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SURFACE CHEMISTRY
# SURFACE CHEMISTRY: The branch of chemistry which deals with the nature of surface and
changes occurring on the surface is called surface chemistry. Adsorption on solid or on solution
surface, corrosion and colloidal properties are important surface effects.
# Some important points regardind surface chemistry:
1. Adsorption: The tendency of accumulation of molecular species at the surface than in the bulk of
solid or liquid is termed adsorption. Adsorption is a surface phenomenon. E.g. when a small amount of
finely divided charcoal is put into a vessel containing a gas, it is observed that pressure of the gas
decrease rapidly at first and then gradually. The decrease in pressure of the gas is due to the
accumulation of the gas on the surface of charcoal.
2. Adsorbate: The substance which gets adsorbed on any surface is called adsorbate.
3. Adsorbent: The substance on the surface of which the adsorption takes place is called adsorbent. In
the above example Charcoal surface is adsorbent while gas is adsorbate.
4. Desorption: The removal of the adsorbed substance from the surface is called desorption.
5. Positive and Negative adsorption: When the concentration of adsorbate is more on the surface of
the adsorbent, then in the bulk, it is called positive adsorption. If the concentration of the adsorbate is
less relative to its concentration in bulk, it is called negative adsorption.
6. Sorption: The process in which adsorption and absorption takes place simultaneously is known as
sorption.
7. Absorption: When the molecules of a substance are uniformly distributed through out the body of
other substance is known as absorption. E.g. Absorption of water vapours by anhydrous calcium
chloride.
- Difference between Adsorption and Adsorption: The main difference between Adsorption and
absorption are as followsAdsorption
1. Adsorption occurs on the surface in of the
Absorption
1. Absorption occurs throughout the solid i.e. the
adsorbent.
bulk of the material.
2. Concentration of the adsorbate is different
2. Concentration of adsorbate is same through
from that in the bulk.
out the material.
3. Rate of adsorption decreases until
3. Rate of absorption is same throughout the
equilibrium is reached.
4. Examples, Adsorption of O2, H2, NH3,
SO3 on activated charcoal.
process.
4. Absorption of NH3 & CO2 by water vapour
forming NH4OH & H2CO3.
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# Type of Adsorption: Adsorption is classified into two types on the basis of attraction between
molecules of adsorbate and adsorbent.
1. Physical Adsorption
2. Chemical Adsorption
1. Physical Adsorption: When adsorbate is held on the surface of the solid (adsorbent) by weak
Vander Waal’s forces, it is called physical adsorption. E.g. gases adsorbed on activated charcoal and
N2 adsorbed on mica.
2. Chemical Adsorption: When adsorbate is held on the surface of a solid by forces similar to those
of chemical bond, this type of adsorption is called chemical adsorption/chemisorption.
- Comparison of physic-sorption and chemi-sorption:
Physical Adsorption
Chemical Adsorption
1. It is caused by intermolecular vanderwaal’s
1. It is caused by chemical bond formation.
forces.
2. It is not specific.
2. It is highly specific.
3. It is reversible.
3. It is irreversible.
4. It depends on the nature of gas. More easily
4. It does not depend on the nature of the gas.
liquefiable gases are adsorbed readily.
5. Low temperature is favourable. It decreases
5. High temperature is favourable. It increases
with increase in temperature.
with increase in temperature.
6. High pressure is favourable. It increases
6. Increase of pressure does not affect
with increases in pressure.
adsorption.
7. It forms multi-layers on adsorbent surface
7. It forms uni-molecular layer.
under high pressure.
8. Heat of adsorption is low 20-40 KJ mole.
8. Heat of adsorption is high 200-400 KJ mole.
9. The state of adsorbate is same as in bulk.
9. State of adsorbate molecules may be different
from that in the bulk.
10. E.g. N2 adsorbed on mica.
10. E.g.CO2 or O2 adsorbed on tungusten, H2 on Ni,
O2 adsorbed on Ag, Au, etc.
- Adsorption of a gas on a solid surface is effected by following factors(i) Nature of gas
(iv) Activation of adsorbate
(ii) Nature of adsorbent
(iii) Effect of temperature
(v) Effect of pressure
(i) Nature of gas: The adsorption depends upon the nature of the gas adsorbed. The easily liquefiable
gases such as HCI, NH3, Cl2, CO2, and CO2 etc are adsorbed more than the permanent gases such as
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H2, N2, and O2 etc. The ease of liquefaction of a gas depends upon its critical temperature. The critical
temperature of a gas is the minimum temperature above which a gas cannot be liquefied however high
the pressure may be applied. The higher the critical temperature, the more easily a gas is liquefied and
hence more readily it will be adsorbed.
(ii) Nature of adsorbent: The extent of adsorption of a gas depends upon the nature of adsorbent.
Adsorption increase with increase of surface area of the adsorbent, therefore finely divided and porous
solids are best adsorbents as they provide large surface area.
Examples – Finely divided metals like Ni, Pt and porous substance – charcoal, silica gel.
(iii) Effect of temperature: The process of adsorption is exothermic, therefore desorption is
endothermic. The rate of adsorption decreases with the increase in temperature, at constant pressure,
and with decrease in temperature rate of adsorption increases. As a result the adsorption of gas by
solid results in emission of energy.
The energy is called adsorption energy. Thus adsorption energy is the energy emitted by
adsorption of one mole of gas or vapours on solid.
(iv) Activation of adsorbent: Activation of adsorbent means the increase in adsorbing power of the
adsorbent. This can be done by the following methods(a) Metallic adsorbents are activated by mechanical rubbing or by subjecting it to some chemical
reactions.
(b) To increase the adsorbing power of adsorbents, they are sub-divided into smaller pieces.
(c) Some adsorbents are activated by strong heating in them with superheated steam.
(v) Effect of pressure: The extent of adsorption of a gas per unit mass of adsorbent depends upon the
pressure of gas. The relation amount of substance adsorbed by adsorbent (x/m) and the equilibrium gas
pressure (p) at constant temperature is called as adsorption isotherm.
