Surface Chemistry - Notes, Topics, Formula, Books, FAQs

Surface chemistry explores what happens at the interface between two phases—like solid–liquid or liquid–gas. It explains why adsorption (where particles cling to a surface) and colloids (tiny particles suspended in another substance) behave differently than substances in the bulk.

• Adsorption occurs because surface molecules experience unbalanced attractions, so they pull in nearby particles.

• There are two kinds:

  • Physisorption: Weak forces (like van der Waals) hold multiple layers, reversible, and common at low temperatures.

  • Chemisorption: Strong chemical bonds form a single layer, often needing activation energy, and is usually irreversible.

Adsorption depends on factors like surface area, pressure, temperature, and the nature of the substances involved. It’s a key part of processes like catalysis, coatings, water purification, gas separation, and more.

This unit will introduce:

• How adsorption works on a molecular level,

• The difference between physisorption and chemisorption,

• Factors influencing these processes,

• And why they’re so important in science and industry.

Important Topics of Surface Chemistry

Surface chemistry covers important areas like adsorption (how molecules stick to surfaces), colloids and emulsions (tiny particles or liquids dispersed in another medium), catalysis (how reactions speed up on surfaces), and surface tension, wetting and diffusion (how liquids interact with solid surfaces). These topics help explain behavior at interfaces that differs from the bulk material.

Surface Chemistry

Surface chemistry studies phenomena occurring at the interfaces of phases, such as solid-liquid, liquid-gas, or solid-gas. It deals with the processes like adsorption, catalysis, and the behaviour of colloidal particles, which are critical in industries and biological systems.

Difference Between Adsorption and Absorption

Adsorption is the accumulation of molecules on a surface while absorption is the uniform distribution of molecules throughout a substance. Adsorption is a surface phenomenon, whereas absorption occurs throughout the bulk of a material.

Physisorption and Chemisorption

Physisorption involves weak van der Waals forces and occurs at low temperatures, whereas chemisorption involves strong chemical bonds and is specific to the nature of the adsorbate and adsorbent. Chemisorption usually requires activation energy and higher temperatures.

Adsorption Isotherm

An adsorption isotherm depicts the relationship between the amount of adsorbate on the surface and the pressure or concentration of the adsorbate at constant temperature. It helps in understanding adsorption mechanisms.

Freundlich Isotherm

The Freundlich isotherm describes adsorption on heterogeneous surfaces, expressing that adsorption increases with pressure but levels off at high pressures.

Adsorption

Adsorption is the process where molecules adhere to the surface of a solid or liquid. It is widely used in water purification, gas masks, and heterogeneous catalysis. Adsorption depends on factors like temperature, pressure, and surface area.

Catalysis

Catalysis involves substances called catalysts that alter the rate of a chemical reaction without being consumed. Heterogeneous catalysis occurs on the surface of a solid, while homogeneous catalysis occurs in the same phase as the reactants.

Colloids

Colloids are mixtures where particles (1–1000 nm) are dispersed in a medium. Colloids exhibit unique properties like the Tyndall effect and Brownian motion, making them essential in industries like pharmaceuticals, food, and cosmetics.

Preparation and Purification of Colloidal Solutions

Colloidal solutions can be prepared by methods like dispersion (e.g., milling, peptization) or condensation (e.g., chemical reactions). Purification techniques include dialysis, ultrafiltration, and centrifugation to remove impurities.

This Story also Contains

1. Important Topics of Surface Chemistry
2. Overview of the Chapter Surface Chemistry
3. Formulae Chart For Surface Chemistry:
4. How to Prepare for Surface Chemistry?
5. Role of Surface chemistry in various chemical processes
6. Some important tips to prepare surface chemistry:
7. Books Prescribed:
8. Important PYQs
9. Conclusion:

Overview of the Chapter Surface Chemistry

It has been known that the surface of a liquid is in a state of unsaturation due to the unbalanced or residual forces which act along the surface of a liquid. Similar to it, the surface of a solid may also have residual forces. Thus, the surface of a solid has a tendency to attract and to retain molecules of gas or liquids with which such surfaces come in contact. This phenomenon of surfaces is termed as adsorption. Differences between Adsorption and Absorption is that Adsorption is a surface phenomenon whereas absorption is a bulk phenomenon in which the substance is symmetrically distributed throughout the body of a solid or liquid to form a solution or a compound.

In the case of adsorption, the equilibrium is easily attained in a very short interval of time whereas in absorption the equilibrium takes place slowly.

According to Freundlich adsorption isotherm:

Taking the logarithm of the above equation we get :

Examples of adsorption and absorption

(i) Water vapour is absorbed by anhydrous calcium chloride while it is adsorbed by silica gel.

(ii) Ammonia is adsorbed by charcoal while it is absorbed by water to form ammonium hydroxide.

NH₃+ H₂O $\rightarrow$ NH₄OH

(iii) Decolourisation of sugar solution by activated charcoal is another example of adsorption. In this example, charcoal adsorbs the colouring material and thus decolourises the solution.

Following them, one will get to know about different types of adsorption, physical adsorption and chemical adsorption.

