Condensation methods:
When the substance for colloidal particle is present as small sized particle, molecule or ion, they are brought to the colloidal dimension by condensation methods. Here care should be taken to produce the particle with colloidal size otherwise precipitation will occur.Various chemical methods for the formation of colloidal particles.
(i) Oxidation:
Sols of some non metals are prepared by this method.
(a) When hydroiodic acid is treated with iodic acid, I₂ sol is obtained.
\(\mathrm{HIO}_{3}+5 \mathrm{HI} \rightarrow 3 \mathrm{H}_{2} \mathrm{O}+\mathrm{I}_{2}(\mathrm{Sol})\)
(b) When O₂ is passed through H₂Se, a sol of selenium is obtained.
\(\mathrm{H}_{2} \mathrm{Se}+\mathrm{O}_{2} \rightarrow 2 \mathrm{H}_{2} \mathrm{O}+\mathrm{Se}(\mathrm{sol})\)
(ii) Reduction:
Many organic reagents like phenyl hydrazine, formaldehyde, etc are used for the formation of sols. For Example: Gold sol is prepared by reduction of auric chloride using formaldehyde.
\(2 \mathrm{AuCl}_{3}+3 \mathrm{HCHO}+3 \mathrm{H}_{2} \mathrm{O} \rightarrow 2 \mathrm{Au}(\mathrm{sol})+6 \mathrm{HCl}+3 \mathrm{HCOOH}\)
(iii) Hydrolysis
Sols of hydroxides of metals like chromium and aluminium can be produced by this method.
For example,
\(\mathrm{FeCl}_{3}+3 \mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{Fe}(\mathrm{OH})_{3}+3 \mathrm{HCl}\)
(iv) Double decomposition
For the preparation of water insoluble sols this method can be used. When hydrogen sulphide gas is passed through a solution of arsenic oxide, a yellow coloured arsenic sulphide is obtained as a colloidal solution.
\(\mathrm{As}_{2} \mathrm{O}_{3}+3 \mathrm{H}_{2} \mathrm{~S} \rightarrow \mathrm{As}_{2} \mathrm{~S}_{3}+3 \mathrm{H}_{2} \mathrm{O}\)
(v) Decomposition
When few drops of an acid is added to a dilute solution of sodium thiosulphate, the insoluble free sulphur produced by decomposition of sodium thiosulphate accumulates into small, clusters which impart various colours blue, yellow and even red to the system depending on their growth within the size of colloidal dimensions.
\(\mathrm{S}_{2} \mathrm{O}_{3}{ }^{2-}+2 \mathrm{H}^{+} \rightarrow \underset{\text { sol }}{\mathrm{S}}+\mathrm{H}_{2} \mathrm{O}+\mathrm{SO}_{2}\)

Condensation methods:
When the substance for colloidal particle is present as small sized particle, molecule or ion, they are brought to the colloidal dimension by condensation methods. Here care should be taken to produce the particle with colloidal size otherwise precipitation will occur.
(i) Oxidation:
Sols of some non metals are prepared by this method.
(a) When hydroiodic acid is treated with iodic acid, I₂ sol is obtained.
\(\mathrm{HIO}_{3}+5 \mathrm{HI} \rightarrow 3 \mathrm{H}_{2} \mathrm{O}+\mathrm{I}_{2}(\mathrm{Sol})\)
(b) When O₂ is passed through H₂Se, a sol of selenium is obtained.
\(\mathrm{H}_{2} \mathrm{Se}+\mathrm{O}_{2} \rightarrow 2 \mathrm{H}_{2} \mathrm{O}+\mathrm{Se}(\mathrm{sol})\)
(ii) Reduction:
Many organic reagents like phenyl hydrazine, formaldehyde, etc are used for the formation of sols. For Example: Gold sol is prepared by reduction of auric chloride using formaldehyde.
\(2 \mathrm{AuCl}_{3}+3 \mathrm{HCHO}+3 \mathrm{H}_{2} \mathrm{O} \rightarrow 2 \mathrm{Au}(\mathrm{sol})+6 \mathrm{HCl}+3 \mathrm{HCOOH}\)
(iii) Hydrolysis
Sols of hydroxides of metals like chromium and aluminium can be produced by this method.
For example,
\(\mathrm{FeCl}_{3}+3 \mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{Fe}(\mathrm{OH})_{3}+3 \mathrm{HCl}\)
(iv) Double decomposition
For the preparation of water insoluble sols this method can be used. When hydrogen sulphide gas is passed through a solution of arsenic oxide, a yellow coloured arsenic sulphide is obtained as a colloidal solution.
\(\mathrm{As}_{2} \mathrm{O}_{3}+3 \mathrm{H}_{2} \mathrm{~S} \rightarrow \mathrm{As}_{2} \mathrm{~S}_{3}+3 \mathrm{H}_{2} \mathrm{O}\)
(v) Decomposition
When few drops of an acid is added to a dilute solution of sodium thiosulphate, the insoluble free sulphur produced by decomposition of sodium thiosulphate accumulates into small, clusters which impart various colours blue, yellow and even red to the system depending on their growth within the size of colloidal dimensions.
\(\mathrm{S}_{2} \mathrm{O}_{3}{ }^{2-}+2 \mathrm{H}^{+} \rightarrow \underset{\text { sol }}{\mathrm{S}}+\mathrm{H}_{2} \mathrm{O}+\mathrm{SO}_{2}\)