- Freundlich adsorption isotherm curves: The variation of extent of adsorption (x/m) with pressure
(p) was given mathematically by Freundlich. From the adsorption isotherm, the following observation
can be easily made-
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(a) At low pressure, the graph between x/m & P gives a straight line which indicates that x/m is
directly proportional to the pressure. Thusx
P
m
x
=kP
m
or
(k = Constant)
(b) At high pressure graph becomes almost constant which means that x/m becomes independent of
pressure. Thusx
= Constant
m
or
x
 P0
m
or
x
= k P0
m
( P0 = 1)
(c) In the intermediate range of pressure, x/m will depend upon the power of pressure, which lies
between 0 to 1 Thusx
 P1/n
m
OR
x
k P1/n
m
( here n > 1)
Where x/m depends upon the nature of adsorbate and adsorbent. The above relation is called
Freundlich’s Isotherm equation. The constants k & n can be determined as explained belowx
= k P1/n
m
…….(1)
Taking logarithms on both sidelog
x
1
= log K +
log P
m
n
Thus the plot between x/m and P gives a straight line with slope equal to 1/n intercept log K.
- Adsorption From Solution Phase: Apart from adsorption of the gases, solids have also the capacity
to adsorb substances present in solutions. E.g. if we place a piece of charcoal in a litmus solution taken
in a test tube and shake, the solution becomes colourless. It is because of adsorption of the litmus
which is in fact a dye, by the charcoal. Similarly, animal charcoal decolourises impure sugar solution
by adsorbing colouring dye.
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A few Important observation regarding adsorption from solution are given(i) The extent of adsorption decreases with the increase in temperature.
(ii) The increase in surface of the adsorbent promotes adsorption.
(iii) The extent of adsorption depends upon the concentration of the solute in the solution.
(iv) The extent of adsorption depends upon the nature of adsorbate and adsorbent.
The actual mechanism of adsorption from the solution is not definite. However, it is
believed that it continues till a unimolecular layer is built up on the surface of the adsorbent. The
Freundlich adsorption isotherm is applicable to the adsorption from the solutions in the same way as in
case of gases. The effect of temperature is also similar. However, equilibrium concentration (C) is
used in place of equilibrium pressure and the mathematical expression isx
= k C1/n
m
…….(2)
Taking logarithms on both sidelog
x
1
= log K +
log C
m
n
- Adsorption isobar for physisortion and chemisorption:
- Applications of Adsorption: The phenomenon of adsorption finds extensive applications in
industry, in laboratory and also in various chemical processes. A few important applications are listed
below1. Removal of colouring matter: Many substances such as sugar, juice and vegetable oils are
coloured due to the presence of impurities. They can be decolourised by placing in contact with
adsorbents like activated charcoal or fuller’s earth. Similarly, most of the organic compounds that are
synthesised in the laboratory are coloured. They can also be decolourised with the help of animal
charcoal.
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2. In gas masks: Activated charcoal is generally used in gas masks to adsorb poisonous and toxic
gases from air. These masks are used by the miners since there is atmosphere of CO and other such
gases prevailing in the coal mines.
3. In separating noble gases: Activated coconut charcoal has the capacity to adsorb the noble gases
present in a mixture (except radon) at different temperatures. This helps in the separation of these
gases. The process of separation has been discussed in detail in competition focus.
4. In dying of cloth: Mordents such as alums are used in dying of cloth. They adsorb the dye particles
which otherwise, do not stick to the cloth.
5. In dehumidizers: Silica gel is commonly employed to adsorb humidity or moisture from air. This
is necessary for the storage of delicate instruments which might otherwise be damaged by contact with
moist air.
6. Heterogeneous catalysis: The use of the metal catalysts such as Fe, Ni, Pd, etc., in the
manufacturing processes such as Contact process, Haber process and the hydrogenation of oils etc., is
based upon the phenomenon of adsorption.
7. In ion-exchange resins: The organic polymers containing groups like – COOH, –SO3H, –NH2 etc.,
possess the property of selective adsorption of ions from solutions. These are quite useful in the
softening of water and also in the separation of the elements of the Lanthanoid series (also called rare
earths) from their mixtures.
8. 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.
9. In qualitative analysis: Certain qualitative tests such as the lake test for the confirmation of Al3+
ions are based upon adsorption, i.e. Al(OH)3 has the capacity to adsorb the colour of blue litmus from
the solution.
10. In adsorption indicators: In many titrations involving precipitating reactions, dyes such as eosin
and flourescein are used as adsorption indicators. This is based upon the fact that these are more
adsorbed by the precipitates than by the solution.
11. In creating high vacuum: For some experiments, a very high vacuum is needed which means that
the air from the vessel has to be completely evacuated. Vacuum pumps are used for the purpose.
However, charcoal can be used to remove last traces of air to achieve very high vacuum.
12. In Froth floatation process: In this process commonly used for the concentration of sulphide ores
containing some earthy impurities (gangue) mainly carrying silica, pine oil is mixed with the impure
ore. The oil particles are adsorbed on the surface of the sulphide ore particles leaving behind the
gangue impurities.
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- Catalysis: The substances which alter the velocity of a reaction, themselves remaining chemically
and quantitatively unchanged after the reaction, are called catalysts and the phenomenon is known as
catalysis. When catalyst accelerates the rate of reaction it is said to be a positive catalyst.
NOTE-01: Negative catalyst or inhibitors: If it retard the rate of reaction it is called negative
catalyst or inhibitors.
E.g. (i) The decomposition of H2O2 gets retarded by the presence of glycerol or phosphoric acid.
(ii) A small amount of ethtyl alcohol retards the oxidation of chloroform to phosgene.
(iii) Ethylene glycol decreses the rusting of iron.
NOTE-02: If one of the product of the reaction it self acts as a catalyst, it is known as auto-catalyst.
E.g. The oxidation of oxalic acid by acidic KMnO4 is initially slow but Mn2+ ions formed in the
reaction act as auto-catalyst and increase the rate of reaction.
