ChemistryCBSE Class 12 · Surface Chemistry

Colloids, Coagulation and Emulsions Class 12 Chemistry

Q: What are lyophilic sols?

Liquid-loving sols, formed directly, reversible and stable.

Q: What is an emulsion?

A liquid-liquid colloid, usually of oil in water or water in oil.

Updated 18 October 2026 · Fact-checked

A colloid is a heterogeneous system whose dispersed particles measure 1 to 1000 nm. Lyophilic sols are stable and reversible; lyophobic sols are irreversible and easily coagulated. Colloidal particles carry charge, shown by electrophoresis. By the Hardy-Schulze rule, the higher the valence of the coagulating ion, the greater its power. Emulsions are liquid-liquid colloids, oil in water or water in oil.

For a negative sol, the higher the charge on the added positive ion, the greater its coagulating power: Al³⁺ is greater than Ba²⁺, which is greater than Na⁺.

Illustration drawn for QuestionBuddy.

Understand Colloids, Coagulation and Emulsions

Colloids. A colloid is a heterogeneous system in which a dispersed phase is spread as very fine particles in a dispersion medium. The particles are 1 to 1000 nm (10^-9 to 10^-6 m): bigger than simple molecules, small enough to stay suspended. Suspensions, such as sand in water, have larger particles that settle. The small size gives a huge surface area: a 1 cm cube (surface 6 cm^2) divided into 10^12 equal cubes has a total surface area of 60,000 cm^2 (6 m^2).

Classification. By physical state of dispersed phase and medium there are eight types (a gas in a gas is a true mixture), such as sols, gels, emulsions, foams and aerosols. By interaction with the medium:

Lyophilic (liquid-loving) sols, such as gum, gelatine, starch or rubber, form directly on mixing with a suitable liquid. They are reversible and quite stable.
Lyophobic (liquid-hating) sols, such as metals and their sulphides, need special methods, are easily coagulated by small amounts of electrolyte, heating or shaking, and are irreversible. They need stabilising agents.

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

Charge and electrophoresis. Colloidal particles in a sol carry the same sign of charge. When a potential is applied across platinum electrodes dipping in a sol, positive particles move to the cathode and negative to the anode: electrophoresis. If the particles are prevented from moving, the medium moves: electroosmosis.

Coagulation. The charge keeps lyophobic particles apart; if it is removed, they aggregate and settle. Methods: electrophoresis (particles discharged at the electrodes), mixing two oppositely charged sols in nearly equal amounts (mutual coagulation, for example hydrated ferric oxide and arsenious sulphide), boiling, persistent dialysis and adding electrolytes.

Hardy-Schulze rule. The greater the valence of the coagulating ion, the greater its power to cause precipitation. For a negative sol: Al3+ > Ba2+ > Na+. For a positive sol: [Fe(CN)6]4- > PO4 3- > SO4 2- > Cl-. The coagulation value is the minimum concentration of electrolyte in millimoles per litre needed to precipitate a sol in two hours; the smaller it is, the greater the coagulating power.

Lyophilic stability and protection. Lyophilic sols are stable because of charge and solvation; adding an electrolyte with a solvent such as alcohol or acetone removes the water sheath. Lyophilic colloids coat lyophobic particles and protect them from electrolytes: protective colloids.

Emulsions. An emulsion is a liquid-liquid colloid, usually with water as one liquid. In oil in water (O/W) emulsions, water is the medium (milk, vanishing cream). In water in oil (W/O) emulsions, oil is the medium (butter and cream, as given in the reading material). An emulsifying agent forms a film between droplets and medium: proteins, gums and soaps for O/W; heavy metal salts of fatty acids, long-chain alcohols and lampblack for W/O. Emulsions show Brownian movement and the Tyndall effect and can be broken by heating, freezing or centrifuging.

Colloids around us. Blue colour of the sky from scattering by dust and water; fog, mist and clouds as aerosols; milk, butter, ice cream; blood (alum or ferric chloride stops bleeding by coagulating it); fertile soil with humus as a protective colloid; delta formation where sea-water electrolytes coagulate clay from rivers.

Colloids, Coagulation and Emulsions: formulas and key facts

Colloid size: $1 nm t o 1000 nm ∣1 0^{-} 9 m t o 1 0^{-} 6 m$; Between true solution and suspension.
Hardy-Schulze rule: Negative sol: Al3+ > Ba2+ > Na+ | Positive sol: [Fe(CN)6]4- > PO4 3- > SO4 2- > Cl-; Higher valence, greater coagulating power, smaller coagulation value.
Lyophilic versus lyophobic: Lyophilic: reversible, stable, direct preparation | Lyophobic: irreversible, easily coagulated, special methods; Examples: gelatine; metal sulphides.

