Class 12 Chemistry Chapter 5 Notes

The chapter on Surface Chemistry in Class 12 mainly focuses on the way particles in specific solutions remain suspended throughout. They never settle down, and why does it happen? 

Surface Chemistry Notes will help you understand why and how. Moreover, it will teach what occurs at the interface between different phases. Thus, Surface Chemistry deals with the processes which occur at interfaces, predominantly between liquid and gases. This chapter will also explore its various applications in different fields.

Surface Chemistry plays a significant role in our day-to-day lives as it is involved in many phenomena and technological applications. Typical scenarios where you can find Surface Chemistry usage are soap bubbles, foaming, and raindrops stuck on your windows and glasses. You will learn several essential concepts in the Class 12 Chemistry Chapter 5 Notes. These  notes are proven to be helpful, and it makes it easier for students to revise the whole  chapter quickly. Thus giving you an edge over your peers while  preparing  for the competitive examinations such as JEE mains and  Advanced.

Key Topics Covered in Class 12 Chemistry Chapter 5 Notes


As described in the  notes, this  branch of Chemistry, which deals with the nature of a surface and its species, is known as surface Chemistry. Adsorption on solid or solution surfaces and colloidal properties are significant surface effects.

Surface Chemistry works with the chemical reactions on the surface or interface. There is a broad range of practical applications which involve Surface Chemistry. From the production of high vacuum to the manufacturing of Gas Masks, Surface Chemistry principles are used widely. It is also used to understand the catalyst reactions. 

Tyndall Effect and Brownian  Movement, used for various applications:


The phenomenon of a higher concentration of molecular species (gases or liquids) on the surface of solids than in bulk is known as Adsorption.

The solid on the interface of which adsorption develops is called an adsorbent. The substances which get adsorbed on the solid surface are called adsorbates. The adsorbent may be a liquid or a solid, and the adsorbate may be a gas or a liquid. Students may refer to our NCERT Solutions in addition to Class 12 Chemistry Chapter 5 Notes for a more detailed explanation. .


Surface Chemistry in the field of Chemistry studies events created on the surface or interface, which is at the boundary separating both bulk phases. Original chemicals or solutions can make up the two bulk phases.

A hyphen between the two bulk phases involved, solid-liquid and solid/liquid, signifies the interface. Therefore gases are entirely miscible, and there is no interface between them. 

Crystallisation, dissolution, corrosion, heterogeneous catalysis, electrode processes, and other essential phenomena occur at the interface. 

Adsorption & Absorption:

In Adsorption, the adsorbate is the substance whose molecules get adsorbed at the surface. It’s a surface phenomenon where the substance on whose surface the process takes place is called the adsorbent.

Absorption is a separate mechanism from Adsorption, where the molecules undergoing resorption get soaked up but not by the air. Adsorption, whenever it takes place, is based on the surface, where a film of adsorbate is developed on the surface, whereas absorption includes the complete volume of the absorbing agent.


The accumulation of attracting and keeping the molecule of a substance on the surface of a solid is called Adsorption. This results in an enhanced concentration on the surface than in bulk. The adsorption of gas on a solid material depends on the following factors: 

  • The surface area of the solid.
  • The temperature of the gas.
  • Nature of the gas, the pressure of the gas, and the nature of the solid. 

Students are recommended to visit the Extramarks website and refer to our Class 12 Chemistry Chapter 5 Notes for a more detailed explanation.

  • Adsorption: 

The existence of a substance at an interface in a different concentration than in adjoining bulk is called adsorption.

  • Adsorbate: 

The substance adsorbing on the surface of another substance is called adsorbate.

  • Adsorbent: 

The substance present in bulk on the surface of  adsorption is called adsorbent.

  • Desorption

This process of removing an adsorbed substance from a surface on which it is adsorbed is called desorption. 


As per Class 12 Chemistry Chapter 5 Notes, absorption is the phenomenon in which a substance is uniformly distributed all over the bulk of the solid. The concentration is the same throughout the bulk of the solid.

  • Sorption: When adsorption and absorption take place at the same time, it is known as sorption.
  • Enthalpy or Heat of Adsorption: 

Adsorption generally happens with exothermic energy release. The enthalpy change for the  adsorption of 1 mole of an adsorbate on the adsorbent surface is known as enthalpy or heat of  adsorption.

Common adsorbents as given in our Class 12 Chemistry Chapter 5 Notes are clay, silica gel, colloids, metals etc. 

Mechanism of Adsorption:

It is an exothermic process, which means energy is liberated during this process. The amount of heat that evolves when one mole of the adsorbate is adsorbed on the adsorbent is called enthalpy. The change in enthalpy is denoted to be negative. This is because when adsorbate molecules are adsorbed on the surface, the freedom of movement of molecules becomes restricted, resulting in a decrease in entropy. Adsorption can be created spontaneously at constant temperature and pressure.

We have explained the process of adsorption in further detail with the use of visual diagrams in our Class 12 Chemistry Chapter 5 Notes. Students are recommended to register on Extramarks’ website to get full access to our chapter notes and NCERT Solutions.

Types of Adsorption: 

Physical Adsorption: 

When adsorbate is held on a surface of the adsorbent by weak van der Waals’ forces, the  adsorption is known as physical Adsorption or physisorption. It is non-specific and reversible. The amount of gas depends upon its nature. The scope of adsorption increases with an increase in surface area, like porous and finely divided metals are suitable adsorbents. 

Characteristics of Physisorption

  • As any gas can get adsorbed onto the surface, there can be no specificity.
  • Highly liquefiable gases are physically adsorbed more strongly.
  • It is reversible and dependent on pressure and temperature. An increase in pressure decreases the volume of gas and hence increases the adsorption of gas molecules. 
  • Conversely, a decrease in pressure will cause the removal of gas molecules from the solid surface. Therefore the adsorption process is exothermic. 
  • Physical adsorption occurs readily at low temperatures and decreases with increasing temperature (Le-Chatelier’s principle). 
  • Porous substances are better adsorbents as an increased surface area promotes adsorption. 
  • Does not require energy for activation. 

Chemical Adsorption or Chemisorption

When the forces holding the adsorbate are as strong as in chemical bonds, that adsorption process is called chemisorption. 

Characteristics of Chemisorption:

  • It is definite and irreversible. 
  • The exact amount of gas adsorbed is not related to the critical temperature of the gas. 
  • It also increases with surface area. 
  • There is a strong force of attraction similar to chemical bonds. 
  • It forms unimolecular layers. 
  • The process-specific character will occur only if a chemical bond is formed between the adsorbent and adsorbate. 
  • It is an exothermic process, and an increase in temperature accompanies the procedure. 
  • It occurs slowly at low temperatures and at a higher rate with increased pressure. 

As in the case of physisorption, chemisorption is directly proportional to surface area and thus increases with an increase in surface area. Since the process involves chemical bond formation, the enthalpy is high. It requires specific energy of activation. 

Extramarks Class 12 Chemistry Chapter 5 Notes study material is accessible on the Extramarks website for students who want to learn more and to nail the competition with these notes.

Factors affecting Adsorption of Gases by Solids:

  1. i) Nature and surface area of adsorbent: The same gas is adsorbed by different solids at different extents, even at the same temperature. The surface area is directly proportional to the volume of gases adsorbed.
  2. ii) Nature of the gas being adsorbed: Different gases are adsorbed to different extents even by the same solid. As the critical temperature of a gas enhances, it is easier to liquefy, and it is also more readily adsorbed. The Reason is “Higher the required temperature, the easier it is to liquefy the gas”. 