H2C2O4 + 2 KMnO4 3 H2SO4  2 MnSO4 + K2SO4 + 10 CO2 + 8 H2O
- Type of catalysis: Catalytic reactions can be divided into two types(1) Homogeneous catalysis: When the reactants and catalyst are in same phase, the catalysis is said to
be homogeneous catalysis. E.g.
H O ( l )
3
(i) CH3COOC2H5 (l) + H2O (l) 
 CH3COOH (l) + C2H5OH (l)
(ii) SO2 (g) + ½ O2 (g)  SO3 (g)
NO ( g )
(iii) 2CO (g) + O2 (g)  2CO2 (g)
NO ( g )
- Theory of Homogenous catalysis: The homogeneous catalysis is explained with the help of
intermediate compound formation theory. According to this, the catalyst combines with one of the
reacting species (A) to form an intermediate compound and less energy is needed for its formation as
compared to uncatalysed reaction. This compound being unstable may combined with the other
reactant (B) to form the final product accompanied by the release of the catalyst
Reactant + Catalyst
 Intermediate Compound
(A)
Reactant + Intermediate  Product + Catalyst
(B)
Compound
(AB)
The oxidation of sulphur dioxide in the presence of oxides of nitrogen may be shown to
proceed as follows-
O2(g)
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2NO (g) 
+
(Reactant)
NO2 (g)
(Catalyst)
+ SO2 (g) 
(Reactant)
(2)
2NO2 (g)
(Intermediate compound)
SO3(g)
(Product)
+
NO (g)
(Catalyst)
Heterogeneous catalysis: The catalytic process in which the reactants and the catalyst are
in different phases is known as heterogeneous catalysis.
(i) 2H2 (g) + O2 (g) 
 2H2O (g)
Pt (s )
3
(ii) CO (g) + 2H2 (g) 2
 CH3OH (l)
ZnO / Cr O ( s )
 C2H6 (g)
(iii) C2H4 (g) + H2 (g)   
Ni / Pt / Pd ( s )
In heterogeneous catalysis, catalyst is generally solid and substrate is gas, some
times it may be a liquid. This is also called surface catalysis, as reaction is initiated on the surface of
catalyst. Nature of working of these catalysts is specific in nature.
- Theory of Heterogeneous catalysis: The heterogeneous catalysis carried on the surface of metal
catalysis is explained with the help of adsorption theory. According to old concept, the reactants
molecules which are mostly gaseous in nature approach the surface of the catalyst. The bonds
present in them cleave and these are adsorbed on the surface of the catalyst. The adsorption being of
exothermic nature, the heat energy evolved (enthalpy of adsorption) helps in increasing the chemical
activity on the catalyst surface. The adsorbed species mutually combine to form an activated
complex. This finally releases the product from the surface so that new reactant molecules may be
accommodated on the surface.
However, this theory could not account for the specificity of a catalyst. This has been replaced
by Modern Adsorption theory which is briefly discussed.
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The mechanism of the heterogeneous catalysis is explained in the following steps(i) Diffusion of the reactants to the surface of the catalyst.
(ii) Some association between the catalyst surface and the reactants i.e., adsorption.
(iii) Occurrence of the chemical reactions on the catalyst surface.
(iv) Dissociation of the reaction products from the catalyst surface i.e., desorption.
(v) Diffusion of the products from the catalyst surface.
# Important Features of Solid Catalysts: Two important features associated with solid catalysts as
activity and selectivity. These are briefly discussed1. Activity: It is the ability of the catalyst to accelerate the chemical reaction in case of
chemisorption. No doubt, the reactants must get adsorbed reasonably strongly on the surface of
catalyst to become active. However, they must not be adsorbed on the catalyst surface. We all know
that the transition metals (d-block) generally act as heterogeneous catalysts. For the hydrogenation
reactions, the catalytic activity increases from groups 5 to group 11. Maximum catalytic activity is
shown by the metals belonging to groups 7, 8 and 9.
2. Selectivity of catalyst: The action of catalyst is quite selective in the sense that a particular
catalyst may catalyse a specific reaction. Moreover, the nature of the product depends upon the
nature of the catalyst. E.g. In the following reactions, the number of moles of H 2 that are taking part
in the hydrogenation reaction depends upon the nature of the catalyst resulting in different products.
(i)
CO (g) + 3H2(g)
Ni



(ii)
CO (g) + 2H2(g)
2 3


CH3OH(g)
(iii)
CO (g) + H2(g)
Cu


HCHO(g)
Cu / ZnO Cr O
CH4(g) + H2O(g)
# Enzyme catalysis: Enzymes are complex nitrogenous organic compounds which are produced by
living plants and animals. Numerous reactions, occurring in the bodies of animals and plants to
maintain the life process are catalysed by enzymes. The enzymes are thus termed as bio-chemical
catalysts and phenomenon is known as bio-chemical catalysis. Enzymes from colloidal solutions in
water and are very effective catalysts. The following are some of the examples of enzyme catalysis(i) Inversion of cane sugar:
 C6H12O6 (l) + C6H12O6 (l)
C12H22O11 (l) + H2O (l)  
Invertase
Sugar
Glucose
(ii) Conversion of glucose into ethyl alcohol:
C6H12O6 (l)  2C2H5OH (l) + 2CO2 (g)
Zymase
Glucose
Ethyl alcohol
Fructose
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(iii) Decomposition of urea:
NH2CONH2 (l) + H2O (l)  2NH3 (g) + CO2 (g)
Urease
Urea
Ammonia
(iv) Conversion of starch into maltose:
2(C6H10O5)n (l) + H2O (l)   n C12H22O11 (l)
Diastase
Starch
Maltose
(v) Pepsin converts the protein into the peptide in the stomach while tryps in convert protein into
amino acids in the intestine.
(vi) Lactic bacilli convert milk into curd by fermentation of milk.
- Characterstics of enzyme catalysis:
(i) Most efficient catalysts: The enzyme catalysed reactions are very fast as activation energy of the
reaction in presence of enzyme is low. One molecule of enzyme may transform million molecules of
the reactant per minute.
(ii) Highly specific in nature: Each enzyme is specific for given reaction, one catalyst cannot catalyse
more than reaction. E.g. The enzyme Urease catalyses the hydrolysis of urea only.
