product.
ES*  E + P
Mechanism of enzyme catalysed reaction
Colloidal State
• Substances whose solutions could pass through filter paper and animal membrane, higher rate of
diffusion are called crystalloids e.g sugar , urea , common salt.
• Substances whose solutions are heterogeneous but looks homogenous can pass through filter
paper but not animal membrane, also, having slower rate of diffusion are called colloids. Eg. Gum,
glue, starch
• The term colloid does not apply to a particular class of substance but is a state of matter like
solid, liquid and gas. Any substance can be brought into colloidal state by suitable means.
• Mixtures of substances in water, which can neither pass through filter paper nor animal
membrane are called suspensions.

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Comparison of Suspensions Colloids and True Solutions
S.No. Property True Solution Colloids Suspension
(i) Particle size < 10Å 10Å to 103 Å > 103Å
(ii) Visibility Not visible with any of
the optical means
Visible with
ultramicroscope
Visible with naked
eye
(iii) Separation
(a) with filter paper
(b) with membranes
Not possible
Not possible
Not possible
Possible
Possible
Possible
(iv) Diffusion Diffuses rapidly Diffuses very slowly
does not settle
Does not diffuse
(v) Settling Does not settle but it may settle
under Centrifuge
Settles under
gravity
(vi) Nature Hemogeneous heterogeneous Heterogeneous
(vii) Appearance Clear Generally clear Opaque
Colloids and their Classification
A colloidal system is made of two phases. The substance distributed as the colloidal particles is called
Dispersed phase (the internal phase) or the discontinuous phase. The second continuous phase in which
the colloidal particles are dispersed is called dispersion medium.
Classification of Colloids
Colloids are classified on the basis of the following criteria:
(i) Physical state of dispersed phase and dispersion medium
(ii) Nature of interaction between dispersed phase and dispersion medium.
(iii) Type of particle of the dispersed phase.
(i) Classification Based on Physical State of Dispersed Phase and Dispersion Medium
Depending on the physical states of dispersed phase or dispersion medium, colloidal solutions are
of eight types:
Dispersed
phase
Dispersion
Medium
(appearance) Name (e.g.) alloys
Solid Solid (solid) Solid Sol Coloured glasses, Pearl, Ruby,
alloys, gems
Solid Liquid (liquid) Sol Ag, Sol, Au, Sol., S. Sol., Muddy
water, gelatin in water, paint
Solid Gas (gas) Aerosol Smoke, dust, strom
Liquid Liquid (liquid) Emulsion Milk, medicines, oil water, blood
Liquid Solid (solid) Gel shampoo, jelly, cheese, butter, all
fruits and veg, polish, curd
Liquid Gas (gas) Aerosol of liquid Cloud, fog, mist, spray
Gas Liquid (liquid) Foam Soap leather, whipped cream,
shaving cream, soda water, froath
Gas Solid (solid) Solid foam Styrene foam, Foam rubber
A colloidal dispersion of one gas in another is not possible since the two gases would give a
homogenous molecular structure.
Out of the various types of colloidal given in table the most common are sols (solids in liquids), gels
(liquids in solids) and emulsions (liquids in liquids). If the dispersion medium is water, the sol is called
aquasol or hydrosol and if the dispersion medium is alcohol, it is called alcosol and so on.
(ii) Classification Based on Nature of Interaction between Dispersed Phase and Dispersion
Medium
Depending upon the nature of interaction between the dispersed phase and the dispersion
medium, colloidal sols are divided into two categories, namely, lyophilic (solvent attracting) and
lyophobic (solvent repelling). If water is the dispersion medium, the terms used are hydrophilic
and hydrophobic.
(a) Lyophilic Sols
Colloidal solutions in which the dispersed phase has considerable affinity for the dispersion
medium, are called lyophilic sols (solvent–linking). For example – dispersion of gelatin starch, gum
and proteins in water. Such colloidal solutions can be easily prepared in water directly so called
intrinsic colloids. These solutions are stable known as reversible colloids since the residue left on
evaporating can be readily transferred back into colloidal state by adding water. These sols are
quite stable and cannot be easily coagulated.