For a few gases, the intermolecular forces of attraction are more effective on the adsorbent surface and thus, the adsorption will be more. 


As the temperature enhances, adsorption decreases.


When at a constant temperature, the adsorption of a gas enhances with an increase in pressure.

Activation of the Solid Adsorbent: 

It is applied to increase the adsorbing absorbing power of the adsorbent.

As detailed in our Class 12 Chemistry Chapter 5 Notes the activation of the solid adsorbent can be done by:

  • Making the adsorbent surface rough can be done by rubbing the surface or chemical action or depositing fine particles of metals on the surface by electroplating.
  • Dividing the adsorbent into small pieces or grains increases the surface area, but this process has a practical limitation. If the particles are too small, like powder, the adsorption of the gas will become problematic.
  • By removing the already adsorbed gases.

Freundlich Adsorption Isotherm:

In 1909, German scientist Freundlich expressed an empirical relationship between the amount of gas adsorbed by the same amount of mass of solid adsorbent and pressure at a particular temperature. It shows the following equation:

                        x/m = k.P1/n (n > 1)

Here ‘x’ is the mass of the gas adsorbed on mass ‘m’ of the adsorbent at pressure ‘P’. ‘k’ and ‘n’ are constants that depend on the nature of the adsorbent and the gas at a particular temperature. 

Students may refer to NCERT  Solutions in addition to Class 12 Chemistry Chapter 5 Notes for a more detailed explanation of Isotherm.

One gram of the adsorbent is plotted between the pressure in the form of a curve to show the relationship. Here, at a fixed pressure, physical  adsorption decreases with increased temperature. The curves reach saturation at high pressure. Now, if you take the log of the above equation:

Here, log x / m = log k + 1 / n log P

To examine the validity of the Freundlich isotherm, we can plot log x/m on the y-axis and log P on the x-axis. If the plot represents a straight line, the Freundlich isotherm is valid. Otherwise, it is not. 

The straight-line slope gives the value of 1/n, while the intercept on the y-axis shows the importance of log k

Freundlich Isotherm:

Limitations of Freundlich Isotherm:

Freundlich isotherm only approximately describes the characteristics of adsorption. The value of 1/n is between 0 and 1. Hence the equation holds good only over a limited pressure range.

  • If  1/n = 0, x/m is constant, the adsorption is independent of pressure.
  • If  1/n =1, x/m = k P,  so  x/m ∝ P, adsorption is directly proportional to pressure.

Experimental results support both of the conditions mentioned above. At high pressure, the experimental isotherms always achieve saturation. Freundlich isotherm fails to explain this observation, and thus fails at high pressure.

The Freundlich isotherm was followed by two other isotherms: Langmuir adsorption isotherm and BET adsorption isotherm. Langmuir isotherm assumed that Adsorption is monolayer, whereas BET isotherm thought it is multi-layer. More about the Freundlich isotherm, Langmuir adsorption isotherm and BET adsorption isotherm has been explained in our Class 12 Chemistry Chapter 5 Notes.

Application of Adsorption:

  1. Production of high vacuum

The persistent traces of air adsorbed by charcoal from a vessel evacuated by a vacuum pump  give a very high vacuum. A bundle of charcoal cooled in liquid air is combined with a vessel that has already been vacuumed as feasible. The charcoal absorbs the remaining air, resulting in an extremely high vacuum.

  1. Gas masks

A gas mask (a device consisting of activated charcoal or a mixture of adsorbents) is usually used to breathe in coal mines to adsorb poisonous gases. It’s an adsorbent system made of activated charcoal or adsorbents. In coal mines, this device is used to adsorb harmful gases (e.g. CO, Cl2 sulphur oxide, etc.) and purify the air for breathing. 

  1. Control of humidity

Silica gel and aluminium gels are adsorbents for absorbing moisture and controlling humidity.

  1. Removal of colouring matter from solutions

Generally, Animal charcoal absorbs the colours of solutions by adsorbing coloured impurities.

  1. Heterogeneous catalysis

Commonly, the  absorption of reactants on the catalysts’ solid surface increases the reaction rate. There are several gaseous reactions of industrial importance involving solid catalysts. Production of ammonia using iron as a catalyst, developing H2SO4 by contact process, and using finely divided nickel in the hydrogenation of oils are Examples of heterogeneous catalysis reactions.

  1. Separation of noble gases:

Due to the difference in the degree of gas  adsorption because of charcoal, a mixture of noble gases are separated by Adsorption on coconut charcoal at different temperatures.

  1. In curing diseases

Some drugs are used to kill germs by getting adsorbed on them.

  1. Froth floatation process

Low-grade sulphide ore is concentrated by separating it from silica and other earthy matter using pine oil and a frothing agent.

  1. Adsorption indicators

Generally, surfaces of certain sediments, such as(AgX) silver halides, have the property of adsorbing some dyes like eosin, fluorescein, etc., thereby producing a characteristic colour at the endpoint.

The chromatographic analysis is based on the Adsorption phenomenon and finds some applications in analytical and industrial fields. We have described a few useful cases of applications of the adsorption phenomenon in our Class 12 Chemistry Chapter 5 Notes.. 


All reactants need to overcome specific energy. Activation energy is the difference between the energy of the transition state and the reactant species. Some reactant molecules have sufficient kinetic energy to overcome this energy barrier, whereas others don’t. Therefore, not all reactions happen at the same rate in general conditions. Thus, specific reagents are added, lowering the required activation energy to convert reactants to products. These reagents are called catalysts, and this process of reducing the activation energy is called catalysis. 

  • Potassium chlorate, when heated, powerfully decomposes slowly, giving dioxygen. The decomposition between 653 – 873 K.

2 KClO3 → 2 KCl + 3O2

However, with a little MnO, a decomposition occurs only at 473 – 633 K and at a much-accelerated rate. MnO2 is a catalyst for this reaction.

2 KClO3 ——-→ 2 KCl + 3O2

A substance increases the reaction rate, remaining chemically and quantitatively unchanged after the reaction is called a catalyst. 

A promoter is a substance that enhances the activity of a catalyst. At the same time, poison is a substance that decreases the movement of a catalyst in the reaction to manufacture NH3 by Haber’s process.

N2 (g) + 3H2(g)——2NH3(g)

Fe is a catalyst, and Molybdenum (Mo) is a promoter. Students are recommended to refer to our Class 12 Chemistry Chapter 5 Notes for a more detailed explanation. They can easily access our class notes and other study materials by registering on the Extramarks website.

Homogeneous and Heterogeneous catalysis:

If the reactants and the catalyst are in the same phase (liquid or gas), the process is called homogeneous catalysis.

For Example: 2S02(g) + O2(g) 2SO3(g) catalyst is NO

CH3COOCH3(l) + H2O(g) CH3COOH(l) + CH3OH(l)

If the reactants and the catalyst are in different phases(liquid or gas), the process is called heterogeneous catalysis.

2S02(g) + O2(g) 2SO3(g) catalyst is Pt and Fe

N2(g) + 3H2 2NH3(g) catalyst is Fe

Vegetable oil(l) + H2(g) Vegetable Ghee(s) catalyst is powdered Ni

Characteristics of solid catalysts as detailed in our Class 12 Chemistry Chapter 5 Notes:

  • Activity

It relies upon the strength of chemisorption to a maximum extent.

