Surface Chemistry Class 12: Adsorption, Colloids and Emulsions (School-Assessed)

What Surface Chemistry is about

In real board papers: not asked in any of the 8 board papers we hold (2023 to 2026). See the questions

Key concepts

Adsorption and how it differs from absorption

NCERT section CBSE reading material: Surface Chemistry 5.1 Adsorption; 5.1.1 Distinction between Adsorption and Absorption

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 surface material is the adsorbent. Finely divided solids have large surface areas and are good adsorbents: charcoal, silica gel, alumina gel, clay, colloids and powdered metals. Removing an adsorbed substance from a surface is desorption.

Examples from the reading material:

• Gases such as O₂, H₂, CO, Cl₂, NH₃ or SO₂ in a closed vessel with powdered charcoal: the gas pressure falls as molecules collect on the charcoal.
• Methylene blue solution shaken with animal charcoal becomes colourless.
• Raw sugar solution passed over animal charcoal loses its colour.
• Air becomes dry near silica gel.

Absorption is different: 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); broken open, it is white within. Water vapour is absorbed by anhydrous calcium chloride but adsorbed by silica gel. When both happen together, the term sorption is used.

Physisorption, chemisorption and uses of adsorption

NCERT section CBSE reading material: 5.1.2 Types of Adsorption; 5.1.3 Applications of Adsorption

Physical adsorption (physisorption): the gas is held on the surface by weak van der Waals forces.
Chemical adsorption (chemisorption): the gas is held by chemical bonds (covalent or ionic). It needs a high activation energy, so it is also called activated adsorption.

The two can occur together, and physisorption at low temperature may change into chemisorption as temperature rises: hydrogen is first held on nickel by van der Waals forces, then the molecules dissociate and the hydrogen atoms are chemisorbed.

Applications (reading material list):

• Producing a high vacuum (charcoal adsorbs the last traces of air).
• Gas masks with activated charcoal, used in coal mines to adsorb poisonous gases.
• Controlling humidity with silica and aluminium gels.
• Removing colour from solutions with animal charcoal.
• Heterogeneous catalysis: iron in ammonia manufacture, the contact process for H₂SO₄, nickel in hydrogenation of oils.
• Separating noble gases on coconut charcoal at different temperatures.
• Drugs that kill germs by being adsorbed on them.
• Froth floatation of sulphide ores (pine oil and a frothing agent).
• Adsorption indicators (eosin, fluorescein on silver halide precipitates).
• Chromatography.

Catalysis

NCERT section CBSE reading material: 5.2 Catalysis; 5.2.1 Homogeneous and Heterogeneous Catalysis; 5.2.2 Shape-Selective Catalysis by Zeolites

A catalyst speeds up a reaction and is unchanged in mass and composition at the end. Potassium chlorate decomposes slowly at 653–873 K, but with a little manganese dioxide it decomposes faster at 473–633 K, and the MnO₂ is unchanged.

• Promoters increase a catalyst's activity; poisons decrease it. In Haber's process, molybdenum is a promoter for the iron catalyst.
• Homogeneous catalysis: reactants, products and catalyst are in the same phase. Example: oxidation of SO₂ to SO₃ with oxides of nitrogen as catalyst in the lead chamber process (all gases).
• Heterogeneous catalysis: catalyst and reactants are in different phases. Example: oxidation of SO₂ to SO₃ over solid platinum.
• Shape-selective catalysis by zeolites: zeolites are microporous aluminosilicates with a honeycomb-like Al–O–Si framework. Their reactions depend on the size and shape of the reactant and product molecules and on the pores. They are used for cracking and isomerisation of hydrocarbons; the zeolite ZSM-5 converts alcohols directly into petrol by dehydrating them.

Colloids: definition and classification

NCERT section CBSE reading material: 5.3 Colloids; 5.4 Classification of Colloids

A colloid is a heterogeneous system in which one substance (the dispersed phase) is spread as very fine particles in another (the dispersion medium). The difference from a true solution is particle size: colloidal particles are 1 to 1000 nm (10⁻⁹ to 10⁻⁶ m), bigger than simple molecules but small enough to stay suspended. Suspensions (sand in water) have larger particles that settle.