(iii)Temperature dependence: The rate of an enzyme catalysed reaction depends on the temperature.
At optimum temperature the activity of enzyme is maximum. The activity of enzyme is destroyed at
343 K. In body enzyme catalysis occurs at 298-31 K.
(iv) pH dependence: The rate of an enzyme catalysed reaction varies with pH of the system. At
optimum pH activity of enzyme is maximum, in animal body it is 7.4.
(v) Colloidal nature: Enzymes from colloidal solutions in water. Their efficiency is retarded in
presence of large quantities of electrolytes.
(vi) Activators or co-enzymes: The enzymatic activity is increased in the presence of certain
substances. Some metal ions i.e. Co2+, Ni2+, Mn2+ and Cu2+ etc act as activators.
(viii) Inhibitors and poisons: Enzymes are also inhibited or poisoned by the presence of certain
substances called inhibitors and poisons. These substances destroy the catalytic activity of the enzyme.
- Mechanism of enzyme catalysis: There are number of methods given for enzyme catalysis but Lock
and key mechanism is very important. There are two steps in this mechanism(i) Step I: Binding of enzyme [E] to substrate (s) to form an activated complex,
Enzyme [E] + Substrate [S] 
 Enzyme-substrate complex [ES]
(ii) Step II: Product formation in the activate complex,
Enzyme-substrate complex [ES] 

E + Product [P]
[Slow step]
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- Zeolite catalysis: Those catalytic reactions which depend upon the size of the cavities and pores
present in the catalyst are called shape selective catalysis. Zeolites are alumino-silicates of molecular
formula Na2Al2Si2O8. These are three dimensional networks of silicates in which some silicon atoms
are replaced by aluminum atoms. Zeolites to be used as catalysts are heated in vaccum so that the
water of hydration is lost. As a result, Zeolite becomes porous. Thus only those molecules can be
absorbed in these pores whose size is small enough to enter these cavities and can also leave easily.
The reaction taking place in zeolites depend upon the size and shape of reactant and product molecules
are compared to those of pore and cavities of the zeolite. The most remarkable feature of zeolite
catalysis is shape selectivity. Therefore the selectivity of catalyst depends on the pores structure.
Zeolites are being very widely used as catalysts in petro-chemical industries for cracking of
hydrocarbons and isomerization. An important zeolite catalyst used in the petro-chemical industry is
ZSM–5. It converts alcohol directly into gasoline (petrol) by dehydrating them so that a mixture of
hydrocarbons is formed. The hydrated zeolites are used as ion-exchangers in softening of hard water.
# Colloid: According to Thomas Graham the substances like common salt, sugar, urea etc which can
be obtained in the crystalline form and in the dissolved state, diffuse through the vegetable or animal
membrane are termed as crystalloids and the substances like starch, gum, glue etc. which are non
crystalline in nature and in the dissolved state do not diffuse or have a little tendency to pass through
the animal or vegetable membrane are known as colloids.
But the above classification was not perfect since many crystalline substances can be
converted into colloidal form by suitable means. E.g. sodium chloride behaves as a crystalloid when
dissolved in water but behaves as colloid when dissolved in benzene. Similarly soap behaves as
colloid in water and as a crystalloid in alcohol.
Thus the nature (crystalloid or colloidal) of a substance (Solute) depends on the particle
size. A colloidal state of matter is a state in which the size of particle is 1 nm to 100 nm but when the
size of particle is less than 1 nm then it behaves as crystalloid.
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- Types of solution: Depending on the size of the particle, solutions are of three types(i) True solution
(ii) Suspension
(iii) Colloidal solution
(i) True Solution: A homogeneous solution which contains small solute particles (molecules or ions)
dispersed throughout a solvent is termed as true solution. The particle size is less than 1 nm and
particles of solute in true solution are invisible even under microscope. Its particles can pass through
ordinary filter paper as well as through animal membrane.
(ii) Suspension: A heterogeneous solution which contains small insoluble particles is termed as
suspension. The particle size is more than 100 nm. The particles of a suspension may not be visible to
the naked eyes but are visible under a microscope. Due to gravity they get settled at the bottom. The
particles of a suspension can neither pass through an ordinary filter paper nor through semipermeable
membrane.
(iii) Colloidal Solution: Heterogeneous solution which contains particles of intermediate size. The
particles of a colloidal solution have diameters between 1 to 10 nm. Such particles cannot be normally
seen with a naked eye. However light reflected by them can be seen under an ultra microscope. The
particles of a colloidal solution can pass through ordinary filter paper but not through animal
membrane. Thus colloidal solutions are intermediate between true solution and suspensions.
The distinction between characteristics of suspension, colloidal solution and true solution:
S.No. Property
True Solution
Colloidal Solution
1.
Particle size
Less than 1 nm
Between 1 to 100 nm More than 100 nm
2.
Number of phases
3.
Nature
4.
Particle Visibility
1
2
2
Homogeneous
Heterogeneous
Heterogeneous
Invisible through
Visible through
Visible through
microscope
5.
Suspension
microscope
eyes
Separation
(a) With filter paper Not Possible
(b) With membrane Not Possible
Not Possible
Possible
Possible
Possible
6.
Diffusion
Diffuse quickly
Diffuse slowly
7.
Effect of gravity
Negligible
Negligible
Effects
8.
Brownian movement Do not Show
Show
Do not show
9.
Tyndall Effect
Do not Show
Show
Do not show
10.
Effect of Electric
Positive ion of solution All colloidal Particles No effect of
field
moves towards cathode Move towards
and negative ion
oppositely charged
towards anode.
electrodes.
Do not diffuse
electric field.
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# Phases of colloids and their classification: Colloidal solution is heterogeneous nature. It consists of
two phases(i) Dispersed phase
(ii) Dispersion medium
(i) Dispersed phase: It is the component present in small proportion and is just like a solute in a
solution.
(ii) Dispersion medium: It is generally component present in excess and is just like a solvent in a
solution. Thus the particles of the dispersed phase are distributed in the dispersion medium.
