ChemistryCBSE Class 12 · Surface Chemistry

Adsorption and Catalysis Class 12 Chemistry: Types, Uses and Zeolites

Updated 18 October 2026 · Fact-checked

Adsorption is the collection of molecules on the surface of a solid or liquid, unlike absorption, which spreads through the bulk. Physisorption uses weak van der Waals forces; chemisorption uses chemical bonds and needs activation energy. Catalysts speed up reactions without change in mass and composition; they may be homogeneous or heterogeneous, and zeolites give shape-selective catalysis.

Understand Adsorption and Catalysis

Adsorption. Adsorption is the collection of molecules at the surface of a solid or liquid rather than in its bulk. The substance that collects is the adsorbate; the material of the surface is the adsorbent. Finely divided solids with large surface area are good adsorbents: charcoal, silica gel, alumina gel, clay and powdered metals. Removal of an adsorbed substance is desorption. Examples: gases like O2, H2, CO, Cl2, NH3 or SO2 in a closed vessel with powdered charcoal show a fall in pressure; methylene blue shaken with animal charcoal becomes colourless; raw sugar solution loses colour over animal charcoal; air becomes dry near silica gel.

Adsorption versus absorption. In absorption the substance spreads uniformly through the bulk. A chalk stick dipped in ink keeps the colour on its surface (adsorption) while the solvent soaks inside (absorption). Water vapour is absorbed by anhydrous calcium chloride but adsorbed by silica gel. When both occur together the term sorption is used.

Physisorption and chemisorption.

Physical adsorption: the gas is held by weak van der Waals forces.
Chemical adsorption: the gas is held by chemical bonds (covalent or ionic) and needs high activation energy, so it is called activated adsorption.
Both can occur together, and physisorption at low temperature may change to chemisorption as temperature rises; hydrogen is first held on nickel by van der Waals forces, then the molecules dissociate and hydrogen atoms are chemisorbed.

Applications. High vacuum (charcoal adsorbs last traces of air); gas masks with activated charcoal in coal mines; humidity control with silica and aluminium gels; removing colour with animal charcoal; heterogeneous catalysis (iron in ammonia manufacture, contact process, nickel in hydrogenation of oils); separating noble gases on coconut charcoal; drugs adsorbed on germs; froth floatation of sulphide ores; adsorption indicators such as eosin and fluorescein; chromatography.

Catalysis. A catalyst speeds up a reaction and is unchanged in mass and composition at the end. Potassium chlorate decomposes slowly at 653 to 873 K, but with a little manganese dioxide it decomposes faster at 473 to 633 K, and MnO2 is unchanged. Promoters increase a catalyst's activity (molybdenum for iron in Haber's process); poisons decrease it.

Homogeneous catalysis: reactants, products and catalyst are in the same phase, for example oxidation of SO2 to SO3 with oxides of nitrogen in the lead chamber process.
Heterogeneous catalysis: catalyst in a different phase, for example SO2 to SO3 over solid platinum.
Shape-selective catalysis by zeolites: zeolites are microporous aluminosilicates with a honeycomb-like Al-O-Si framework. Their action depends on the size and shape of reactant and product molecules and on pore size. Used for cracking and isomerisation of hydrocarbons; ZSM-5 converts alcohols directly into petrol by dehydration.

Adsorption and Catalysis: formulas and key facts

Physisorption vs chemisorption: Physisorption: van der Waals forces, weak, low temperature | Chemisorption: chemical bonds, needs activation energy, activated adsorption; Can occur together.
Types of catalysis: Homogeneous same phase
Heterogeneous different phase
Shape-selective zeolites, pore size decides; Lead chamber is homogeneous; platinum contact is heterogeneous.
Catalyst terms: Promoter increases activity
Poison decreases activity
Catalyst unchanged in mass and composition; Molybdenum promotes iron in Haber's process.

Homogeneous	same phase
Heterogeneous	different phase
Shape-selective	zeolites, pore size decides

How to solve Adsorption and Catalysis questions

This topic is formative (school) assessment only in 2026-27, but Class 12 school tests ask for distinctions, examples and reasoning. Use fixed heads.

1For adsorption versus absorption, state where the substance goes: surface or bulk, and give chalk-in-ink or CaCl2 versus silica gel.
2For physisorption versus chemisorption, compare forces, activation energy and the nickel-hydrogen example.
3For catalysis type, check the phase of catalyst and reactants.
4For zeolites, mention the Al-O-Si framework, pore size and ZSM-5.
5Close with one application.