  • Selectivity:

Generally, the selectivity of a catalyst is its ability to direct a  chemical reaction to yield a particular product. 

For Example, We get different products starting with H2 and CO from similar catalysts.

CO(g) + 3H2(g)——CH4(g) + H2O(g)   catalyst is Ni

CO(g) + 2H2(g)——CH3OH(g)            catalyst is Cu/ ZnO and- Cr2O3

CO(g) + H2(g) —— HCHO(g)               catalyst is Cu

Hence a catalyst is highly selective by nature,, i.e., a given substance can show as a catalyst only in a specific reaction and not for all the reactions. A catalyst for a specific reaction may fail to catalyse any other reaction. 

 Catalysis of chemical reactions is divided into a few ways:

  • Homogeneous Catalysis

The Homogeneous catalysis of chemical reactions is a process where the reactants are involved in the reaction. The catalyst is in the same phase, for example, hydrolysis of glucose in the presence of acid.

  • Heterogeneous Catalysis

The Heterogeneous catalysis of chemical reactions is a process where the reactants involved in the reaction, and the catalyst, are in different phases. For Example. The reaction of hydrogen and nitrogen in iron to form ammonia.

  • Positive catalysis

If a foreign material accelerates the pace of a reaction, it is known as a positive catalyst, and the phenomenon is known as positive catalysts. Below are some examples of positive catalysts.

In the presence of colloidal platinum, 2H2 decomposes.

2H2O2(l) → 2H2O2(l) + O2(g) 

  • Negative catalysis

It refers to substances that slow down rather than speed up the reaction rate when added to a reaction mixture. Negative catalysts, as well as inhibitors, are used in the following Examples.

If some alcohol is mixed with chloroform, the Oxidation by air is slowed.

2CHCl2(ℓ)+ O2(g)→2COCl2(g)+2HCl(g) catalyst is Alcohol(ℓ)

  • Photocatalysts

It is the phenomenon wherein the catalyst can receive light (such as visible light) and be promoted to an excited state.

  • Auto-catalysis

One of the products behaves as a catalyst in some processes. The reaction is slow initially, but the reaction rate increases after the products are formed. It is the term for this type of event. The following are some autocatalysis Examples.

As the reaction develops, oxalic acid’s oxidation rate by acidified potassium permanganate increases. The existence of 2Mn ions, which are generated during the process, causes this acceleration. As a result, two Mn ions serve as an auto-catalyst.

5H2C2O4 + 2KMnO4 + 3H2SO4→2MnSO4 + K2SO4 + 10CO2 +  8H2O

When nitric acid is poured on Cu, the reaction is slow at first, but it quickly speeds up due to the development of HNO2 nitrous acid, which works as an auto-catalyst during the reaction.

Acetic acid and ethyl alcohol are formed during the hydrolysis of ethyl acetate. The reaction is slow in the beginning, but it picks up pace  as time moves  on. This is because acetic acid is produced, which acts as an auto-catalyst in the reaction.

  • Induced catalyst: 

When the reaction influences the rate of other reactions, which do not occur under ordinary conditions, it acts as an induced catalyst, and the phenomenon is an induced catalyst. 

For Example: If oxalic acid is added to a mixture of KMnO4 and HgCl2, both reduce simultaneously. Thus reduction of KMnO4 causes the reduction of HgCl2

The  Catalysis of a chemical reaction is a complex topic and many struggle to get a firm understanding of this phenomenon. Students can refer to our Class 12 Chemistry Chapter 5 Notes in which our Chemistry subject teachers have explained this concept in great detail.

Mechanism of Heterogeneous Catalysis of Chemical Reactions:

The modern theory of adsorption has a five-step mechanism for the catalysis of chemical reactions. The 5 step process is defined as follows. To get more information about each step students should refer to our Class 12 Chemistry Chapter 5 Notes.

  • General Introduction and diffusion of reactant molecules on the catalytic surface.
  • Adsorption of molecules of reactants on the catalytic surface.
  • Formation of intermediate on a catalytic surface by a chemical reaction between the reactant molecules.
  • Desorption of product molecules from the catalytic surface.
  • Diffusion of product molecules away from the catalytic surface to form final products.

Some Important Definitions and Explanations which are covered in Extramarks Class 12 Chemistry Chapter 5 Notes

Define catalysis

A catalyst is defined as a substance that increases the rate of a chemical reaction without changing itself. The process by which a catalyst enhances the reaction rate is called catalysis. 

How do catalysts work? 

For reactants to react and give a final product, reactant molecules have threshold energy, and the exact number of molecules with this energy should also be more than a threshold value. This energy is known as activation energy. 

Examples of some heterogeneous catalysts: 

Under the Oxidation of sulphur dioxide into sulphur trioxide in front of (platinum metal or vanadium pentoxide )as a catalyst, reactants are in a gaseous state while the catalyst is in a solid state.

                                       2SO2 (g ) + O2 (g) → Pt (s) + 2SO3

Shape-Selective Catalysis by Zeolites: 

The catalytic reaction, which depends upon the catalyst’s pore structure and the size of the reactant and product molecules, is called shape-selective catalysis. Zeolites are suitable shape-selective catalysts because of their honeycomb-like design.

An essential zeolite catalyst used in the petroleum industry is ZSM-5. It converts alcohol directly into gasoline (petrol) by dehydrating them to give a mixture of hydrocarbons. The Class 12 Chemistry Chapter 5 Notes compiled by the Extramarks team will help students understand this section better. 


Class 12 Chemistry Chapter 5 also defines enzymes. Enzymes are nitrogenous compounds that plants and animals generate. It is a high molecular mass protein molecule and forms colloidal solutions with water. It is a very effective catalyst. These enzymes catalyse many reactions in the bodies of animals and plants to maintain life processes; Thus, enzymes are called biochemical catalysts. 

Enzyme Catalysis:  

These are complex nitrogenous organic compounds produced by living plants and animals. They are high molecular mass protein molecules and form colloidal solutions in water. The enzymes are also referred to as Biochemical Catalysts as they also occur in plants and animal bodies. This phenomenon is known as Bio-Chemical Catalysis and students can learn more about it in our Class 12 Chemistry Chapter 5 Notes.

Enzyme Catalysed Reactions

Decomposition of Urea converts into Ammonia and Carbon Dioxide. Urease enzyme is used as an enzyme catalyst.

The following are examples of enzyme-catalysed reactions:

  • Inversion of cane sugar:

C12H22O11(l) + H2O(l) C6H12O6(l) + C6H12O6(l) catalyst is Invertase

Cane sugar                     Glucose         Fructose

  • Conversion of glucose into ethyl alcohol:

C6H12O6(l) —-2 C2H5OH(l) +2CO2(g)     catalyst is Zymase

  • Decomposition of urea into ammonia and carbon dioxide CO2:

NH2CONH2(aq) + H2O(l) 2NH3(g) +CO2  (g)catalyst is Urease.