- Colloids are classification: Colloids are classified on the following bases1. On the basis of physical state of Dispersed phase and Dispersion medium: Out of solid, liquid
and gas, each one can act as a dispersed phase and dispersion medium leading to eight types of
colloidal systems. A gas mixed with another gas forms homogeneous mixture and not a colloidal
system. The examples of the various types along with their typical names areTypes of colloidal solution:
S.no. Dispersion medium
Dispersed Phase
Name
Examples
1.
Gas
Liquid
Aerosol
Cloud, mist etc.
2.
Gas
Solid
Aerosol
Smoke, dust, etc.
3.
Liquid
Gas
Foam
Froth, whipped cream,
soda water etc.
4.
Liquid
Liquid
5.
Liquid
Solid
Emulsion
Sol
Milk, Hair cream
Paints, Muddy water,
Sulphur sol.
6.
Solid
Gas
Solid Foam
Pumice stone, foam, rubber
7.
Solid
Liquid
Gel
Cheese, butter, jellies.
8.
Solid
Solid
Solid sol
Gems, coloured glasses,
minerals etc.
2. On the basis of nature of interaction between dispersed phase and the dispersion medium: On
this basis, colloids are of two types(i) Lyophilic Colloid
(ii) Lyophobic Colloid
(i) Lyophilic Colloid: The colloidal solutions in which the particles of the dispersed phase have a
great affinity for the dispersion medium, are called lyophilic colloids. These solutions are easily
formed and the lyophilic colloids are reversible in nature. In case water acts as the dispersion medium,
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the lyophilic colloid is called hydrophilic colloid. The common examples of lyophilic colloids are
glue, starch, gelatin, protein etc.
(ii) Lyophobic Colloids: The colloidal solutions in which there is no affinity between particles of the
dispersed phase and the dispersion medium are called lyophobic colloids. Such solutions are formed
with difficulty. These are irreversible in nature. In case, the dispersion medium is water, the lyophobic
sol is called hydrophobic colloid. The common examples of lyophobic colloids are certain metals,
metal oxides, metal sulphides and metal hydroxides.
The difference between the lyophilic sol and lyophobic sols are:
S.No.
1.
Nature
Lyophilic colloid
Preparation of
Prepared easily by directly
solution
mixing with the liquid
Lyophobic colloid
Prepared by special methods only
dispersion medium.
2.
Particle
Particles are of molecular size.
Particles are aggregates of
many molecules.
3.
Visibility
Particles are not easily visible
even under ultra microscope.
Particles are easily detected
under ultra microscope.
4.
Nature
Reversible
Irreversible
5.
Stability
More stable
Less stable
6.
Viscosity
Much higher than that of
Same as that of the
the dispersion medium.
dispersion medium.
Lower than that of the
Same as that of the
7.
Surface tension
dispersion medium.
8.
dispersion medium.
Action of
Addition of large amount of
Addition of less amount of
Electrolytes
electrolyte causes precipitation
electrolyte causes precipitation.
9.
Tyndall effect
Show Tyndall Effect
Do not show Tyndall effect.
10.
Migration in
Their particles may migrate in
Their particles migrate only in
electric field
either direction or may not
one particular direction in the
migrate at all.
electric field.
3. On the basis of the molecular size: On this basis, colloids are of three types(i) Multi-molecular colloid
(ii)
Macromolecular colloid
(iii)
Associated colloid
(i) Multi-molecular colloid: Colloids in which the dispersed phase consists of aggregates of atoms or
molecules with molecular size less than 1 nm are multi-molecular colloids. E.g. Sols of gold atoms and
Sulphur (S8) molecules. In these colloids, the particles are held together by Vander Waal’s forces.
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(ii) Macromolecular colloid: Substances having big size molecules are called macromolecules having
large molecules masses are dissolved in a suitable liquid they form a solution in which the molecules
of the substance i.e. the dispersed particles have size in the colloidal range such substances are called
macromolecular colloid. Their colloidal solutions are quite stable. Examples of naturally occurring
macromolecules are starch, cellulose, proteins, enzymes and gelatin. Man made macromolecules are
polyethylene, nylon, polystyrene, synthetic rubber etc.
(iii)Associated colloid: The substances which when dissolved in a medium at low concentration
behave as normal, strong electrolytes but at higher concentrations exhibit colloidal state properties due
to the formation of aggregated particles are called associated colloids. The aggregated particles thus
formed are called micelles. The maximum concentration above which these micelles are formed is
called critical Micelle Concentration (cmc). Different micelles have different values of cmc. Micelles
have 100 or more molecules.
- Mechanism of micelle formation: Micelles are generally formed by the aggregation of several ions
or molecules with lyophobic as well as lyophilic parts. For example, sodium stearate (C 17H35COONa)
is example of such type of molecule. The micelle may contain as many as 100 molecules or more.
When sodium stearate is dissolved in water, it gives Na+ and C17H35COO¯ ions.
C17H35COONa
Sodium Stearate
C17H35COO¯ + Na+
Stearate
The stearate ions associate to form ionic micelles
of colloidal size. It has long hydrocarbon part of
C17H35 radical which is lyophobic end and COO¯
part which is Lyophilic. In the figure, the chain
corresponds to Stearate ion C17H35 COO¯. When
the concentration of the solution is below its
CMC, it behaves as normal electrolytes. But above
this concentration, it is aggregated to behave as micelles.
# Preparation of colloid:
1. Preparation of lyophilic colloid: The lyophilic colloids have strong affinity between particles of
dispersed phase and dispersion medium. Therefore, these colloidal solutions are readily formed by
simply mixing the dispersed phase and dispersion medium under ordinary conditions. Examples
gelatin, gum, egg albumin etc.
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2. Preparation of Lyophobic colloids: Lyophobic colloids can be prepared mainly by the following
two type of methods(i) Condensation methods
(ii) Dispersion methods
(i) Condensation Methods: In this method, smaller particles of dispersed phase are condensed
suitable to be of colloidal size. This is done by the following methods(A) Chemical Methods: The following chemical reactions may be used to prepare lyophobic colloidal
solutions(a) Oxidation: A colloidal solution of Sulphur is obtained by bubbling H2S gas through the solution of
bromine water, Sulphur dioxide etc.