Adsorption and Catalysis in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Adsorption in a water filter

Suppose a household filter contains granules of activated charcoal. A coloured vegetable dye in the water becomes pale after passing through.

Name the phenomenon and explain why finely divided charcoal works better than a lump.

Try it first, then show the solutionSolution

Dye molecules collect on the charcoal surface: adsorption.
Fine division gives a larger surface area.
More surface means more adsorption.

Answer: The dye is adsorbed on the surface of the charcoal. Finely divided charcoal has a much larger surface area than a lump, so more dye molecules can collect on it.

Memory hook: More surface, more adsorption.

Gas mask

Suppose a mine rescuer wears a mask containing activated charcoal.

Explain how the mask protects and whether the bonding is physical or chemical.

Try it first, then show the solutionSolution

Poisonous gases collect on the charcoal surface.
Weak forces are the case in physisorption, strong bonds in chemisorption.

Answer: The mask adsorbs poisonous gases on the surface of activated charcoal, so the air breathed is cleaner. Whether the adsorption is physical (van der Waals forces) or chemical (bonds) depends on the gas and temperature.

Memory hook: Charcoal traps on its surface.

Tricks to remember Adsorption and Catalysis

Short memory aids. Say each one aloud once, then use it on a question the same day.

AD-sorb on the Add side
Adsorption adds to the surface; absorption goes in the bulk.
Use it when: Distinction.
Physi = weak, Chemi = bond
Physisorption: van der Waals; chemisorption: chemical bonds.
Use it when: Distinction.
Homo same, Hetero different
Homogeneous catalysis: same phase; heterogeneous: different phase.
Use it when: Catalysis types.

Adsorption and Catalysis: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Adsorption or absorption? · 1 mark · MCQBoard level

Water vapour is absorbed by anhydrous calcium chloride but adsorbed by:
(a) charcoal
(b) silica gel
(c) chalk
(d) ink

Try it first, then show the solutionSolution

Silica gel holds water vapour on its surface; the reading material names silica gel (1 mark).

Answer: (b) silica gel

Example 2: Physisorption vs chemisorption (2 marks) · 2 marks · Very short answerBoard level

Distinguish physisorption from chemisorption on the nature of forces and activation energy. (2 marks)

Try it first, then show the solutionSolution

Physisorption: weak van der Waals forces, no high activation energy (1 mark).
Chemisorption: chemical bonds, needs high activation energy, hence activated adsorption (1 mark).

Answer: Physisorption is held by weak van der Waals forces, while chemisorption is held by chemical bonds and needs a high activation energy, so it is called activated adsorption.

Example 3: Hydrogen on nickel (4 marks) · 4 marks · Short answerHarder board level

Explain why hydrogen on nickel shows both types of adsorption as temperature rises. (4 marks)

Try it first, then show the solutionSolution

At low temperature hydrogen molecules are held by van der Waals forces: physisorption (1 mark).
With rise in temperature, the activation energy for chemisorption is supplied (1 mark).
H2 molecules dissociate into hydrogen atoms (1 mark).
These atoms are bonded chemically to nickel: chemisorption (1 mark).

Answer: At low temperature hydrogen is held on nickel by van der Waals forces (physisorption). As temperature rises, the activation energy needed for chemisorption is supplied, the H2 molecules dissociate, and the hydrogen atoms are chemically bonded to nickel (chemisorption).

Example 4: Catalysis types (5 marks) · 5 marks · Long answerHarder board level

Classify the oxidation of SO2 to SO3 as homogeneous or heterogeneous in (i) the lead chamber process using oxides of nitrogen and (ii) the use of solid platinum. Explain, and state what a promoter and a poison do. (5 marks)

Try it first, then show the solutionSolution

(i) All substances are gases, same phase: homogeneous (1 mark).
(ii) Platinum is solid while SO2 and O2 are gases: heterogeneous (1 mark).
Reason must refer to phase of catalyst and reactants (1 mark).
Promoter increases catalytic activity; example molybdenum with iron in Haber's process (1 mark).
Poison decreases catalytic activity (1 mark).

Answer: (i) With oxides of nitrogen all reactants and the catalyst are gases in the same phase, so it is homogeneous. (ii) With solid platinum the catalyst and the gaseous reactants are in different phases, so it is heterogeneous. A promoter increases the activity of a catalyst, as molybdenum does for iron in Haber's process, while a poison decreases it.