(b) Lyophobic Sols
Colloidal solution in which the dispersed phase has no affinity or attraction for the dispersion
medium are called Lyophobic colloid (solvent hating) solutions. Colloidal solutions of metals which
have negligible affinity for solvents are examples of this type. Lyophobic colloidal solutions are less
stable. On evaporation of solvent the residue cannot be easily transferred back into colloidal state
by ordinary mean hence also called extrinsic colloid. Therefore, lyophobic colloids are also called
irreversible colloids. Lyophobic sols need stabilizing agents for their preservation. Such sols are
readily precipitated on addition of small amount of electrolytes by heating or by shaking.

9
Comparison of Lyophobic and Lyophilic sols
SN Property Lyophobic sol (Suspensoid) Lyophilic sol (Emulsoid)
1 Preparation Cannot be prepared easily, special
methods are required
Can be easily prepared by shaking
or warming the substance with
solvent
2 Stability are less stable are more stable
3 Reversibility are irreversible are reversible
4 Hydration or solvation These are less solvated as the
particles have less affinity for the
solvent
These are highly solvated as the
particles have great affinity for
solvent
(iii) Classification Based on Type of Particles of the Dispersed Phase
Depending upon the type of the particles of the dispersed phase, colloids are classified as:
(a) Multimolecular Colloids:
On dissolution, a large number of atoms or smaller molecules of a substance aggregate together to
form species having size in the colloidal range (diameter < 1 nm). The species thus formed are
called multimolecular colloids. For example, a gold sol may contain particles of various sizes
having many atoms. Sulphur sol consists of particles containing a thousand or more of S8 sulphur
molecules.
(b) Macromolecular Colloids
These are formed by macromolecules which have bigger size than the colloidal particle but as soon
as they are put in suitable solvents, they get dissociated to form smaller particle of the colloidal
range. Such systems are called macromolecular colloids. These colloids are quite stable and
resemble true solutions in many respects. Examples of naturally occurring macromolecules are
starch, cellulose, proteins and enzymes; and those of man–made macromolecules are polythene,
nylon, polystyrene, synthetic rubber, etc.
(c) Associated Colloids (Micelles)
There are some substances which at low concentrations behave as normal strong electrolytes, but
at higher concentrations exhibit colloidal behaviour due to the formation of aggregates. The
aggregated particles thus formed are called micelles. These are also known as associated
colloids. The formation of micelles takes place only above a particular temperature called kraft
temperature (Tk) and above a particular concentration called critical micelle concentration
(CMC). Surface active agents such as soaps and synthetic detergents belong to this class.
Mechanism of Micelle Formation
Let us take an example of soap solutions. Soap is sodium or potassium salt of a higher fatty acid and may
be represented as RCOO Na+ (e.g., sodium stearate CH3(CH2)16COO–Na+, which is a major component of
many bar soaps). When dissolved in water, it dissociates into RCOO– and Na+ ions. The RCOO– ions,
however, consist of two parts – a long hydrocarbon chain R (also called non – polar ‘tail’) which is
hydrophobic (water repelling), and a polar group COO– (also called polar–ionic ‘head’), which is
hydrophilic (water loving).
Hydrophobic tail and hydrophilic head of stearate ion
The RCOO– ions are, therefore, present on the surface with their COO– groups in water and the
hydrocarbon chains R staying away from it and remain at the surface. But at critical micelle concentration,
the anions are pulled into the bulk of the solution and aggregate to form a spherical shape with their
hydrocarbon chains pointing towards the centre of the sphere with COO– part remaining outward on the
surface of the sphere. An aggregate thus formed is known as ‘ionic micelle’. These micelles may contains
as many as 100 such ions.

10
Similarly, in case of detergents. e.g., sodium laurylsulphate, CH3(CH2)11 O ,NaSO3

the polar group is

 3OSO along with the long hydrocarbon chain. Hence, the mechanism of micelle formation here also is
same as that of soaps.