H2S + Br2 
 2HBr + S
2H2S + SO2 
 2H2O + 3S
Colloidal Solution
Colloidal Solution
(b) Reduction: The colloidal solutions of metals are obtained by reduction of their compounds.


2AuCl3 + 3SnCl2
2Au + 3SnCl4
Gold Sol
(c) Hydrolysis: A colloidal solution of ferric hydroxide is prepared when a concentrated solution of
ferric chloride is added drop wise to hot water.
FeCl3 + 3H2O


Fe(OH)3 (Sol) + 3HCl
(d) Double decomposition: As2S3 sol is obtained by passing H2S through dilute solution of arsenious
oxide in water.
As2O3 + 3H2S


As2S3 + 3H2O
Colloidal Sol
(B) Physical Methods:
(a) By exchange of solvent: Substances which are fairly soluble in alcohol and less soluble in water,
when their alcoholic solutions are poured in water, their colloidal solution is obtained. E.g. alcoholic
solution of Sulphur on pouring into water gives milky colloidal solution of Sulphur.
(b) By excessive cooling: A colloidal solution of ice in an organic solvent like ether or chloroform can
be prepared by freezing a mixture of water and ether.
(c) By change of physical state: Sols or substances like mercury and sulphur are prepared by passing
their vapours through a cold water containing a suitable stabilizer such as ammonium salt or citrate.
(ii) Dispersion Methods: In these methods, larger particles of a substance are broken into smaller
particles. The following methods are employed(A) Mechanical dispersion: In this method, the Substance is first ground to coarse particles. It is then
mixed with the dispersion medium to get a suspension. The suspension is then grinded in a colloidal
mill. It consists of two metallic discs nearly touching each other and rotating in opposite directions at a
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very high speed (about 7000 revolutions per minute). The particles are ground down to colloidal size.
Colloidal solutions of black into, paints, varnishes, dyes etc are obtained by this method.
(B) Electrical Dispersion or Bredig’s method: This method is used to prepare sols of metals such as
platinum, silver, copper or gold. The metal whose sol is to be prepared is made as two electrodes
immersed in dispersion medium such as water. The dispersion medium is kept cooled by surrounding
it with a freezing mixture. An electric arc is struck between the electrodes.
The tremendous heat generated by the are vapourises the metals which are condensed
immediately in the liquid to give colloidal solution. The colloidal solution prepared is stabilized by
adding a small amount of KOH to it.
(c) Peptisation: The process of converting a freshly prepared precipitate into colloidal form by the
addition of a suitable electrolyte is called peptisation. The electrolytes used for the purpose are called
peptizing agents. On treating a precipitate of ferric oxide with a small amount of FeCl3 solution gives a
reddish brown coloured ferric hydroxide Sol. In this case Fe+3 ions from ferric chloride are adsorbed
by Fe(OH)3 precipitate.
Fe(OH)3 +
Precipitate
Fe+3


[Fe(OH)3] Fe+3
electrolyte
Colloidal sol
The charged particles repel one another and form colloidal solution.
Similarly a precipitate of silver chloride can be peptised by shaking with a dilute solution
of silver nitrate to give a colloidal solution of silver chloride sol of BaSO4 or CuS is obtained by
treatment of precipitated BaSO4 or CuS with water.
# Purification of colloidal solutions: When a colloidal solution is prepared, it contains certain
impurities of electrolytes, which tend to destabilize the solution. Hence their removal is very essential.
The following methods are used for the purification of colloidal solutions1. Dialysis: The separation of crystalloids from the colloids is based upon the principle that the
particles of the crystalloids pass through parchment paper or cellophane membrane whereas those of
the colloids do not.
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The process of separating the particles of colloids from those of crystalloids by diffusion of the
mixture through a parchment or an animal membrane is known as dialysis. The apparatus used is
called a dialyser. A bag of suitable membrane containing the colloidal solution is suspended in a vessel
through which fresh water is continuously flown. The molecules and ions diffuse through the
membrane into the outer water and pure colloidal solution is left behind. The above process can be
quickened if an electric field is applied. The process is then called Electro-dialysis.
2. Ultra filtration: It is the process of removing the impurities from the colloidal solution by passing
it through graded filter papers called ultra filter papers.
3. Ultra Centrifugation: In this method, colloidal solution is filled in a tube and kept in Ultra
centrifuge. Ultracentrifuge rotates at a very high speed as a result colloidal particles settle down and
impurities are left in solution. The settled colloidal particles are added to suitable dispersion medium
to get back the solution.
# Properties of colloidal solutions: Colloidal solutions have following properties:
1. Physical Properties:
(i) Heterogeneous nature: The colloidal solutions are heterogeneous in nature consisting of two
phases- (a) dispersed phase and (b) dispersion medium.
(ii) Filterability: Colloidal particles do not pass through animal and vegetable membranes and Ultra
filter papers.
(iii) Stable nature: The colloidal solutions are quite stable. Their particles are in a state of motion and
donot settle down at the bottom of the container.
(iv) Visibility: The particles in colloidal solution are not visible to the naked eye or under ordinary
microscope. But they are visible under ultra microscope.
(v) Surface Area: The surface area of colloidal particle in colloidal solution is very large as a result
colloidal solutions are good absorbs.
(vi) Colour: Generally colloidal solutions are coloured. Their colour depends on wavelength of light
scattered by dispersed phase. The wavelength depends on nature of the dispersed phase and size of
particles.
2. Colligative Properties: Colloidal solution Colligative properties such as osmotic pressure,
elevation in boiling point, depression in freezing point and relative lowering of vapour pressure.
Colloidal particles have very high average value molecular masses and hence the number of moles
present in solution will be extremely small. Thus the value for any of the Colligative properties for a
particular substance will be smaller as compared to its value when it is a part of true solution.