Some enzymatic Reactions:

Enzyme Source Enzymatic Reaction
Invertase Yeast Sucrose → Glucose and fructose
Zymase Yeast Glucose → Ethyl alcohol and carbon dioxide
Diastase Malt Starch → Maltose
Maltase Yeast Maltose → Glucose
Urease Soybean Urea → Ammonia and carbon dioxide
Pepsin Stomach Proteins → Amino acids

Characteristics of enzyme catalysis  are provided with a comprehensive and detailed explanation  in our Class 12 Chemistry Chapter 5 Notes:

Enzyme catalysis is unusual in its efficiency and a high degree of specificity. Enzyme catalysts exhibit the following characteristics:

  • Most highly efficient

An enzyme molecule may transform one million molecules of the reactant per minute.

  • Peculiar nature

Several enzymes are typical for a given reaction, i.e., one catalyst cannot catalyse more than one process. 

For Example,  the  urease enzyme catalyses the hydrolysis of urea. It doesn’t  catalyse the hydrolysis of any other amides.

  • Highly dynamic nature under optimum pH

The rate of an enzyme-catalysed reaction is maximum at a certain pH called optimum pH, which is between pH values 5 to 7.

  • Highly active under optimum temperature

The rate of an enzyme reaction becomes enhanced at a definite temperature, known as the optimum temperature. On the other side of the optimum temperature, the enzyme activity is decreased. The best temperature range for enzymatic activity is 298 to 310 K. Human body temperature being 310 K, is suited to enzyme-catalysed reactions.

  • Increasing activity in the presence of activators and coenzymes:  

When the enzymatic activity is increased in the presence of certain substances, called coenzymes, it has been observed that the catalytic activity is increased considerably like a tiny non-protein (vitamin) is present along with an enzyme.

Activators are commonly metal ions such as Na+ Mn2+, Co2+, Cu2+, etc. These metal ions enhance their catalytic activity when weakly bonded to enzyme molecules. Amylase enzyme in the presence of sodium chloride, i.e., Na+ ions, is a very active catalytic.

  • Influence of inhibitors and poisons

Commonly, ordinary catalysts and enzymes are inhibited or poisoned by the +nce of certain substances. The inhibitors or poisons attached to the active functional groups on the enzyme interface often reduce or completely vanish the catalytic activity of the enzymes. The usage of many drugs is related to their action as enzyme inhibitors in the body. 

Mechanism of Enzyme Catalysis: 

Several cavities are present on the surface of colloidal particles of enzymes. These cavities are of a characteristic shape and possess active groups such as – NH2, – COOH, – SH, – OH etc. These are the vibrant centres on the surface of enzyme particles. The reactant molecules (substrate), which have complementary shapes, fit into these cavities just like a key fits into a lock. 

An activated complex is created, which then decomposes to yield the products in two steps, as shown below:

Step I.

E + S → ES* Binding of enzymes to substrate to form an activated complex

Step II.

ES* → E + P Decomposition of the activated complex to create a product

While Class 12 Chemistry Chapter 5 Notes provide a detailed explanation, students may also refer to a vast repository of study materials such as NCERT Solutions on the Extramarks’ website. 

Some Important  Industrial Catalytic Processes as covered in our Class 12 Chemistry Chapter 5 Notes:


1. Haber’s process for the production of ammonia N2(g) + 3H2(g) → 2NH3(g)


1. Finely divided iron, molybdenum as a promoter; conditions: 200 bar pressure and 723 to 773K temperature.

2. Ostwald’s process for the production of Nitric acid.

4NH3 (g) + 5O2(g)   → 4N0(g) + 6H2O(g)

2NO(g)  + O2(g)   → 2NO2(g)

4NO2(g)  + 2H2O(l) + O2(g)   → 4HNO3(aq) nitric acid

2. Platinised Asbestos, the temperature is 573K.
3. Contact process for the production of Sulphuric acid.

2SO2(g) + O2 (g)  ⇌ 2SO3(g)

SO3(g) + H2SO4 (l)  → H2S2O7 (l)  oleum

H2S2O7 (l)+H2O (l)  → 2H2SO4 (aq) 

3. Platinised asbestos or vanadium pentoxide (V205): The temperature is 673 to 723K.


A heterogeneous system in which a substance is dispersed (dispersed phase) as very fine particles in another substance is known as a dispersion medium.

The vital difference between a solution and a colloid is particle size. Their size is in between that of proper solution and suspension. 

Classification of Colloids: 

 Colloids are classified  on the basis of  following  criteria:

  • The physical state of the dispersed phase as well as dispersion medium.
  • Nature of interaction between the dispersed phase as well as dispersion medium.
  • Types of particles of the dispersed phase.

The physical state of the dispersed phase and dispersion medium

Eight colloidal systems are possible depending upon whether the dispersed phase and the dispersion medium are solids, liquids or gases.

Apart from Class 12 Chemistry Chapter 5 Notes, students may refer to other study materials such as CBSE revision notes, CBSE past year question papers, and important questions while preparing for their Board and other competitive examinations.

Classification of Colloids:

Under Class 12 Chemistry Chapter 5 Notes, Colloids are Classified into the physical state of the dispersed phase and dispersed medium.

S.No Dispersed Phase Dispersed Medium Name of Colloid Examples
1. Solid Solid Solid sol Gemstones, some coloured glasses
2. Solid Liquid Sol Muddy water, Paints, cell fluids
3. Solid Gas Aerosol Smoke, dust
4. Liquid Solid Gel Cheese,  jelly
5. Liquid Liquid Emulsion Milk, Hair cream, butter
6. Liquid Gas Aerosol Fog, mist, cloud,insecticide sprays
7. Gas Solid Solid sol  Pumice Stone, foam rubber
8. Gas Liquid Foam Froth, soap lather, whipped cream

Based on the characteristics of Interaction Between the Dispersed Phase and Dispersed Medium:

  • Lyophilic Colloids (liquid loving)

A few substances can form colloids directly by adding them to a suitable liquid(dispersion medium). These colloids are called lyophilic colloids. Examples of these substances are gum, gelatine, starch and rubber. It is also known as reversible sols. In these sols(colloids), if the dispersion phase is separated from the dispersion medium (by evaporation), the sol can be formed by combining the dispersion phase with the medium again. They are also very much stable and cannot be coagulated.

  • Lyophobic colloids (liquid hating)

A few substances cannot form colloids by directly adding them to a liquid. Colloidal sols are made up of unique methods known as lyophobic colloids. Examples of these substances are metals and metal sulphides. They are also known as Irreversible colloids. The colloidal solution whose dispersed phase particles whatsoever have no attraction but rather have repulsion for the dispersion medium are known as lyophobic colloids. The keys of metals like Ag and Au, hydroxides like Al (OH)3 and Fe (OH)3 and metal sulphides like As2S3 are some of the examples of lyophobic colloids. Such sols are formed with difficulty. They are irreversible.

Based on Types of Particles of the Dispersed Phase:

  • Multimolecular Colloids: 

Many particles(atoms or molecules) of the dispersed phase combine to form colloidal particle sizes (1-1000nm). These colloids are known as multimolecular colloids.

For Example, Sulphur sol and gold sol.

  • Macromolecular colloids: 

Substances with(macromolecules) large molecules in suitable solvents form solutions, but these macromolecules might be in the colloidal range. These solutions are known as macromolecular colloids and resemble real solutions in many ways. 

For  example, Starch, Cellulose, and Proteins are natural macromolecules. Nylon,synthetic rubber, polythene, and polystyrene are man-made or artificial macromolecules.