1. For a chemical reaction, catalyst is needed in very small quantity. Generally, a pinch of catalyst is enough for a reaction in bulk.
2. There may be some physical changes, but the catalyst remains unchanged in mass and chemical composition in a chemical reaction.
3. A catalyst itself cannot initiate a reaction. It means it can not start a reaction which is not taking place. But, if the reaction is taking place in a slow rate it can increase its rate.
4. A solid catalyst will be more effective if it is taken in a finely divided form.
5. A catalyst can catalyse a particular type of reaction, hence they are said to be specific in nature.
6. In an equilibrium reaction, presence of catalyst reduces the time for attainment of equilibrium and hence it does not affect the position of equilibrium and the value of equilibrium constant.
7. A catalyst is highly effective at a particular temperature called as optimum temperature.
8. Presence of a catalyst generally does not change the nature of products.
For example. \(2 \mathrm{SO}_{2}+\mathrm{O}_{2} \rightarrow 2 \mathrm{SO}_{3}\)

Food:
Food stuffs like milk cream, butter, etc are present in colloidal form.
Medicines:
Antibodies such as penicillin and streptomycin are produced in colloidal form for suitable injections. Colloidal gold and colloidal calcium are used as tonics. Milk of magnesia is used for stomach troubles. Silver sol protected by gelatine known as Argyrol is used as eye lotion.
In Industry:
Colloids find many applications in industries
(i) Water purification:
Purification of drinking water is activated by coagulation of suspended impurities in water using alums containing Al³⁺.
(ii) In washing:
The cleansing action of soap is due to the formation of emulsion of soap molecules with dirt and grease.
(iii) Tanning of leather:
Skin and hides are protein containing positively charged particles which are coagulated by adding tannin to give hardened leather for further application. Chromium salts are used for the purpose. Chrome tanning can produce soft and polishable leather.
(iv) Rubber industry:
Latex is the emulsion of natural rubber with negative particles. By heating rubber with sulphur, vulcanized rubbers are produced for tyres, tubes, etc.
(v) Sewage disposal:
Sewage contains dirt, mud and wastes dispersed in water. The passage of electric current deposits the wastes materials which can be used as a manure.
(vi) Cortrell's precipitator:
Carbon dust in air is solidified by Cottrell's precipitator. In it, a high potential difference of about 50,000V is used. The charge on carbon is neutralized and solidified. Thus the air is free from carbon particles.
(vii) The blue colour of the sky in nature is due to Tyndall effect of air particles.
(viii) Formation of delta:
The electrolyte in sea and river water coagulates the solid particles in river water at their intersection. So, the earth becomes a fertile land.
(ix) Analytical application:
Qualitative and quantitative analysis are based on the various properties of colloids.

Electrophoresis:
(i) When electric potential is applied across two platinum electrodes dipped in a hydrophilic sol, the dispersed particles move toward one or other electrode.
(ii) This migration of sol particles under the influence of electric field is called electrophoresis or cataphoresis.
(iii) If the sol particles migrate to the cathode, then they posses positive (+) charges, and if the sol particles migrate to the anode then they have negative charges(-).
(iv) Thus from the direction of migration of sol particles we can determine the charge of the sol particles.
(v) Hence electrophoresis is used for detection of presence of charges on the sol particles

Helmholtz double layer:
(i) The surface of colloidal particle adsorbs one type of ion due to preferential adsorption.
(ii) This layer attracts the oppositely charged < ions in the medium and hence at the boundary separating the two electrical double layers are setup.
(iii) This is called as Helmholtz electrical double layer.
(iv) As the particies nearby are having similar: charges, they cannot come close and condense.