Note: The soaps and detergents form micelles in water only because of the presence of charge on their
molecules. Micelles formation does not occur in solvent like ethyl alcohol since it is not as polar as soaps.
That is why only water is used for the washing of dirty clothes.
Method of Preparation of Colloidal Solution
Lyophilic sols may be prepared by simply warming the solid with liquid dispersion medium e.g., starch
with water. On the other hand lyophobic sols have to be prepared by special methods. These methods fall
into two categories:
(a) Dispersion methods in which large macrosized particles are broken down to colloidal size.
(b) Condensation methods in which colloidal sized particles are built up by aggregating single ions or
molecules. This method is also known as condensation method.
Sr.
No.
Dispersion method Aggregation or condensation
method
1 Mechanical dispersion 1. Exchange of solvents
2 Electro–dispersion 2. Change of physical state
3 Ultrasonic dispersion 3. Chemical method
4 Peptization (a) Double decomposition
(b) Oxidation
(c) Reduction
(d) Hydrolysis
(a) Dispersion Method
1. Mechanical dispersion (e.g. black ink, paint, varnish) dye
Solid material is first finely grounded by usual methods. It is
then mixed with dispersion medium which gives a coarse
suspension. The suspension is now introduced into the colloid
mill. The simplest form of colloid mill consist of two metal
discs held at a small distance apart from one another and
capable of revolving at a very high speed (about 7000
revolutions per minute) in opposite directions. The particles
are grounded down to colloidal size and are then dispersed in
liquid.
2. Electrical Dispersion (Bredig’s arc Method) (e.g. metal sol.)
This method is suitable for the preparation of colloidal
solutions of metals like gold, silver, platinum, etc. An arc is
struck between the metal electrodes under the surface of
water containing some stabilizing agent such as a trace of
KOH. The water is cooled by immersing the container in a ice
bath. The intense heat of the arc vaporizes some of the metal
which condenses under cold water.
3. Peptization
The dispersion of a freshly precipitated material into colloidal
solution by the action of an electrolyte in solution is termed
peptization, the electrolyte used is called a peptizing agent.
During Peptization, the precipitate adsorbs one of the ions of
the electrolyte on its surface. This causes the development of
Colloidal mill

11
positive or negative charge on precipitates, which cause
electrostatic repulsion and ultimately break up into smaller
particles of the size of a colloid.
Ex:
Freshly prepared ferric hydroxide on treatment with a small amount of ferric chloride
solution at once forms, a dark raddish brown solution. Ferric chloride acts as a peptizing
agent.
Purification of Colloidal Solutions
Colloidal solutions prepared by above methods generally contain excessive amount of electrolytes and
some other impurities. The purification of colloidal solution is carried out by the following methods:
• Dialysis:
Animal membranes (bladder) or those made of parchment
paper and cellophane sheet have very fine pores. These
process permit ions or very small molecules to pass
through but not large colloidal particles. Dialysis is a
process of removing a dissolved substance (impurities)
from a colloidal solution by means of diffusions
through semipermiable which is a bag of suitable
membrane containing colloidal solution to be purified,
placed in a vessel (or continuous flow of water). The
ions or molecules of impurities diffuse through membrane
and get dissolved in outer water and pure colloidal solution
is left in the bag. Blood is a colloidal solution and is purified
by dialysis.
When potential difference is applied across the membrane,
ions in the solution move faster towards opposite electrode.
This process is called electrodialysis. The colloidal solution
is placed in a bag of suitable membrane while pure water is
taken outside. Electrodes are fitted in the compartment as
shown in figure. The ions present in the colloidal solution
migrate out towards the oppositely charged electrodes.