After studying about adsorption the candidate will learn about catalyst. Catalyst is a substance which can change the speed of chemical reaction without undergoing any change in itself whether be it mass or chemical composition at the end of the reaction. This phenomenon is known as catalysis. Catalyst can be broadly divided into two categories:

1. Homogeneous catalysis: Here reactants and catalyst are in the same phase (liquid or gas)

2. Heterogeneous catalysis: Here reactants and catalyst are in a different phase.

Talking about the important features of solid catalysts the aspirant will learn about activity and selectivity. By activity of the catalyst, we mean its capacity to increase the speed of chemical reaction. Dependence of activity is on the strength of chemisorption. By selectivity of a catalyst we mean its ability to form particular products excluding others. Moving further in this chapter the candidate will learn about the classification of colloids on the basis of:

1. Size of particles of solute

2. The physical state of the dispersed phase and the dispersion medium

3. Nature of interaction between the dispersed phase and the dispersion medium

4. Types of particles of the dispersed phase

In this part of surface chemistry, the candidate will know about micelle formation, cleansing action of soaps, different methods of preparation of colloids, methods of purification of colloidal solutions and their respective properties which include

• Electrophoresis: Electrophoresis is done to confirm the charge on colloid particles. On applying the electric potential across the electrodes, colloid particles move towards one of the electrodes as shown in the figure given below:

• Coagulation or Precipitation(learning about Hardy-Schulze rule): Coagulation is the process when the charge is removed from the colloid particles and these particles come closer to each other and precipitate or coagulate.

• Brownian movement: This is the movement of colloidal particles which is observed under a powerful microscope. In this motion, colloidal particles move in zig-zag motion in solution.

• Charge on colloidal particles: Colloidal particles carry an electric charge. This charge can be negative or positive but it is always the same for all the particles.

Lastly, one will learn about Emulsions. The emulsion is a colloidal system consisting of immiscible liquids. e.g. milk is an emulsion in which particles of liquid fat are dispersed in water. In common, however, one of the liquids is water and the other, and oily substance insoluble in it. An emulsion is a heterogeneous system consisting of more than one immiscible liquids dispersed in one another in the form of droplets. Such systems possess extremely small stability which is made by the addition of surface-active agents, finely divided solids, etc.

Formulae Chart For Surface Chemistry:

1. Freundlich adsorption isotherm:

2. Langmuir adsorption isotherm: $\Theta =\, \frac{{K}'P}{1+KP} \, or\, \frac{x}{m}= \, \frac{aP}{1+bP} where\, a ,b\, are \, constant.$

3. $\frac{x}{m}= \, KC^{\frac{1}{n}};\, C= concentration\, of\, solute\, in\, solution$

4. Zeta potential, $Z= \frac{4\pi \eta \mu }{D}$

How to Prepare for Surface Chemistry?

To excel in surface chemistry, focus on high-value topics like adsorption (including isotherms), catalysis, and colloids. Master colloid essentials—types, preparation methods, stability factors (like zeta potential), and purification techniques such as electrophoresis and dialysis. Use NCERT as your base, then strengthen your concepts by solving past JEE questions, mock tests, and daily practice papers.

Role of Surface chemistry in various chemical processes

• In chemical industries, the principle of catalysis is used for the preparation of chemicals.

• Drug selection by doctors like syrup or tablets is based on the reactivity of medicine which depends on surface properties.

• In electrical industries, the use in the surface and interface of microchips used in computers.

• Protein adsorption on the walls of the intestine.

• Adsorption surface phenomena are used for vacuum formation.

Some important tips to prepare surface chemistry:

1. The candidate must spend ample amount of time in understanding and learning the concepts that too from NCERT.

2. An aspirant must practice theoretical problems from this chapter

3. One should practice more than one correct type questions from this chapter.

4. Various chemical phenomena like electrophoresis must be understood with utmost sincerity.

Books Prescribed:

First, you must finish the class XII NCERT textbook and solve each and every example and unsolved question given in it. Then for advanced level preparation like JEE and NEET, you must follow R.C. Mukherjee and P. Bahadur. You must definitely solve the previous year papers. Meanwhile, in the preparation, you must continuously write the mock tests for the depth of knowledge. Our platform will help you to provide with the variety of questions for deeper knowledge with the help of videos, articles and mock tests.

Also read,

Important PYQs

Question 1: Lyophilic sols are more stable than lyophobic sols because:

1) There is a strong electrostatic repulsion between the negatively charged colloidal particles.

2) The colloidal particles are solvated.

3) The colloidal particles have no charge.

4) The colloidal particles have a positive charge.

Solution:

Lyophilic sols are more stale than lyophilic sols due to their more effective solvation by the solvent molecules.

Hence, the answer is the option (2).

Question 2: Which one of the following is correct for the adsorption of a gas at a given temperature on a solid surface?

1)

2)

3)

4)

Solution:

As we have learnt,

Adsorption proceeds with a release of energy due to the formation of new intermolecular forces and proceeds with a decrease in the entropy of the system.

Thus,

Hence, the answer is the option (4).

Practice More Questions From The Link Given Below:

Conclusion:

Surface chemistry explores reactions at phase boundaries—solid/liquid, liquid/gas—focusing on adsorption, catalysis, colloids, surface tension, and emulsions. Adsorption happens when molecules stick to surfaces, either via weak van der Waals forces (physisorption) or strong chemical bonds (chemisorption). Core applications include industrial catalysis, water purification, coatings, and drug delivery. For JEE Advanced, this topic offers scoring potential: master adsorption isotherms, colloid types/preparation, surfactants, and surface tension theory to tackle high-yield numericals and past-year questions efficiently.