How to solve Colloids, Coagulation and Emulsions questions

Surface Chemistry is formative (school) assessment only in 2026-27. Expect distinctions, rule-based ordering and short numerical reasoning.

1Identify the sign of the sol's charge before choosing the coagulating ion.
2Pick the opposite-charge ion with the highest valence for the best coagulation.
3Remember: higher valence means a smaller coagulation value.
4For surface area problems, find the edge of the small cubes first, then the total area.
5For emulsions, state which liquid is the medium.

Colloids, Coagulation and Emulsions 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.

Delta formation

Suppose a muddy river carrying negatively charged clay particles flows into the sea, whose water contains dissolved salts.

Explain why silt gets deposited near the river mouth.

Try it first, then show the solutionSolution

River water carries colloidal clay.
Sea water has electrolytes.
Positive ions neutralise the charge, so clay coagulates and settles.

Answer: The electrolytes in sea water neutralise the charge on the colloidal clay particles, so they coagulate and settle, forming a delta.

Memory hook: Salt breaks the sol.

Stopping bleeding

Suppose a footballer scrapes a knee and the first-aid helper presses a ferric chloride-soaked cotton pad on the wound.

Why does the oozing blood clot quickly?

Try it first, then show the solutionSolution

Blood is a colloidal solution.
Ferric chloride (or alum) supplies ions that neutralise the charge.
The blood coagulates and seals the cut.

Answer: Blood is a colloidal solution. Ferric chloride supplies an electrolyte whose ions neutralise the charge on the colloidal particles, so the blood coagulates and the bleeding stops.

Memory hook: Ferric chloride or alum coagulates blood.

Tricks to remember Colloids, Coagulation and Emulsions

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

Lyo-philic = loves liquid
Philic loves; phobic hates the liquid.
Use it when: Naming.
Opposite charge, high valence
Pick the ion with opposite charge and highest valence.
Use it when: Hardy-Schulze.
O/W = milk, W/O = butter
Medium is the second word: oil in WATER (milk).
Use it when: Emulsions.

Colloids, Coagulation and Emulsions: solved examples

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

Example 1: Size range · 1 mark · MCQBoard level

A particle of diameter 5 x 10^-8 m dispersed in water belongs to which class?
(a) true solution
(b) colloid
(c) suspension
(d) none of these

Try it first, then show the solutionSolution

5 x 10^-8 m = 50 nm, which lies within 1 to 1000 nm, so it is colloidal (1 mark).

Answer: (b) colloid

Example 2: Direction of electrophoresis (2 marks) · 2 marks · Very short answerBoard level

In electrophoresis, towards which electrode do positively and negatively charged sol particles move? What is electroosmosis? (2 marks)

Try it first, then show the solutionSolution

Positive particles to the cathode; negative to the anode (1 mark).
If particles are prevented from moving, the medium moves: electroosmosis (1 mark).

Answer: Positive particles move to the cathode and negative particles to the anode. If the particles are prevented from moving, the dispersion medium moves instead, which is electroosmosis.

Example 3: Surface area of divided cube (3 marks) · 3 marks · Short answerHarder board level

A cube of edge 1 cm is divided into 10^12 identical small cubes. Find the edge of each small cube and the total surface area, and compare with the original. (3 marks)

Try it first, then show the solutionSolution

Edge of small cube = 1 cm / (10^12)^(1/3) = 1 cm / 10^4 = 10^-4 cm (1 mark).
Area of one small cube = 6 x (10^-4)^2 = 6 x 10^-8 cm^2; total = 6 x 10^-8 x 10^12 = 6 x 10^4 = 60,000 cm^2 (1 mark).
Original area = 6 cm^2; 60,000 / 6 = 10,000 times larger (1 mark).

Answer: Edge = 10^-4 cm. Total area = 6 x 10^-8 x 10^12 = 60,000 cm^2 (6 m^2), which is 10,000 times the original 6 cm^2.

Example 4: Coagulating ions (4 marks) · 4 marks · Short answerHarder board level

A sol of arsenious sulphide is negatively charged. Arrange NaCl, BaCl2 and AlCl3 in increasing order of coagulating power for the same molar concentration, and state the rule. Which has the smallest coagulation value? (4 marks)

Try it first, then show the solutionSolution

Negative sol is coagulated by positive ions: Na+, Ba2+, Al3+ (1 mark).
Hardy-Schulze rule: the greater the valence, the greater the coagulating power (1 mark).
Increasing order: NaCl < BaCl2 < AlCl3 (1 mark).
Smallest coagulation value: AlCl3 (1 mark).

Answer: For a negative sol the coagulating ions are the cations. By the Hardy-Schulze rule a higher valence gives a greater coagulating power, so the order is NaCl < BaCl2 < AlCl3, and AlCl3 has the smallest coagulation value.