Because they are so small, colloidal particles have an enormous surface area: a 1 cm cube (6 cm²) divided into 10¹² equal cubes has a total surface area of 60,000 cm² (6 m²).

Classification:

• By physical state of the dispersed phase and medium: eight types are possible (a gas in a gas is a true mixture, not a colloid), such as sols, gels, emulsions, foams and aerosols.
• By interaction with the medium:
• Lyophilic (liquid-loving) sols form directly by mixing substances like gum, gelatine, starch or rubber with a suitable liquid. If the medium is removed, the sol re-forms on remixing: reversible sols. They are quite stable.
• Lyophobic (liquid-hating) sols, such as those of metals and their sulphides, form only by special methods. They are easily coagulated by small amounts of electrolyte, by heating or shaking, and do not re-form on adding the medium: irreversible sols. They need stabilising agents.

With water as the medium, the terms are hydrophilic and hydrophobic.

Charge on colloidal particles, electrophoresis and coagulation

NCERT section CBSE reading material: 5.4.2 (iii) Charge on colloidal particles; Electrophoresis; Coagulation; Protection of colloids

Colloidal particles always carry an electric charge, the same sign on every particle in a given sol (positive or negative).

Electrophoresis: when a potential is applied across platinum electrodes dipping in a sol, the particles move: positive particles to the cathode, negative ones to the anode. If the particles are prevented from moving, the dispersion medium moves instead: electroosmosis.

Coagulation (precipitation): the charge keeps lyophobic particles apart. If it is removed, they aggregate and settle. Ways to coagulate a lyophobic sol:
1. Electrophoresis (particles are discharged at the electrodes).
2. Mixing two oppositely charged sols in nearly equal amounts, such as hydrated ferric oxide (positive) and arsenious sulphide (negative): mutual coagulation.
3. Boiling (more collisions disturb the adsorbed layer).
4. Persistent dialysis (removes the stabilising traces of electrolyte).
5. Adding electrolytes: ions of opposite charge neutralise the particles.

Hardy–Schulze rule: the greater the valence of the coagulating (flocculating) ion, the greater its power to cause precipitation.

• For a negative sol: Al³⁺ > Ba²⁺ > Na⁺
• For a positive sol: [Fe(CN)₆]⁴⁻ > PO₄³⁻ > SO₄²⁻ > Cl⁻

The coagulating value is the minimum concentration of electrolyte (millimoles per litre) needed to precipitate a sol in two hours; the smaller it is, the greater the coagulating power.

Lyophilic sols are stable because of both charge and solvation. To coagulate them, add an electrolyte and a suitable solvent such as alcohol or acetone (which removes the water sheath). Lyophilic colloids can coat lyophobic particles and protect them from electrolytes: they act as protective colloids.

Emulsions and colloids around us

NCERT section CBSE reading material: 5.5 Emulsions; 5.6 Colloids Around Us

An emulsion is a liquid–liquid colloid: fine droplets of one liquid spread through another, usually with water as one liquid. Two types:

• Oil in water (O/W): water is the medium; examples are milk (liquid fat in water) and vanishing cream.
• Water in oil (W/O): oil is the medium; the reading material gives butter and cream as examples.

Emulsions tend to separate, so an emulsifying agent is added; it forms a film between the droplets and the medium. For O/W emulsions: proteins, gums, natural and synthetic soaps; for W/O: heavy metal salts of fatty acids, long-chain alcohols, lampblack. Emulsions can be diluted with any amount of the dispersion medium, show Brownian movement and the Tyndall effect, and can be broken by heating, freezing or centrifuging.

Colloids around us:

• The blue colour of the sky: dust particles and water in air scatter blue light.
• Fog, mist and clouds are aerosols of water droplets; rain can be caused artificially by spraying an oppositely charged sol or electrified sand.
• Foods such as milk, butter, halwa, ice cream and fruit juices.
• Blood is a colloidal solution; alum and ferric chloride stop bleeding by coagulating it.
• Fertile soils are colloidal, with humus as a protective colloid.
• Delta formation: electrolytes in sea water coagulate the colloidal clay in river water, depositing it.