• Ultra–filtration: Ultra–filtration is the process of separating colloidal particles from the solvent
and soluble solute in the colloidal solution by especially prepared filters, which are permeable to
all substances except colloidal particles. Such filter are called ultrafilters. However, the pores of
filter paper can be reduced in size by impregnating with colloidion solution to stop the flow of
colloidal particles. The usual colloidion is a 4 % solution of
nitro–cellulose in a mixture of alcohol and ether. An ultra–filter paper may be prepared by
soaking the filter paper in a colloidal solution, hardening by formaldehyde and then finally drying
it.
• Ultracentrifugation: In this method, the impure sol is taken in a tube which is placed in an
ultracentrifuge. In this machine, the tube is rotated at a very high speed. As a result, the colloidal
particles settle down at the bottom of the tube whereas the crystalloids and other soluble
impurities remain in the solution. This solution is decanted off and the colloid particles are
remixed with the dispersion medium to give the pure colloidal sol.
Properties of Colloidal Solution
• Heterogeneous:
Colloidal particles in a solution differ in sizes and are not homogeneously distributed throughout
the solution.
• Visibility:
Colloidal particles cannot be seen with naked eyes or with the help of microscope. It is a well
known fact. No particle is visible if its diameter is less than half the wavelength of light used. The
visible light has greater wavelength than the size of colloidal particle.
• Colour:
The colour of hydrophobic sol depends on the wavelength of the light scattered by the dispersed
particles. The wavelength of the scattered light again depends on the size and the nature of
particles. The colour of colloidal solution also changes with the manner in which the observer
receives the light.
For example, a mixture of milk and water appears blue when viewed by reflected light and red
when viewed by the transmitted light. Finest gold sol is red in colour, as the size of particles
increases, it appears purple, then blue and finally golden. When light emitted by the setting sun
passes through the blanket of dust, the blue part of the light is scattered away from out eyes and
at the same time the red colour is seen. This sun appears red while setting.
Colour of Ag Sol Particle diameter
Orange Yellow 6 × 10–5 mm
Orange Red 9 × 10–5 mm
Purple 13 × 10–5 mm

12
Violet 15 × 10–5 mm
• Colligative Properties
These properties depend on the number of solute particles in solution. In case of colloidal
solutions, colloidal particles are the aggregates of many ions or smaller molecules and when
compared to true solutions or normal solutions, the total number of particles of solute in solution
are very less and hence these solutions exhibit colligative properties to lesser extent.
• Optical Properties: Tyndalll effect
Sols exhibit Tyndall effect. When a beam of light is passed through a sol and viewed at right angles.
The path of the light shows up a hazy beam of cone. This was first observed by Farraday and later by
Tyndall and is known as Tyndall effect. It may be defined as the scattering of light by the colloidal
particles in a colloidal sol. The bright cone of the light is called “Tyndall cone”. The Tyndall effect is due
to the fact that the colloidal particles absorb light and scatter it in all colloidal dispersion. Tyndall effect is
observed only when the following two conditions are satisfied
(i) The diameter of the dispersed particles is not much smaller than the wavelength of light used.
(ii) The refractive index of dispersed phase & the dispersion medium differ greatly in magnitude.
Some example of Tyndall effect are:
(i) Blue colour of sky and sea water.
(ii) Visibility of tails of comets.
(iii) Twinkling of stars.
• Kinetic Properties: (Brownian Movement)
When a sol is examined with an ultramicroscope, the suspended particles are seen as shining
speeks of light. By following an individual particle, it is observed that the particle is in a state of
continuous motion in zig–zag directions. The continuous rapid zig–zag motion of a colloidal
particle in the dispersion medium is called “Brownian movement of motion” (first observed by
British botanish Robert Brown).
The Brownian movement has been explained to be due to the unbalanced bombardments of the particles
by the molecules of dispersion medium. This motion is independent of the nature of the colloid but
depend on the size of the particles and viscosity of the solution. Smaller the size and lesser the viscosity,
faster is the motion. The Brownian movement has a stirring effect which does not permit the particles to
settle and thus, is responsible for the stability of sols.