  • Associated Colloids: 

A few substances at high concentrations act as colloids due to the formation of aggregates. Therefore at low concentrations, they behave like normal strong electrolytes. These aggregates formed are called micelles. Such colloids are called associated colloids. 

Refer to our Class 12 Chemistry Chapter 5 Notes for more information on this topic classification of colloids.

Kraft Temperature: 

The formation of micelles only occurs above a particular temperature known as Kraft’s Temperature.

Critical micelle concentration(CMC) – The critical concentration above which micelle development takes place

For  example, Soaps, synthetic detergents etc.

General methods to prepare Sols:

Lyophilic sols are readily formed by mixing the dispersed phase and the dispersion medium under ordinary conditions. 

Two methods can generally prepare lyophobic sols.

  1. Condensation methods
  2. Dispersion methods
  3. Condensation methods

In this process, the smaller particles are condensed suitably to be of colloidal size. This can be achieved by a chemical process or by solvent exchange.

  1. Dispersion methods

The large particles of a substance, suspension, in this case, are broken into smaller particles. This can be done by mechanical dispersion, electrical dispersion, Bredig’s arc method, and peptisation. 

Mechanism of Micelle Formation:

Soap is the sodium(Na) or potassium (K)salts of fatty acid and may be shown as RCOO- Na (e.g. sodium stearate, (CH3(CH2)16COONa+]). When dissolved into water, it dissociates into  RCOO and  Na+ ions.

The RCOOion consists of two parts – a long hydrocarbon chain (also called the non–polar tail), which is hydrophobic (water-repelling), and a polar group COO (polar head), which is hydrophilic (water-loving )

At higher concentrations (CMC), RCOO ions create an aggregate of spherical shape with the hydrocarbon chains pointing towards the centre and the COO part facing outward on the sphere’s surface. This aggregate formation is called an ionic micelle. It may have as many as a hundred ions. Students may refer to NCERT and CBSE Solutions in addition to Class 12 Chemistry Chapter 5 Notes for a more detailed explanation of the mechanism of micelle formation.

The Cleansing Action of Soaps:

Generally, the cleansing action of soap is due to the development of micelle by the soap molecules so that the hydrophobic part is in the oil droplet(dirt) and the hydrophilic part projects out. Since the polar groups interact with water surrounded by soap, ions are pulled from the surface and pulled into the water. Students may refer to various study materials including  NCERT Solutions, CBSE Revision Notes, and Class 12 Chemistry Chapter 5 Notes for a more detailed explanation for thorough preparation.

Preparation of Colloids:

A few methods to prepare colloids are shown below:

  1. Chemical method

Colloids can be developed by chemical reactions leading to the formation of molecules by double decomposition,  oxidation, reduction or hydrolysis. These molecules then aggregate to form sols. 

As2O3 + 3H2S—–→ As2S3(sol) + 3 H2O           (Double decomposition)

SO2 + 2H2S—–→ 3S(sol) + 2H2O                    (Oxidation)

2AuCl3 + 3HCHO + 3H2O—–→ 2Au(sol) + 3 HCOOH + 6HCl (Reduction)

FeCl3 + 3H2O—–→ Fe(OH)3(sol) + 3HCl        (Hydrolysis)

  1. Electrical Disintegration or  Bredig’s  Arc method

This method is applied to obtain colloidal sols of metals like gold(Au), silver (Ag)and platinum(Pt). An electric arc is struck beside the electrodes of the metal immersed in the dispersion medium. The intense heat evaporates the metal, which then condenses to form particles of colloidal size.

Peptization converts a precipitate into a colloidal sol by shaking it with a dispersion medium and a small amount of electrolyte. The electrolyte used for this is called a peptising agent. The precipitate absorbs one of the electrolyte ions on its surface during peptisation. This will cause a positive or negative charge to develop on the residue, which ultimately will break up into small particles of the colloid size. 

Purification of colloidal solutions:

The colloidal solutions developed usually contain impurities, especially electrolytes that do not let the solutions stabilise. Such impurities must be eliminated to make the colloidal solution stable.

As described in our Class 12 Chemistry Chapter 5 Notes, the following process is generally used for the purification of colloidal solutions. 

  1. Dialysis

The method is based on the fact that colloidal particles cannot pass through a parchment or cellophane membrane while the electrolyte ions can transfer through it. The pure colloidal solution is taken in a bag made of cellophane or parchment.

So the bag is filled and suspended in fresh water, and the impurities slowly diffuse out of the bag, leaving behind a pure colloidal solution. For example, dialysis can remove HCl from the ferric hydroxide sol.

The process of separating the particles of a colloid by diffusion through a suitable membrane is called dialysis.


An apparatus called a dialyser is used. A bag with a suitable membrane containing the colloid is suspended in a vessel through which freshwater flows continuously. The impurities get diffused through the membrane into the water, leaving behind the colloid. 

  1. Electrodialysis

Dialysis is a slow method. It is made faster by this method. In this method, an electric field is applied through metal electrodes. These metal ions present in the colloidal solution migrate out to the oppositely charged  electrodes. It is possible only when the dissolved substance in the impure colloid is an electrolyte.  

The dialysis is carried out by applying an electric field to speed up the purification process. This process is called electrodialysis. 

  1. Ultra-filtration

It removes the impurities from the colloidal solution by passing it through graded filter paper called ultrafilter papers. These graded filter papers are made from ordinary filter papers by impregnating them with colloidal solutions. This separates colloidal particles from the soluble solutes(impurities) using specially prepared filters, permeable to all substances except the colloid. 

Colloidal particles can pass through filter papers because of the large pores. An ultrafilter paper can be made by soaking the filter paper in a colloidal solution, hardening it with formaldehyde, and drying it. As this is a slow process, pressure or suction is applied to speed it up. The colloidal particles that remain on the ultra-filter paper are stirred with a fresh dispersion medium(solvent) to form a pure colloid. 

Properties of Colloids:

Colligative properties:

The colloid particles are bigger aggregates than those in a proper solution. Suppose the number of particles in a colloid is less than a proper solution of the exact concentration. The values of colligative properties like (osmotic pressure, lowering of vapour pressure, depression in freezing point, and elevation in boiling point are of small order compared to values shown by proper solution at the same concentration.

Tyndall effect:

Tyndall phenomena are referred to as light scattering due to the particles present in the colloidal solution when viewed at right angles to the passage of light.

It is observed only when-

  • The diameter of the dispersed particles is much smaller than the wavelength of light applied.
  • The refractive index of the dispersed phase and dispersed medium has a significant difference.
  • This effect was used to make an ultramicroscope and differentiate between true solution and colloids.


The colour of the colloidal solution depends on the wavelength of the light scattered by the dispersed particles, the size and nature of the dispersed particles, and how it is viewed. 

For  example,  the finest gold sol is red, and as the size of the particle keeps increasing, its colour changes to blue, then purple and finally gold. 

Properties of Colloidal Solutions as explained in our Class 12 Chemistry Chapter 5 Notes:

  1. Heterogeneous nature

The colloidal solutions are heterogeneous, consisting of the dispersed phase and the dispersion medium. 

  1. Visibility

The colloidal is not visible to the naked eye, which can be seen with ultra microscopes.