Example 5: Zeolite reasoning · 4 marks · Case-basedHOTS / competency

Zeolite ZSM-5 converts alcohols directly into petrol. (i) What is a zeolite? (1) (ii) Why is its catalysis called shape-selective? (2) (iii) Name the process by which alcohols are converted. (1)

Try it first, then show the solutionSolution

(i) A microporous aluminosilicate with a honeycomb-like Al-O-Si framework (1 mark).
(ii) The reaction depends on the size and shape of reactant and product molecules and on the pores (2 marks).
(iii) Dehydration (1 mark).

Answer: (i) A microporous aluminosilicate with a honeycomb-like Al-O-Si framework. (ii) The reaction depends on the size and shape of reactant and product molecules and on the pores of the zeolite. (iii) Dehydration of the alcohols.

Where students lose marks in Adsorption and Catalysis

The slips examiners see most often, and what to write instead.

Mistake: Reversing the two adsorption types
Names are similar.
Wrong working
Physisorption involves chemical bonds.
Right working
Physisorption: van der Waals forces; chemisorption: chemical bonds.
Do this: Chemi = chemical bond.
Usually costs: 1 mark.
Mistake: No phase reason in catalysis
Only the example given.
Wrong working
Platinum catalysis is heterogeneous.
Right working
Heterogeneous because solid platinum and gaseous reactants are in different phases.
Do this: Always state the phases.
Usually costs: 1 mark.
Mistake: Calling a catalyst a reactant
Loose wording.
Wrong working
MnO2 is consumed.
Right working
MnO2 is unchanged in mass and composition at the end.
Do this: Say unchanged.
Usually costs: 1 mark.

Exam tips

Surface Chemistry is formative (school) assessment only in 2026-27.
Give one example with every distinction.
Name the phase of catalyst and reactants when classifying catalysis.

Practice questions on Adsorption and Catalysis

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Which form of the same mass of charcoal adsorbs the most gas?
(a) one large lump
(b) finely powdered charcoal
(c) a pressed block
(d) charcoal soaked in water
Hint
Surface area.
Answer
(b) finely powdered charcoal
Q2 · 2 marks · Very short answerBoard level
Give two applications of adsorption. (2 marks)
Hint
Masks, humidity.
Answer
Any two: gas masks with activated charcoal; controlling humidity with silica or alumina gel; removing colour with animal charcoal; chromatography; froth floatation.
Q3 · 4 marks · Short answerHarder board level
A student shakes methylene blue solution with animal charcoal, filters it and finds the filtrate colourless. Explain, state the adsorbent and adsorbate, and say what would happen if the charcoal were replaced by the same mass of a lump of charcoal. (4 marks)
Hint
Surface area.
Answer
The dye molecules collect on the surface of the charcoal, which is the adsorbent, and the dye is the adsorbate. A lump of the same mass has less surface area, so less dye is adsorbed and the filtrate stays coloured.
Q4 · 5 marks · Long answerHarder board level
Compare adsorption and absorption using chalk and ink, and calcium chloride and silica gel, and explain the term sorption. (5 marks)
Hint
Surface versus bulk.
Answer
In adsorption the substance collects on the surface, in absorption it spreads uniformly through the bulk. A chalk stick dipped in ink retains colour on its surface (adsorption) while the solvent soaks inside (absorption); broken open, the chalk is white within. Water vapour is absorbed by anhydrous CaCl2 but adsorbed by silica gel. When both processes occur together, it is called sorption.
Q5 · 1 mark · Assertion-reasonHOTS / competency
Assertion (A): Chemisorption needs a high activation energy.
Reason (R): Physisorption involves chemical bonds.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Hint
Which type uses bonds?
Answer
(c) A is true, but R is false. Physisorption involves van der Waals forces.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What is desorption?Show answer
Removal of an adsorbed substance from the surface.
2.What does a promoter do?Show answer
Increases the activity of a catalyst.
3.Name a shape-selective zeolite catalyst.Show answer
ZSM-5.
4.Which gas is adsorbed by charcoal in gas masks?Show answer
Poisonous gases in the air.

Adsorption and Catalysis: frequently asked questions

What is the difference between adsorption and absorption?

Adsorption occurs on the surface; absorption spreads through the bulk.

What is activated adsorption?

Another name for chemisorption, since it needs a high activation energy.

What are zeolites?

Microporous aluminosilicates with a honeycomb-like Al-O-Si framework.