Example 5: Protective colloids and mixing sols · 4 marks · Case-basedHOTS / competency

(i) Hydrated ferric oxide (positive) and arsenious sulphide (negative) sols are mixed in nearly equal amounts. What happens and why? (2) (ii) How does adding gelatine to a gold sol change its response to an electrolyte? (2)

Try it first, then show the solutionSolution

(i) Mutual coagulation: the opposite charges neutralise each other (2 marks).
(ii) Gelatine, a lyophilic colloid, coats the lyophobic particles and protects them from the electrolyte (2 marks).

Answer: (i) The sols coagulate mutually because their opposite charges neutralise each other. (ii) The lyophilic gelatine coats the lyophobic particles and protects them from the electrolyte, so the sol is not easily coagulated.

Where students lose marks in Colloids, Coagulation and Emulsions

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

Mistake: Choosing the same-charge ion to coagulate
Ignoring the sol's sign.
Wrong working
Al3+ coagulates a positive sol.
Right working
A positive sol is coagulated by anions; Al3+ coagulates a negative sol.
Do this: Opposite charge.
Usually costs: 2 marks.
Mistake: Forgetting the cube-root step
Rushing the area calculation.
Wrong working
Edge = 1 / 10^12 cm.
Right working
Edge = 1 cm / (10^12)^(1/3) = 10^-4 cm.
Do this: Take the cube root of the number of cubes.
Usually costs: 2 marks.
Mistake: Higher valence = higher coagulation value
Mixing power and value.
Wrong working
Al3+ has the highest coagulation value.
Right working
Al3+ has the smallest coagulation value but the greatest power.
Do this: Power and value are opposites.
Usually costs: 1 mark.

Exam tips

Surface Chemistry is formative (school) assessment only in 2026-27.
Write the sign of the sol before using Hardy-Schulze.
Give an example with each classification.

Practice questions on Colloids, Coagulation and Emulsions

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
Milk is an example of:
(a) water in oil emulsion
(b) oil in water emulsion
(c) gel
(d) foam
Hint
Fat droplets in water.
Answer
(b) oil in water emulsion
Q2 · 2 marks · Very short answerBoard level
Give two differences between lyophilic and lyophobic sols. (2 marks)
Hint
Reversibility and stability.
Answer
Lyophilic sols form directly on mixing, are reversible and quite stable; lyophobic sols need special methods, are irreversible and easily coagulated by small amounts of electrolyte.
Q3 · 3 marks · Short answerHarder board level
A cube of edge 2 cm is divided into 1000 identical small cubes. Find the total surface area of the small cubes and the number of times it exceeds the original area. (3 marks)
Hint
Edge 0.2 cm.
Answer
Original area = 6 x 2^2 = 24 cm^2. Small cube edge = 2/10 = 0.2 cm; area of one = 6 x 0.04 = 0.24 cm^2; total = 0.24 x 1000 = 240 cm^2, which is 10 times the original.
Q4 · 5 marks · Long answerHarder board level
A positive sol needs to be coagulated using sodium chloride, sodium sulphate or sodium phosphate. Order the three in increasing coagulating power, give the rule and explain why electrophoresis shows the sol's charge. (5 marks)
Hint
Anions with higher valence.
Answer
A positive sol is coagulated by anions: Cl-, SO4 2- and PO4 3-. By the Hardy-Schulze rule, higher valence means greater power, so the order is NaCl < Na2SO4 < Na3PO4. In electrophoresis positive particles move to the cathode and negative to the anode, so the direction of movement shows the sign of the charge.
Q5 · 1 mark · Assertion-reasonHOTS / competency
Assertion (A): A negative lyophobic sol is precipitated more effectively by BaCl2 than by NaCl at the same molar concentration.
Reason (R): The coagulating power of an ion increases with its valence.
(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
What keeps them apart?
Answer
(a) Both A and R are true, and R is the correct explanation of A.

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 a protective colloid?Show answer
A lyophilic colloid that coats lyophobic particles and protects them from electrolytes.
2.What is electroosmosis?Show answer
Movement of the dispersion medium when particles are prevented from moving.
3.Name an O/W emulsion.Show answer
Milk (or vanishing cream).
4.What is mutual coagulation?Show answer
Coagulation on mixing oppositely charged sols in nearly equal amounts.

Colloids, Coagulation and Emulsions: frequently asked questions

What is the Hardy-Schulze rule?

The greater the valence of the coagulating ion, the greater its power to cause precipitation.

What are lyophilic sols?

Liquid-loving sols, formed directly, reversible and stable.

What is an emulsion?

A liquid-liquid colloid, usually of oil in water or water in oil.