• Charge on Colloidal Particles:
Colloidal particles always carry an electric charge. The mutual forces of repulsion between
similarly charged particles prevent them from aggregating and settling under the action of gravity.
This gives stability to the sol. A list of common sols with the type of charge on their particles is
given below.
SN Positively Charged Negatively Charged
1. Hydrated metallic oxides
e.g.: Al2O3.xH2O, CrO3.xH2O and Fe2O3.xH2O etc
Metals e.g. copper, silver, gold sols.
2. Basic dye stuff example – Methylene blue sol Metallic sulphides like As2S3, Sb2S3, CdS
3. Proteins in acidic medium Hemoglobin (blood) Acid dye stuff example – Congored sols, eosin,
albumin.
4. Oxides like TiO2, etc Sols of starch, gum gelatin, clay & charcoal
The charge on colloidal particles is due to one or more of the following reasons:
(i) The sol particles acquire positive or –ve charge by preferential adsorption of +ve or –ve ions from
the dispersion medium.
Preferential adsorption of ions is the most accepted reason. The sol particles acquire positive or negative
charge by preferential adsorption of +ve or –ve ions. When two or more ions are present in the dispersion
medium, preferential adsorption of the ion common to the colloidal particle usually takes place. This can
be explained by taking the following examples:
(a) When silver nitrate solution is added to potassium iodide solution, the precipitated silver iodide
absorbs iodide ions from the dispersion medium and negatively charged colloidal solution results.
However, when Kl solution is added to AgNO3 solution, positively charged sol results due to
adsorption of Ag+ ions from dispersion medium.

13
Agl/l– Agl/Ag+
Negatively charged Positively charged
(b) If FeCl3 is added to excess of hot water, a positively charged sol of hydrated ferric oxide is formed
due to adsorption of Fe3+ ions. However, when ferric chloride is added to NaOH a negatively
charged sol is obtained with adsorption of OH– ions.
Fe2O3.xH2O/Fe3+ Fe2O3.xH2O/OH–
Positively charged Negatively charged
Having acquired a positive or a negative charge by selective adsorption on the surface of a colloidal
particle as stated above, this layer attracts counter ions from the medium forming a second layer,
as shown below.
Agl/l– K+ Agl/Ag+ l–
The combination of the two layers of opposite
charges around the colloidal particle is called
Halmholtz electrical double layer. According to
modern views, the first layer of ions is firmly held
and is termed fixed layer while the second layer is
mobile which is termed diffused layer. Since
separation of charge is a basis of potential, the
charges of opposite signs on the fixed and diffused
parts of the double layer results in a difference in
potential between these layers. This potential
difference between the fixed layer and the diffused
layer of opposite charges is called the electrokinetic
potential or zeta potential.
The presence of equal and similar charges on colloidal particle is largely responsible in providing
stability to the colloidal solution, because the repulsive forces between charged particles having
same charge prevent them from coagulating or aggregating when they come closer to one another.
Electrical Properties:
(a) Stability of colloidal sols – Electrical charge on colloidal particles: The dispersed phase
particles carry either +ve and –ve charge and dispersion medium has an equal and opposite charge.
The particles repel one another and hence do not coagulate, thus making the colloid stable.
Origin of electrical charge on colloidal particles:
a. Due to frictional electrification
b. due to electron capture by sol particles.
c. Due to preferential adsorption of ions from the solution.
d. Due to dissociation of molecule electrolytes adsorbed on the surface of the particles.
e. Due to dissociation of molecules followed by aggregation of ions
(b) Cataphoresis or Electrophoresis is the movement of
colloidal particles either towards cathode or anode,
depending on their charge, under the influence of an electric
field. Electrophoresis is used to determine nature of charge
Positive sol. Sol Fe(OH)3, Ca(OH)2, Al(OH)3, basic dye stuffs
(methylene blue, methyl violet, Haemoglobin).
Negative Sol. Metal & sulphides: CdS, As2S3, Cu, Ag, Pt and
dyes (congored, Prussian blue, silicic acid, gum, charcoal).