  1. Brownian movement

The colloidal particles have a continuous zigzag motion called Brownian movement. Brownian movement may be defined as a constant zigzag motion of colloidal particles in a solution. It depends on the size and the viscosity of the particles and colloids, respectively. The smaller the size of the particle and the lesser its viscosity, the faster will be its motion. This movement is necessary for the stability of sols.  A repository of study materials is available on the Extramarks website and Class 12 Chemistry Chapter 5 Notes for a more detailed explanation of colloids and relevant topics. Students can supplement their learning with required study material and step up their preparation to ace the exam.

Charge on Colloids:

Colloidal particles always have an electric charge. The nature of this charge is equal to all the particles in a known colloidal solution and maybe either +ve  or –ve.

Positively Charged solutions:

  • Haemoglobin.(Blood)
  • Oxides-TiO2 solution.
  • Hydrated metallic oxides like Al2O3.xH2O, Fe2O3.xH2O.
  • Basic Dyes-methylene blue solutions.

Negatively Charged solutions:

  • Metals- Cu, Ag, Au sols.
  • Metallic sulphides-As2S3, CdS. sols
  • Acid dye kinds of stuff- Eosin,congo red sols
  • Sols of starch, gelatin,gum,clay etc.
  1. Electrical properties:

The particles of the colloidal solutions hold electrical charges like positive or negative charges. The +nce of charge is responsible for the stability of these solutions. It may be concluded that only the sol particles carry some charge while the dispersion medium has no charge. 

For  example,  the colloidal solutions of gold and arsenious sulphide (AS2S3) are negatively charged, while the colloidal solution of Fe (OH)3 and Al (OH)3 have a +ve charge. The particles can either be positively or negatively charged in silver chloride sol.

The presence of the charge on the soil particles and their nature, whether positive or negative, can be understood with the help of a phenomenon called electrophoresis. In this example, the colloidal particles move towards positive or negative electrodes depending upon their charge under the influence of an electrical field.

The movement of all colloidal particles under an applied electric field is known as electrophoresis. This phenomenon is explained at greater length in our Class 12 Chemistry Chapter 5 Notes for easy and better understanding of students.


When the particles combine near the negative electrode, the charge on the particles is +ve. On the other hand, When the sol particles get near the positive electrode, the charge on the particles is negative.

The movement of colloidal particles with the influence of an electric field is known as electrophoresis. Negatively charged particles move toward the cathode, and  positively charged particles move towards the anode.

When the movement of particles is obstructed, it is observed that the dispersion medium starts to move in the electric field. This is called electroosmosis. 

Origin of Charge:

The charge on the colloidal particles may be due to the selective  adsorption of metal ions. The particles contributing to the dispersed phase adsorb only those ions preferentially, which are familiar with their lattice ions. 

For  example,  suppose silver nitrate solution is added to an aqueous potassium iodide solution. In that case, the silver iodide will absorb negative ions (I) from the dispersion medium to form a negatively charged sol.

AgI + I   → AgI. I negative sol

However, suppose silver iodide is formed by adding potassium iodide to silver nitrate solution. In that case, the sol will be positively charged due to the  adsorption of Ag+ ions present in the dispersion medium.

AgI + Ag+→ AgI. Ag+ positive sol

Coagulation of Colloidal Solution: 

Coagulation or flocculation is the precipitation phenomenon of a colloidal solution where an electrolyte is added in excess.

 Factors governing coagulation as explained in our Class 12 Chemistry Chapter 5 Notes:

  1. Nature of the Electrolytes:

The coagulation capacity of various electrolytes depends upon the valency of the active ion, called a flocculating ion. It is the metal ion carrying charge opposite to the charge on the colloidal particles. Based on Hardy Schulz’s law, the greater the valency of the active ion or flocculating ion, the greater its coagulating power. Hence, to coagulate the negative sol of As2S3, the coagulating power of different cations has been found to decrease in the order as

Al3+ > Mg2+ > Na+

Generally, to coagulate a positive sol such as Fe (OH)3, the coagulating power of various anions has been found to decrease in the order as:

 Cl < SO42- < PO4 3- < [Fe(CN)6] 4-

The most negligible concentration of an electrolyte required to cause the coagulation or flocculation of a sol is the flocculation value. It is usually expressed as millimoles/litres.

Protection of colloids: 

Protecting the lyophobic colloidal solution from precipitation by the electrolytes due to the previous addition of some lyophilic colloids is known as protection. The colloid added to achieve such protection is called protecting colloid. Students may refer to NCERT and CBSE Solutions in addition to Class 12 Chemistry Chapter 5 Notes for a more detailed and elaborate explanation of colloid protection.

Gold Number: 

The various protecting colloids differ in their protecting powers. Zsigmondy introduced a term called the gold number to describe the protective ability of other colloids. This is explained as the minimum number of milligrams of the protective colloid required to prevent the coagulation of a 10 ml of a given gold sol when 1 ml of a 10% sodium chloride solution is added. Concerning Class 12 Chemistry, Chapter 5 Notes, The coagulation of gold sol is indicated by a change in colour from red to blue. The gold number of less protective colloids are as shown below:

Gelatin Haemoglobin Egg albumin  Starch Dextrin
0.005- 0.01 0.03 0.08 25 6-123

It may be noted that the smaller the value of the gold number, the greater will be the protecting power of the protective colloid. Therefore, the reciprocal of the gold number is a measure of the protective power of a colloid. Thus, gelatin is the best protective colloid out of the list given above. 


Emulsions are colloids where both the dispersion phase and dispersion medium tend to be liquids. These liquids are immiscible or partially miscible, in which one of the liquids is water. 

Types of Emulsions:

In simple words, emulsions are the colloidal solutions of two immiscible liquids in which the liquid acts as both the dispersed phase and the dispersion medium. They usually are obtained by mixing oil with water. Thus the two do not mix well. The emulsion is generally not stable and is stabilised by adding a suitable outside reagent called an emulsifier or emulsifying agent. The substances commonly employed for the purposes are gum, soap, glass powder, etc. Students may refer to a repository of  study materials available on Extramarks website, in addition to Class 12 Chemistry Chapter 5 Notes. 

They are of two types:

Oil dispersed in water

(o/w type): 

Water is a dispersion medium for milk and vanishing cream.

Water is dispersed in oil (w/o type), where oil acts as a dispersion medium.

For Example, Butter, Cream. 

  • Oil-in-water emulsions: In this case, oils act as the dispersed phase (small amount) and water as the dispersion medium (excess). For  example, Milk is an emulsion of soluble fats in water. Casein and vanishing cream are emulsifiers, and these emulsions are known as aqueous emulsions.
  • Water-in-oil emulsions: Water acts as the dispersed phase while the oil behaves as the dispersion medium, for  example,  butter, cod liver oil, cold cream and so on. These types of emulsions are called oily emulsions. 


It is the method of decomposing an emulsion back into its constituent liquids. The demulsification could be done by centrifugation, filtration, boiling, freezing, and chemical processes.

Following are the noteworthy  examples of colloids we encounter daily:

  • The blue colour of the sky: Dust particles and water suspended scatter blue light due to which the sky looks blue too, us.
  • Fog, Mist and Rain: Clouds are aerosols. It is possible to due to artificial rain by throwing electrified sand.
  • Food articles like milk, butter, and fruit juices are colloids.
  • Blood: A colloidal solution of an albuminoid substance. Alum and iron chloride FeCl3 solution stop oozing blood because of coagulation.
  • Soils: Fertile soils are colloidal in which humus acts as a protective colloid.
  • Formation of delta: River water is an example of a colloidal solution of clay.