Chemistry in real life: Surface Chemistry

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.

Topics in this chapter

• Adsorption and Catalysis
• Colloids, Coagulation and Emulsions

Surface Chemistry formula sheet

Colloidal particle size

$1\text{nm}-1000\text{nm}(10^{-9}-10^{-6}\text{m})$

Smaller: true solution; larger: suspension.

Hardy–Schulze rule (negative sol)

$A{l}^{3+}>B{a}^{2+}>N{a}^{+}$

Coagulating power rises with the valence of the opposite ion.

Hardy–Schulze rule (positive sol)

$[Fe(CN{)}_6{]}^{4-}>P{O}_4^{3-}>S{O}_4^{2-}>C{l}^{-}$

Anions coagulate positive sols.

Physisorption versus chemisorption

$\text{van der Waals forces (physisorption)}\text{vs}\text{chemical bonds (chemisorption)}$

Chemisorption needs high activation energy (activated adsorption).

Emulsion types

$\text{O/W: milk, vanishing cream};\text{W/O: butter, cream}$

As listed in the CBSE reading material.

Tricks to remember Surface Chemistry

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

• Physi = Feeble, Chemi = Chemical bond

  Physisorption uses feeble van der Waals forces; chemisorption makes real chemical bonds and needs activation energy.

  Use it when: Comparing the two types of adsorption.

• Lyo-PHILIC = Loves the liquid = Reversible

  Philic sols form on their own and come back after drying; phobic sols are fussy, need special methods and do not come back.

  Use it when: Lyophilic vs lyophobic questions.

• Hardy–Schulze: higher charge, harder hit

  Al³⁺ beats Ba²⁺ beats Na⁺ on a negative sol; [Fe(CN)₆]⁴⁻ beats PO₄³⁻ beats SO₄²⁻ beats Cl⁻ on a positive sol.

  Use it when: Ordering coagulating power.

• Milk is O in W

  Milk: oil (fat) in water. Butter: the reverse (as in the reading material).

  Use it when: Emulsion types.

Surface Chemistry: previous year questions

• Not asked in any of the 8 board papers we hold (2023 to 2026).

Counted from the real papers on our previous year papers page, not estimated. Average marks are over every board paper we hold, with an OR pair counted once. Chapter tags are ours.

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. 1.Define adsorbate and adsorbent.Show answer

   The adsorbate is the substance that collects on the surface; the adsorbent is the material on whose surface it collects.

2. 2.How does silica gel differ from anhydrous CaCl₂ in taking up water vapour?Show answer

   Silica gel adsorbs water (surface); anhydrous CaCl₂ absorbs it (bulk).

3. 3.Why is chemisorption called activated adsorption?Show answer

   It involves chemical bond formation and needs a high activation energy.

4. 4.What is the promoter in Haber's process?Show answer

   Molybdenum, which promotes the iron catalyst.

5. 5.Name the zeolite that converts alcohols into petrol.Show answer

   ZSM-5.

6. 6.What is the size range of colloidal particles?Show answer

   1 to 1000 nm (10⁻⁹ to 10⁻⁶ m).

7. 7.What is electrophoresis?Show answer

   The movement of charged colloidal particles towards an electrode under an applied potential.

8. 8.State the Hardy–Schulze rule.Show answer

   The greater the valence of the coagulating ion, the greater its power to precipitate the sol.

9. 9.What is a protective colloid?Show answer

   A lyophilic colloid that forms a layer around lyophobic particles and protects them from coagulation by electrolytes.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you give three examples of adsorption from the reading material?
• ☐Can you explain the chalk-and-ink observation?
• ☐Can you give two differences between physisorption and chemisorption?
• ☐Can you give one example each of homogeneous and heterogeneous catalysis?
• ☐Can you explain why lyophilic sols are more stable than lyophobic sols?
• ☐Can you list five ways to coagulate a lyophobic sol?
• ☐Can you arrange ions by coagulating power for a positive and a negative sol?
• ☐Can you name the two types of emulsions with examples and emulsifying agents?

Surface Chemistry: frequently asked questions