(c) Electro Osmosis:
A sol is electrically neutral therefore the dispersion medium carries an equal but opposite charge
to that of dispersed particles. Thus the medium will move in opposite direction to the dispersed
phase under the influence of applied electric potential if colloidal solution is enclosed within
semipermeable membrane. The movement of dispersion medium under the influence of applied
potential is known as “Electro–osmosis.”
Electro–osmosis is direct consequence of the existence of zeta potential between the sol particle
and the medium. When the applied potential exceeds the Zeta potential, the diffused layer move
and causes electro–osmosis.
(d) Coagulation or flocculation: [Process of setting of colloidal particle]
We known that the stability of a lyophobic sol is due to the adsorption of positive or negative ions
by the dispersed particles. The repulsion forces between the charged particles do not allow them to

14
settle. If somehow, the charge is removed there is nothing to kept the particles apart from each
other. In such cases they aggregate or flocculate and settle down under the action of gravity.
The flocculation and setting down of the discharged sol particles is called coagulation or the
precipitation and can be brought about in following ways:
(a) By addition of electrolyte. (b) By electrophoresis
(c) By mixing two oppositely charged sols. (mutual precipitation)
(d) By boiling (e) Persistent dialysis.
(a) By Addition of Electrolytes:
When an electrolyte is added in excess to a sol, the electrolyte furnishes both types of ions in
solution. The oppositely charged ions get adsorbed on the surface of colloidal particles. This causes
neutralization and thereby the size and mass of colloidal particle increases and it becomes a
suspension particle. Due to greater volume and greater mass these suspension particles settle
down i.e., they coagulate. The ion irresponsible for neutralization of charge on the particle is called
flocculating ion.
• Coagulation or flocculation is the process of bringing colloidal particles closer so that they
aggregate to form larger particle that precipitate and settle down or float on the surface. It is
usually done by addition of an electrolyte. Effective ions or electrolyte are those which carry
charge opposite to colloids.
• Hardy – Schulze rule states that “greater is the valency of the oppositely charged ion of electrolyte
being added, faster is the coagulation: e.g. for a negatively charged sol, the order is: Al3+ > Ba2+ >
K+, for a positively charged sol the order is: 
 ClSOPO)CN(Fe 2
4
3
4
4
6
• Coagulating value is the minimum amount of electrolyte in milli moles required to coagulate a 1
lit of solution of sol in two hours. The smaller the quantity needed, the higher will be the
coagulating power of anion.
(b) By Electrophoresis:
During electrophoresis the charged sol particles migrate towards the electrode of opposite sign.
There they deposit their charge and then get coagulated (As the neutral particles can aggregate and
change to suspension particles).
(c) By Mixing Two Oppositely Charged Sols:
The mutual coagulation of two sols of opposite charge can be effected by mixing them. For e.g.
Fe(OH)3 (positive sol) and Arsenious sulphide (negative sol) when mixed join and coagulate.
(d) By Boiling
Sols such as sulphur and silver halides disperse in water, get coagulated when boiled, due to
increased collisions between sol particles and water molecules, which remove the adsorbed layer
from the sol and therefore the sol particles settle down.
(e) Persistent dialysis:
The stability of a colloidal sol is due to presence of a small amount of the electrolyte. On prolonged
dialysis, the electrolyte is completely removed. As a result, the colloidal sol becomes unstable and
gets coagulated.
Coagulation of Lyophillic Sols:
Lyophilic sols are stable due to charge and solvation of the colloidal particles. When these two factors
are removed, a lyophilic sol can be coagulated. This is done (i) by adding electrolyte; (ii) by adding
suitable solvent.
When solvents such as alcohol and acetone are added to hydrophilic sol, the dehydration of dispersed
phase occurs. Under this condition a small quantity of electrolyte can bring about coagulation.
Protection of Colloids:
• Lyophilic sols are more stable than lyophobic sols hence they
are used as protective colloids to increase the stability of
lyophobic sols, e.g. addition of gums, gelatin etc. to certain metal
sols. These sols are called Protective colloids.