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3. Mechanical Properties:
(i) Brownian movement: Robert Brown observed under a powerful ultramicroscope that the colloidal
particles are moving in a zigzag motion called Brownian moment. The Brownian movement is due to
unbalanced bombardment of the particles by the molecules of the dispersion medium. As a result the
colloidal particle is then displaced in one direction. The another molecule strikes it, displacing it to
another direction and so on.
Brownian movement is inversely proportional to size of the particle. If the size of the dispersed
phase particles increases, then the chances of unequal bombardment decrease. Brownian movement
opposes the force of gravity and does not allow the colloidal particles to settle down. Thus it is
responsible for the stability of the colloidal solution.
4. Optical Properties: Tyndall in 1869, observed that when a beam of light is passed through a
colloidal solution placed in a dark room, the path of beam gets illuminated with a bluish light. This
phenomenon is known as Tyndall effect. The path of the light is made visible by the scattering of
light by the colloidal particles. Thus the phenomenon of scattering of light by colloidal particles as
a result of which the path of the beam becomes visible is called Tyndall Effect.
Since the size of particle in true solutions is small, hence they do not show scattering of light. As a
result true solution does not show Tyndall Effect.
5. Electrical Properties:
(i) Charge on colloidal particles: The stability of a colloidal solution is due to the fact that the
colloidal particles in the sol are electrically charged. The particles therefore, repel one another and
donot coalesce to from large non colloidal particles. Colloidal particles carry either positive or
negative charge. All the dispersed particles in a colloidal solution carry the same charge while the
dispersion medium has an equal and opposite charge. On the basis of the type of charge on the
colloidal particles, the colloidal solution is classified into two parts-
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(A) Positively charged colloidal solution: The colloidal solution of Fe(OH)3, Al(OH)3, Cr(OH)3
comes under this category. Similarly dyes such as methylene blue and Bismarc brown, Haemoglobin
are positively charged colloidal solution.
(B) Negatively charged colloidal solution: The colloidal particles of metals (such as Ag, Pt, Cu and
Au), metal sulphide (Such as As2S3, Sb2S3, CdS) and acidic dyes (Such as eosin, congo red) are
negatively charged.
- Origin of Electrical charge on colloidal particles: The origin of electrical charge on the colloidal
particles is due to preferential adsorption of ions from solution. The particles constituting the dispersed
phase adsorb only those ions preferentially which are common with their own lattice ions. E.g. the
Fe(OH)3 Sol prepared by the hydrolysis of FeCl3 has positive charge because it adsorbs Fe+3 ion
preferentially on its surface from the solution. The nature of charge on the colloidal particles also
depends on the process of formation of the colloidal solution. E.g. If AgNO3 Solution is added to an
aqueous solution of KI, the AgI will adsorb negative I¯ ion (common ions) from the dispersion
medium to form a negatively charged sol. However if Ag I is formed by adding KI to AgNO 3 solution,
the sol will be positively charged due to the adsorption of Ag+ ions (common ions) present in the
dispersion medium.
(ii) Electrophoresis: The movement of colloidal particles under the influence of an electric field is
called electrophoresis. The presence of the charge on the sol particles and its nature whether positive
or negative can be determined with the help of a phenomenon known as electrophoresis. If the
particles accumulate near the negative electrode, the charge on the particles is positive. On the other
hand, if the sol particles accumulate near the positive electrode, the charge on the particles is negative.
The apparatus consists of a U – tube with two platinum electrodes in each limb. On passing, electric
current, the colloidal particles moves towards oppositively charged electrodes.
If As2S3 sol is taken in a U – tube then the As2S3 particles gets accumulated near the positive
electrode. This indicates that the particles of As2S3 are negatively charged. Similarly when an electric
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current is passed through positively charged Fe(OH)3 sol, it is observed that they move towards
negatively charged electrode and gets accumulated there. This indicates particles of Fe(OH)3 are
positively charged.
6. Coagulation: The phenomenon of the precipitation of a colloidal solution by addition of the
excess of an electrolyte is called coagulation or flocculation. The coagulation can be carried out by
following methods(i) Excessive Dialysis: On prolonged dialysis, the traces of the electrolyte present in the sol are
removed almost completely and the colloids becomes unstable.
(ii) Mutual coagulation: When oppositively charged sols are mixed in almost equal proportions, their
charges are neutralized and both gets coagulated. Smoke (positively charged colloidal solution) mixed
with cloud (negatively charged solution) results in rain. This is an example of mutual coagulation.
(iii)By adding electrolyte: When excess of an electrolyte is added, the colloidal particles are
precipitated. The reason is that colloidal particles take up ions carrying charge opposite to that present
on themselves. This causes neutralization leading to their coagulation. The ion responsible for
neutralization of charge on the particles is called the flocculating ion. A negative ion causes the
precipitation of positively charged sol and vice versa.
- Hardy Schulze Rule: The greater the valency of the flocculating ion added, the greater is its power
to cause precipitation. This is known as hardy Schulze rule.
E.g. In the coagulation of As2S3 (negative sol), the flocculating power of ions is of order.
Si+4 > Al+3 > Mg+2 > Na+
E.g. Similarly in coagulation of Fe(OH)3 (a positive sol) the flocculating power of ion is of order.
[Fe (CN)6]4- > PO43- > SO42- > ClThe minimum concentration of an electrolyte required to cause precipitation of a sol is called
flocculation value. It is expressed in forms of moles per litre.
Flocculation value =
quantity
of electrolyt e  molarity
Total volume
# Protective colloids and gold number: It has already been explained that lyophobic sols are unstable
and easily precipitated by addition of electrolytes. Whereas lyophilic colloids are not precipitated by
addition of an electrolyte.
The addition of lyophilic colloids to lyophobic colloids renders lyophobic colloids difficult
to coagulate by addition of electrolyte. Thus the lyophilic colloids have a unique property of protecting
lyophobic colloids. The process is known as protection and the lyophilic colloids are termed as
Page 22 of 24
Protective colloids. The protective power is measured in terms of gold numbers. Gold number is
defined as“The number of milligrams of a lyophilic colloid that will prevent the precipitation of 10
ml of a gold sol on the addition of 1 ml of 10% sodium chloride solution”.