Seawater contains several electrolytes. When river water meets seawater, the electrolytes present in seawater coagulate the colloidal solution of clay, resulting in its deposition with the delta formation.

Identification of Emulsions:

There are various methods for determining if an emulsion is an oil-in-water emulsion. The characteristics of an emulsion as given in our Class 12 Chemistry Chapter 5 Notes:

  • Dilution Test:

In other words, it dilutes the emulsion with water. When the emulsion can be diluted with water, water acts as the dispersion medium, and the emulsion can be said to be an oil-in-water emulsion. If the oil is not diluted, it functions as a dispersion medium, resulting in a water-in-oil emulsion. 

  • Dye Test

The emulsion is shaken with an appropriate oil-soluble dye. It is an oil-in-water emulsion type if colour is visible while gazing at a drop of the emulsion. It is an example of a water-in-oil type if the entire background is coloured.

  • Conductivity Test

Adding a tiny amount of an electrolyte (e.g.KCl) to the emulsion. Water is the dispersion medium if this test renders the emulsion electrically conductive.

Properties of Emulsion:

  • Emulsions exhibit various properties of colloidal solution, including Brownian movement, Tyndall effect, and electrophoresis, among others.
  • The addition of electrolytes constituting polyvalent metal ions coagulates the globules, demonstrating their -ve charge.
  • Emulsion dispersed particles are more significant than sol dispersed particles. It ranges from 1000 – to  10,000. Though, the particles are smaller than those found in suspensions.
  • Emulsions can be separated into two liquids by heating, centrifuging, freezing, and various methods. Demulsification is another name for this process. Students may refer to detailed study notes given in our Class 12 Chemistry Chapter 5 Notes to get a more precise understanding of Demulsification.

Applications of Emulsions:

  • In metallurgy, ore concentration.
  • In medicine, Emulsion water-in-oil type.
  • Soap’s cleansing function.
  • Milk is a fat-in-water emulsion, an essential part of our nutrition.
  • Emulsification is utilised to digest lipids in the gut.

Applications of Colloids:

Colloids are broadly used in the industry. Some examples are as follows:

  • Cottrell Smoke Precipitator:

Smoke is considered a colloidal solution of solid particles, same as arsenic and carbon compounds, dust etc., in the atmosphere. The smoke is passed through a chamber containing plates with a charge opposite smoke particles. The particles coming into contact with these plates lose their charge and precipitate. The particles, therefore, settle down on the floor of the chamber.

  • Purification of drinking water

Alum is mixed with water to solidify the suspended impurities present in water from natural resources to make it fit for drinking purposes.

  • Medicines

Most medicines or drugs are colloidal. Milk of magnesia(emulsions) is used for a stomach disorder. Argyrol is a silver sol added to an eye lotion. Colloidal Antimony is also used in curing Kala Azar. Colloidal gold is used primarily for intramuscular injection. Colloidal medicines are more effective because of their large surface area and are easily assimilated.

  • Tanning

Due to their colloidal nature, animal hides bear a positive charge when soaked in tanning or chromium salts that contain negatively charged colloidal particles, and mutual coagulation occurs, leading to the hardening of leather. The process is known as tanning.

  • The cleansing action of soaps and detergents:

Soaps and detergents act as emulsifiers and remove greasy impurities and dust from washed-up clothes.

  • Photographic plates and films are developed by coating a light-sensitive emulsion of AgBr silver bromide in gelatin over glass plates or celluloid films.
  • Rubber industry:

Latex, a form of rubber, is a colloidal solution of rubber particles that are negatively charged. Rubber is obtained by coagulation of latex.

  • Industrial products:

Industrial products like paints, inks, synthetic plastics and rubber, graphite lubricants, cement, etc., are colloidal solutions. 

A Summary of Key Topics as covered in our Class 12 Chemistry Chapter 5 Notes:

Surface Chemistry primarily deals with phenomena that occur at the surface or interfaces.

  1. This process of attracting and retaining the molecules of a substance by a solid (or a liquid) on its surface, resulting in a higher concentration of the molecules on the surface, is known as adsorption. In physisorption, adsorbate is held by the adsorbent by weak van der Waals forces, whereas in chemisorption, adsorbate is held together by the adsorbent by a strong chemical bond.
  2. The adsorbed substance is called adsorbate, which is called adsorbent.
  3. Desorption removes an adsorbed substance from a surface on which it is adsorbed.
  4. Absorption differs from adsorption, as in absorption, the substance is constantly distributed across the body of a solid or a liquid.
  5. If the adsorbate is held on the surface by weak van der Waals forces, the method is known as physical  adsorption. This physical adsorption type can be reversed by heating or decreasing the pressure.
  6. If the forces holding the adsorbate on the surface area are of the magnitude of chemical bond forces, the process is called chemical adsorption or chemisorption. This type of Adsorption is irreversible.
  7. The evolution of heat generally accompanies Adsorption, signifying that it is an exothermic method.
  8. The amount of Adsorption of gas on a solid is based upon the below factors are:
  • Nature of the adsorbate.
  • Nature of the adsorbent.
  • Temperature.
  • Pressure.
  1. At a constant temperature, the relation or a plot between the magnitude of  adsorption and the gas pressure is an isothermal  adsorption or adsorption isotherm.
  2. Freundlich adsorption isotherm:

A plot of log x/m Vs log P will be a straight line with a slope of 1 In. It holds good at moderate temperature. At low pressure, n = 1.

  1. Langmuir adsorption isotherm is based on the following assumptions:
  • Every adsorption site is the same in all respects.
  • The tendency of a particle to bind at a particular site is independent of whether the nearby areas are occupied or not.
  1. Langmuir derived the following relation.


Here a and b, are the Langmuir parameters.

  1. A substance that can influence a chemical reaction rate but remains unchanged chemically at the end is called a catalyst.
  2. In homogeneous catalysis, the catalyst is +nt in the same phase as the reactants.
  3. In heterogeneous catalysis, the catalyst is +nt in a different phase than the reactants.
  4. Enzymes, also known as biological catalysts, are proteins that catalyse living systems’ reactions.
  5. The colloidal solutions are intermediate connecting true solutions and suspensions. The diameter of colloidal particles differs from 1 – to 1000 nanometers.
  6. The colloidal system is a heterogeneous system that consists of a dispersed phase and dispersion medium.
  7. The dispersed phase creates the colloidal particles, whereas the dispersion medium includes the medium in which the colloidal particles are dispersed.
  8. There are eight different types of colloidal systems depending on the dispersed phase and the dispersion medium.
  9. Sols are the colloidal system where the solid is the dispersed phase, and the liquid is the dispersion medium.
  10. Hydrosols-Colloids in water. Alcohols – Colloids in alcohol.
  11. Lyophilic colloids are those substances that pass into the colloidal state whenever brought in contact with solvents, e.g., proteins, starch, rubber, etc.

These sols are stable due to solid and attractive forces among the particles and their dispersion medium.

  1. Lyophobic colloids (solvent hating) are substances which do not form the colloidal sol readily when combined with the dispersion medium. These sols are of less stability than the lyophilic sols.
  2. The colloids are also  classified as multi molecular, macromolecular and associated colloids.
  3. The following methods can prepare lyophobic sols:

(a) Chemical methods:

  • Oxidation
  • Hydrolysis
  • Double Decomposition
  • Reduction

(b)Physical methods:

  • Exchange of solvent
  • Excessive cooling:

A colloidal sol of ice in organic solvent chloroform (CHCl3 or ether) can be obtained by freezing a water solution in the solvent.