(i) The presence of electric field increases the speed of removal of electrolytes from colloidal solution.
(ii) The colloidal solution containing an electrolyte as impurity is placed between two dialysing membranes enclosed into two compartments filled with water.
(iii) When current is passed, the impurities pass into water compartment and get removed periodically.
(iv) This process is faster then dialysis, as the rate of diffusion of electrolytes is increased by the application of electricity.

Condensation Methods:
Various chemical methods for the formation of colloidal particles.
(i) Oxidation:
Sols of some non-metals are prepared by this method. (a) When hydroiodic acid is treated with iodic acid, 12 sol is obtained.
\({ HIO }_{ 3 }+5HI\longrightarrow { 3H }_{ 2 }O+{ I }_{ 2 }\) (Sol)
(ii) Reduction:
Many organic reagents like phenyl hydrazine, formaldehyde, etc are used for the formation of sols. For example: Gold sol is prepared by reduction of auric chloride using formaldehyde.
\(2{ AuCl }_{ 3 }+3HCHO+{ 3H }_{ 2 }O\longrightarrow 2Au\left( sol \right) +6HCl+3HCOOH\) 
(iii) Hydrolysis: 
Sols of hydroxides of metals like chromium and aluminium can be produced by this method.
For example,
\({ FeCl }_{ 3 }+3H_{ 2 }O\longrightarrow Fe(OH)_{ 3 }+3HCl\) 
(iv) Double decomposition: 
For the preparation of water insoluble sols this method can be used. When hydrogen sulphide gas is passed through a solution of arsenic oxide, a yellow coloured arsenic sulphide is obtained as a colloidal solution.
\({ As }_{ 2 }{ O }_{ 3 }+3{ H }_{ 2 }S\longrightarrow { AS }_{ 2 }{ S }_{ 3 }+{ 3H }_{ 2 }O\) 
(v) Decomposition:
When few drops of an acid is added to a dilute solution of sodium thio sulphate, the insoluble free sulphur produced by decomposition of sodium thiosulphate accumulates into small, clusters which impart various colours blue, yellow and even red to the system. depending on their growth within the size of colloidal dimensions.
\({ S }_{ 2 }{ O }_{ 3 }^{ 2- }+{ 2H }^{ + }\longrightarrow \underset{sol}S+{ H }_{ 2 }O+{ SO }_{ 2 }\) 
By exchange of solvent:
(i) Colloidal solution of few substances like phosphorous or sulphur is obtained by preparing the solutions in alcohol and pouring them into water.
(ii) As they are insoluble in water, they form colloidal solution.
P in alcohol + water ⟶ Psol.

(i) Reactant selectivity:
When bulkier molecules in a reactant mixture are prevented from reaching the active sites within the zeolite crystal, this selectivity is called reactant shape selectivity.
(ii) Transition state selectivity:
If the transition state of a reaction is large compared to the pore size of the zeolite, then no product will be formed.
(iii) Product selectivity:
It is encountered when certain product molecules one too big to diffuse out of the zeolite pores.

(i) Suppose the reactant of a reaction is present in one solvent and the other reactant is present in an another solvent.
(ii) The reaction between them is very slow, if the solvents are immisible.
(iii) As the solvents from separate phases the reactants have to migrate across the boundary to react.
(iv) But migration of reactants across the boundary is not easy. For such situations a third solvent is added which is miscible with both.
(v) So, the phase boundary is eliminated reactants freely mix and react fast.
(vi) But for large scale production of any product, use of a third solvent is not convenient as it may be expensive.
(vii) For such problems phase transfer catalysis provides a simple solution, which avoides the use of solvents.
(viii) It directs the use a phase transfer catalyst (a phase transfer reagent) to facilitate transport of a reactant in one solvent to the other solvent where the second reactant is present.
(ix) As the reactants are now brought together they rapidly react and form the product.