Protective capacity of any colloid lyophilic 
Gold
1
Number
That is smaller the value of the gold number, greater will be protecting power of the protective
colloid.
The gold number of gelatin is least (0.005 – 0.01) and that of starch is maximum (25). Hence
gelatin is the best protective colloid.
# Application of colloids:
1. In Daily Life:
(i) Food Products: Milk, butter curd, ice cream, egg, jelly, fruit juices, etc are all colloids in one form
or the other.
(ii) Purification of water: Water obtained from natural sources contain sand, bacteria and other
insoluble impurities. These impurities are negatively charged. These impurities are neutralized by
adding alum. The impurities get coagulated and settle down and pure water can be decanted off.
(iii) Cleansing action of soap – The soap solution is colloidal in nature. It removes the dirt particles
either by adsorption or by emulsifying the greasy matter sticking to the cloth.
(iv) To stop bleeding – Blood is a colloidal solution of an albuminoidal substance. The styptic action
of alum and ferric chloride solution is due to coagulation of blood forming a clot which stops further
bleeding.
2. In medicines: Most of the medicines in use are colloidal in nature. Milk of magnesia (colloidal
solution of Mg(OH)2 in water) is used as an antacid. A few important medicines are colloidal gold,
manganese, sulphur, copper, calcium etc. colloidal sulphur is used in treatment of skin disease and
silver sol in treatment of eye disease.
3. In Agricultural field: Colloids play an important role in agricultural field. Fertile soil has a
colloidal nature. It can easily adsorb moisture and hence provides nutrition for the plant growth.
4. In Industrial field:
(i) Tanning Industry: Animal hides are colloidal in nature. When a hide, which has positively
charged particles, is soaked in tannin, which contains negatively charged colloidal particles, mutual
coagulation takes place, which results in the hardening of leather. The process is termed as tanning.
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(ii) Rubber industry: Latex is a colloidal solution of negatively charged colloidal rubber particles.
Rubber can be obtained from latex by coagulation. These rubber particles can be deposited over
articles to be rubber plated by electrophoresis. The article to be rubber plated is made the anode in the
rubber plating bath.
(iii) Ink production: In India ink is made from carbon. The Gum of babool is added to carbon and
prevents coagulation, thus provides stability. The Gum of babool acts as protective colloid. As a result
the sticky nature of ink is removed.
(iv) Photography – The photographic films or plates are prepared by coating a colloidal solution of
AgBr in gelatin on glass plate. The particles of AgBr are colloidal in nature.
(v) Precipitation of smoke – In big cities, pollution due to smoke is a major problem. Smoke contains
a lot of unburnt carbon particles which are injurious to health and have to be precipitated from the
smoke. Smoke is a colloidal system in which the carbon particles are suspended in air. In this the
carbon particles are negatively charged and do not get coagulated. The precipitation of smoke particles
is carried out by Cottrell smoke precipitator. In this method, smoke is allowed to pass through a
chamber having a series of plates charged to very high potential. Charged particles of smoke get
attracted by charged plates get precipitated and the gases coming out of chimney become free of
charged particles.
(vi) Sewage Disposal: The sewage disposal contains dust particles suspended in water. They are
colloidal in nature and also cloud. They can be separated from the dirty water by placing it in gang
tank fitted with electrodes. The colloidal particles migrate towards the oppositively charge electrodes
where their charge is neutralized and the coagulated. The coagulated mass can be used as manure.
5. In nature:
(i) Formation of Delta: River water is muddy and contains charged colloidal particles of clay, sand
and many other materials, when river water meets the sea water, the electrolytes present in sea water
coagulate the colloidal solution of clay which gets deposited with the formation of delta.
(ii) Blue Colour of sky: Blue colour of sky is due to the scattering of light by colloidal dust particles
present in the air.
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(iii) Rain: Clouds are aerosols having small droplets of water suspended in air. On account of
condensation in the upper atmosphere, the colloidal droplets of water grow bigger and bigger in size,
till they come down in form of rain. The rainfall occurs when two oppositely charged clouds meet.
Artificial rain can be caused by spraying oppositely charged colloidal dust or sand particles over a
cloud. The colloidal water particles present in the cloud will get neutralized and coagulate of form
bigger water drops causing artificial rains.
(iv) Tail of comets is seen as a Tyndall cone due to scattering of light by the tiny solid particles lift by
the comet in its path.
# Emulsions: Emulsions are the colloidal solutions in which both the dispersed phase and the
dispersion medium are liquids.
1. Oil/Water Emulsions: In this case, oil acts as the dispersed phase and water as dispersion
medium. E.g. milk, vanishing cream etc.
2. Water/oil Emulsion: In this case, water acts as the dispersed phase while the oil behaves as the
dispersion medium. E.g. cold cream, butter, cod liver oil etc.
The above of emulsions can be inter-converted by simply changing the ratio of the
dispersed phase and dispersion medium. E.g. On oil/water emulsion can be converted to water/oil
emulsion by simply adding excess of oil in the first case.
- Emulsifiers: Emulsions are the colloidal solutions in which both dispersed phase and the dispersion
medium are liquids. Any two immiscible liquids form an emulsion. Since the two do not mix well, the
emulsion is generally unstable. Therefore to form stable emulsions small quantities of certain other
substances are added during their preparation. The substances which are added to stabilize the
emulsions are called emulsifiers or emulsifying agents.
The principal emulsifying agents for oil/water emulsions are protein, gum, natural and
synthetic soaps etc. For water/oil emulsions, the principle emulsifying agents are heavy metal salts of
fatty acids, long chain alcohols, lampblack etc.
- Applications of Emulsions:
(i) Milk which is an important constituent of our diet is an emulsion of liquid fats in water.
(ii) The various pharmaceuticals and cosmetics available in liquid form such as cod liver oil,
B-complex, ointments etc. are from of emulsions.
(iii) The concentration of sulphide ore of a metal by froth flotation process involves the use of some
oil such as palm oil. The oil forms emulsion with ore particles. When air is bubbled through the
emulsion, it rises to the surface as foam and is skimmed off.