• Protective action of lyophilic sols is explained due to formation
of a thin layer around the lyopobic sol particles, thus preventing
them from coming closer.
Emulsions
Emulsion is a colloidal system involving one liquid dispersed in another, provided both are immiscible. Some
commonly known emulsions are Milk, butter, milk cream, cold cream, vanishing cream, etc. There are
many drugs and medicines which are also in the form of emulsions e.g. various ointments, cod liver oil etc.
• Types of emulsions:
Depending upon the type of proportions in which these constituents are present, emulsion have
been classified into two types.
1. Oil in water emulsions (O/W) are those in which oil is the dispersed phase and water is the
dispersion medium. Milk is a common example in which liquid fats are dispersed in water.
Similarly, if a few drops of nitrobenzene (oil) is added to water, an emulsion results. Varnishing
cream is another example of this type.

15
2. Water in oil emulsions (W/O) are those in which water is the dispersed phase and oil is the
dispersion medium, Butter is an example of water in oil emulsion in which water is dispersed in oil.
Cod liver oil and cream are the other examples of these emulsions.
• Detection of the types of Emulsions
The nature of an emulsion whether O/W or W/O can be detected with the help of following tests.
Dilution test. To the emulsion taken in a beaker, add a small amount of water. In cases it gets
diluted, it is an example of oil in water type emulsion. If water forms a separate layer then the
emulsion is water in oil type. Milk, for example, can be readily diluted with water. It is therefore, oil
in water type emulsion.
• Emulsifier and Emulsifying Agents
An emulsifier or emulsifying agent is a substance which helps in stabilizing an emulsion of oil and
water and, thus, prevents them from getting apart. The commonly used emulsifying agents for
O/W emulsion are soaps, detergents, lyphilic colloids, proteins, gums and agar etc. & for W/O
emulsion are heavy metal salt of long chain fatty acids etc. The preparation of emulsion in the
presence of an emulsifier is known emulsification. The role of an emulsifier in stabilizing an
emulsion can be explained as :
It is believed that an emulsifier gets concentrated at
the oil–water interface i.e., the surface at which oil and
water come in contact with each other. It forms a
protective coating around each drop of oil and thus,
prevents the oil drops from coming in contract with
one another. The oil drops remain suspended in water
and are not coagulated.
Gels
• These are colloidal systems where liquids are the dispersed phase and solids act as dispersion
medium, e.g. curd, cheese etc. When a warm solution of gelatin is cooled it sets to a semisolid mass
which is gel. This process is called gelation.
Applications of Colloidal System
Colloids are widely used in the industry. Following are some examples.
1. Medicines in colloidal form can be easily adsorbed by body tissues & hence are more effective e.g.
colloidal antimony is used in curing kalaazar, argyrols is a silver sol used as an eye lotion.
Colloidal gold is used for intramuscular injection Neutralization of excess acidity by Al (OH)3, Milk
of magnesia, an emulsion, is used for stomach disorders, colloidal sulphur – germ killer, kalolin
used to remove toxins from intestine. Cod liver oil is emulsion of oil in water. Some ointment,
antibiotics, Penicillin, streptomycin are produced in colloidal form. Colloidal medicines are more
effective because they have large surface area and are therefore easily assimilated.
2. Food: Gelatin in ice–cream. It preserve smoothness and prevent ice crystals formation, protein,
milk, cheese are colloids, fruits, jelly, cream, bread is dispersion of air in baked dough.
4. Purification of water by alums: Water from rivers or lakes are sometimes used for domestic and
industrial purpose after purification. The water from lake or river is turbid due to the presence of
fine clay particles which are negatively charged. These can be removed by adding potash alum or
aluminium sulphate. Al3+ ions from potash alum o aluminium sulphate neutralize the negative
charge on clay particles. This causes the coagulation of clay particles which settle down leaving
water which is clear of further treatment.