(i) Effective and efficient conversion is the special characteristic of enzyme catalysed reactions. An enzyme may transform a million molecules of reactant in a minute 
For Eg: \({ 2H }_{ 2 }{ O }_{ 2 }\longrightarrow { 2H }_{ 2 }O+{ O }_{ 2 }\)
For this reaction, the activation energy is 18k cal/mole without a catalyst  With colloidal platinum as a, catalyst the activation energy is 11.7kcal /mole. But with the enzyme catalyst the activation energy of this reaction is less than 2kcal/ mole.
(ii) Enzyme catalysis is highly specific in nature.
(iii) Enzyme catalysed reaction has maximum rate at optimum temperature
(iv) The rate of enzyme catalysed reactions varies with the pH of the system. The rate is maximum at a pH called optimum pH.
(v) Enzymes can be inhibited i.e. poisoned activity of an enzyme is decreased and destroyed by a poison. The physiological action of drugs is related to their inhibiting action.
(vi) Catalytic activity of enzymes is increased by coenzymes or activators.

(i) For a chemical reaction, catalyst is needed in very small quantity.
(ii) There may be some physical changes, but the catalyst remains unchanged in mass and chemical composition in a chemical reaction.
(iii) A catalyst itself cannot initiate a reaction. 
(iv) A solid catalyst will be more effective if it is taken in a finely divided form.
(v) A catalyst are specific in nature.
(vi) In an equilibrium reaction, presence of catalyst reduces the time for attainment of equilibrium and hence it does not affect the position of equilibrium and the value of equilibrium constant.
(vii) A catalyst is highly effective at a particular temperature called as optimum temperature.
(viii) Presence of a catalyst generally does not change the nature of products

Freundlich adsorption isotherm:
According to Freundlinch
\(\frac { x }{ m } =kp^{ \frac { 1 }{ n } }\) 
where x is the amount of adsorbate or adsorbed on 'm' gm of adsorbent at a pressure of p. K and n are constants Value is always less than unity.
This equation is applicable for adsorption of gases on solid surfaces. The same equation becomes  \(\frac { x }{ m } =Kc^{ \frac { 1 }{ n } }\) when used for adsorption in solutions with c as concentration.
These equation quantitively predict the effect of pressure(or concentration) on the adsorption of gases(or adsorbates) at constant temperature.
Taking log on both sides of equation \(\frac { x }{ m } ={ kp }^{ \frac { 1 }{ n } }\) 
\(log\frac { x }{ m } =logK+\frac { 1 }{ n } logP\) 

Hence the intercept represents the value of log k and the slope \(\frac { b }{ q } \) gives \(\frac { 1 }{ n } \)
This equation explains the increase of \(\frac { x }{ m } \) with increase in pressure. But experimental values show the deviation at low pressure.

Factors affecting adsorption:
The adsorption is well understood by considering I the various factors affecting it. Qualitatively, the extent of surface adsorption depends on
(i) Nature of adsorbent  
(ii) Nature of adsorbate
(iii) Pressure
(iv) Concentration at a given temperature.
(i) The adsorption is a surface phenomenon: it depends on the surface area of adsorbent.  i.e., higher the surface area, higher is the amount adsorbed.
(ii) Nature of adsorbate:
The nature of adsorbate can influence the adsorption. Gases like SO₂, NH₃, HCI and  CO₂ are easily liquefiable as have greater vander waal's force of attraction. On the other hand, permanent gases like H₂, N₂ and O₂ cannot be liquefied easily. These permanent gases are having low critical temperature and adsorbed slowly, while gases with high critical temperature are adsorbed readily.
(iii) Effect of temperature:
Chemical adsorption is fast with increase pressure, it cannot alter the amount. In Physisorption, when pressure increases the amount of adsorption increases.
(iv) Effect of pressure:
When pressure increases, adsorption becomes steadily increase in case of chemisorption and increases in case of physisorption.

Characteristics of adsorption:
(i) Adsorption can occur in all interfacial surfaces i.e. the adsorption can occur in between gas-solid, liquid solid, liquid liquid, solid- solid and gas-liquid.
(ii) Adsorption is always accompanied by decrease in free energy. When ΔG reaches zero, the equilibrium is attained.
(iii) Adsorption is a spontaneous process
(iv) When molecules are adsorbed, there is always a decrease in randomness of the molecules.
We know, ΔG = ΔH - T ΔS where ΔG is Change in Free energy.
ΔH is Change in enthalpy and ΔS- Change in entropy.
Hence, ΔH = ∆G + TΔS
(v) Adsorption is exothermic as there an interaction between adsorbate adsorbent.