(c) Dispersion methods:

  • Mechanical dispersion:
  • Bredin’s arc method
  • Peptisation method
  1. Lyophilic sols are prepared by warming the substance with a dispersion medium, e.g., starch, gelatin, gum Arabic, etc., and are quickly brought into the colloidal state by warming with water.
  2. The method of separating a soluble crystalloid from a colloid is called dialysis.
  3. Characteristics of colloidal solution:

(a) The zigzag and random motion of the colloidal particles is known as the Brownian movement.

(b) When a beam of light is transferred through a colloidal solution, its path becomes visible. This process is called the Tyndall effect.

It happens because of the scattering of light by colloidal particles.

(c) This movement of colloidal particles under an applied electric field is called electrophoresis.

(d) Diffusion of colloidal particles occurs from a higher concentration region to a lower concentration region.

  1. Emulsions: Emulsion is a colloidal system in which the dispersed phase and the dispersion medium are liquids; e.g., milk consists of tiny drops of liquid fat dispersed in water.
  2. Emulsification is the method of making an emulsion.
  3. Types of Emulsions:

(a) Oil-in-water type in which tiny droplets of oil are dispersed in water, e.g., milk, cod-liver oil, etc.

(b) Water-in-oil type where water droplets are diffuse in an oil medium, e.g., butter.

  1. The amount of protective colloids necessary to prevent the duplication of the colour of 100 ml of 0.01 % of Congo Rubin dye by adding 0.16 equivalent of KCl is called the Congo Rubin number.
  2. Isoelectric point: pH above which sol is negatively charged and below which positively charged at that pH uncharged is called the isoelectric point of colloids. At an isoelectric point, the colloidal particles do not migrate under the influence of an electric field.
  3. The colloidal sol of cellulose nitrate in ethyl alcohol is called collodion.


The summary notes of Surface Chemistry Class 12 from Chapter 5 are given above. Our Class 12 Chemistry Chapter 5 Notes consist of all the information students need to revise this chapter. This  chapter  is important as it has more theoretical and fewer numerical problems. . Students can expect two or five mark questions from it, and  they can easily score marks   if one thoroughly prepares the chapter using Extramarks study materials.

Class 12 Chemistry Chapter 5: Exercise &  Solutions

The solutions provided in this section help students understand how to answer  questions and they are capable of answering  any question without any hassle. .

Some of the essential exercise questions and answers for Class 12 Chemistry Chapter 5 include Demulsification, Coagulation, Dialysis colloidal particles, etc. The exercises and  solutions provided by Extramarks are comprehensive and provide detailed explanations  to ensure that the  students  enjoy the  learning process and can handle any tricky question with ease. 

The Class 12 Chemistry Chapter 5 Notes prepared by the Extramarks experienced faculty are precise  , accurate and reliable for the students to face questions of any difficulty level during Board Examinations  or any other competitive examinations. . 

Students can click on the links given  below to access the solutions for exercises they may need to prepare for the examination.

Students can refer to the respective exercise to access the NCERT solutions Class 12 Chemistry Chapter 5. Students can also explore all types of educational content on the Extramarks website. Click on the respective links below to know more. 

Every small  detail that a student may require  to comprehend Surface Chemistry , a thorough explanation is clearly available  in Class 12 Chemistry Chapter 5 Notes.

Extramarks study materials on Surface Chemistry add to students’ learning skills and test their information recall, comprehension, analytical thinking, and problem-solving ability. It is the most comprehensive study material students can rely on to study, practice various questions, and prepare for Board and other entrance examinations.. A repository of resources are available at Extramarks for all the classes and subjects. Whether students are preparing for tests, exams, or any other competitive exam, they need not get anxious or stressed regarding the preparation or the notes, we have all the answers to your queries. You just need to register with Extramarks and we’ll make sure you get all the assistance you need and feel confident after using our resources.

NCERT Exemplar Class 12 Chemistry

The NCERT Class 12 Chemistry Exemplars are offered to help students succeed in their CBSE Class 12 examinations and graduate entrance examinations. NCERT Exemplar for Class 12 Chemistry contains solved Chapter-by-Chapter questions to assist students  with a quick review of the entire  syllabus and  complete their revision ahead of the  examinations.

 Given below is the list of additional study materials students can refer to and supplement their learning as per their need.

Students can obtain a more extensive understanding of the subject by practising problems from this guide. Extramarks subject expert team uses all of the questions to answer the questions by following the NCERT Books and  CBSE   guidelines (2021-2022).

Key Features of Class 12 Chemistry Chapter 5 Notes

Surface Chemistry is an important  chapter from the examination point of view.  For instance  IIT, JEE and NEET. Hence, while studying Chemistry, the best quality Notes and solutions  provided by Extramarks prove beneficial  for students, especially for last minute preparation. 

Class 12 Chemistry Chapter 5 Notes created by the Extramarks have the following advantages:

  •  Experienced subject matter experts prepare notes in accordance with the latest CBSE guidelines and exam pattern. 
  • These critical notes include a detailed explanation of each and every topic  in the chapter. 
  • They can be used to study right after school or revision  notes before  examinations. 
  • The Class 12 Chemistry Chapter 5 Notes are helpful for all types of Boards and various competitive examinations.

FAQs (Frequently Asked Questions)

1. Can the Chapter 5 Chemistry Class 12 Notes be used as Revision Notes?

Definitely, Chemistry Chapter 5 Class 12 Notes can be used as Revision Notes as it gives all the information students need to know and understand the  chapter.

2. How vital is Chemistry Chapter 5 for the CBSE Board Examination?

Chapter 5 is a vital topic which t may  not appear that easy. . It is crucial to note that Surface Chemistry has a lot of weightage, and students can expect questions from this chapter in the board examinations. Students may refer to Class 12 Chemistry Chapter 5 Notes and other study materials at the Extramarks website.

3. Does Class 12 Chemistry Chapter 5 Notes cover all topics of Surface Chemistry?

Surface Chemistry is a vast topic,  however, every concept is covered in the Extramarks Class 12 Chemistry Chapter 5 Notes.

4. How vital are Class 12 Chemistry Chapter 5 Notes?

The Class 12 Chapter 5 Chemistry Notes help students understand the basic concepts of solutions. It gives a detailed explanation of various laws and ideas mentioned in the chapter. 

5. What is the importance of catalyst poison in the Rosenmund reaction?

In the Rosenmund reaction, the aldehyde is produced by reducing acid halides with hydrogen gas in the presence of catalyst palladium. If a catalyst is not poisoned, the process is not stopped at the aldehyde level. 

6. What is the role of promoters or accelerators in Haber's process?

Promoters or accelerators enhance the catalyst activity in a reaction. In Haber’s method of manufacturing Ammonia, Nitrogen reacts with hydrogen to form ammonia. Nitrogen is significantly less reactive, and the yield of ammonia is very less. To increase the percentage yield of ammonia produced, NO is used as a promoter.

7. What is the significance of autocatalysis?

Autocatalysis is a self-catalysis process. In this reaction, one of the products formed acts as a catalyst and enhances the reaction rate.