5. Stoppage of bleeding from a fresh cut: The bleeding from a fresh cut can be immediately
stopped by applying a concentrated solution of ferric chloride or potash alum. Blood consists of
colloidal particles of haemoglobins which carry positive charge on them. When ferric chloride or
alum is applied on the cut these colloidal particles get their positive charge neutralized by the
anions available from these substances in solution. In the absence of the charge, they get
coagulated and the bleeding stops.
6. Delta formation: Formation of delta due to coagulation of colloidal particles of river water by sea
salt. The river water contains colloidal particles of sand and clay which carry negative charge. The
sea water, on the other hand, contains positive ions such as Na+, Mg2+ and Ca++. As the river water
meets sea water, these ions discharge the sand or clay particles which are precipitated as delta.

16
7. Photographic plates and films: Photographic plates or films are prepared by coating an
emulsion of the light sensitive silver bromide in gelatin over glass plates or celluloid films.
8. Fog, mist and rain. In extremely cold weather, the temperature is very low. The water droplets
(moisture) present in air condense on the surface of the dust particles that are suspended. Since
these are colloidal in nature, they float in air in the form of mist or fog or extremely low, these
colloidal droplets grow in size. They become bigger and bigger and when atmosphere is not in a
position to hold them, they come down as rain.
In the winter season, the atmosphere generally becomes very foggy and visibility is quite poor.
This leads to lot of inconvenience for vehicular movement and result in major accidents. Farmers
are in the habit of burning husk in the open fields. This results in the release of unburnt carbon
particles in the atmosphere. The water droplets condense on them and they appear as fog. But
actually this is smog and not fog and is extremely injurious to health. This can lead to asthama,
lung as well as throat cancer. It is very essential to educate our farmer community about these
harzards.
9. Purification of Blood: Blood is a colloidal solution. It contains toxic waste products such as urea
and uric acid which pass through the membrane while colloidal sized particles of blood proteins
hemoglobin are retained. Kidney failure, therefore, leads to death due to accumulation of
poisonous waste products in the blood. Blood is purified by dialysis.
10. Chemical warfare: (Smoke screen): Smoke is a colloidal solution of solid particles such as
carbon, arsenic compounds, dust, etc., in air. The smoke, before it comes out from the chimney, is
lead through a chamber containing plates having a charge opposite to that carried by smoke
particles. The particles on coming in contract with these plates lose their charge and get
precipitated. The particles thus settle down on the floor of the chamber. The precipitator is called
Cottrell precipitartor. Colloidal dispersions of certain substance, like TiO2 (as dispersed phase) in
air (as the dispersion medium) are used a smoke screens in warfare for the purpose of
concealment. The dispersed phase particles (i.e. TiO2) being very heavy rapidly drops down in the
form of a curtain of drizzling whiteness.
11. Artificial rain: Tiny water droplets in clouds are electrically charged. In any cloud all such water
particles carry the same charge. Artificial rain can be caused by spraying oppositely charged dust
or fine sand or precipitates like Agl (which has a crystal structure similar to ice and as such
particles of Agl can act as nuclei for precipitation) on to a cloud. Even certain salts in t he form of
fine particles can be sprayed. The neutralization of charge results in coagulation of water droplets
or rain. Cloud–brust–a natural disaster resulting in a very heavy down pour over a very short time
is believed to occur due to the mutual discharge of oppositely charged clouds.
12. Cleaning Action of Soaps and Detergents: Dust and dirt particles on clothes are colloidal in
nature. Soaps being sodium salts of higher fatty acids coagulate them which become suspension
particles called micelle and roll out due to greater volume and greater mass.
13. Rubber Industry: Latex is a colloidal solution of rubber particles which are negatively charged.
Rubber is obtained by coagulation of Latex. Rubber is electroplated on metals when they act as
anode.
14. Tanning: Process of hardening of leather is called tanning. Tannin obtained from plants is
negatively charged sol and animal hides are also sol containing positively charged particle. Mutual
coagulation results in hardening of leather.

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