Colloids

• How to classify colloids by phase (sol, gel, emulsion, aerosol, foam) and by affinity for dispersion medium (lyophilic/lyophobic)
• Why the Tyndall effect occurs and what it reveals about particle size
• The cause of Brownian motion and its role in colloidal stability
• How coagulation and flocculation work, including the Hardy-Schulze rule
• Key processes: peptisation, electrophoresis, dialysis, and the significance of gold number

What is a Colloid?

A colloid (colloidal dispersion) is a heterogeneous system in which one substance (dispersed phase) is distributed as particles of size 1–1000 nm (or 10–1000 Å) in another substance (dispersion medium).

Colloidal particles cannot be seen by naked eye but can scatter light (Tyndall effect). They pass through filter paper but not through semi-permeable membranes.

Classification of Colloids

By physical state of dispersed phase and dispersion medium:

By affinity of dispersed phase for dispersion medium:

When the dispersion medium is water, lyophilic = hydrophilic and lyophobic = hydrophobic.

Lyophilic sols are more stable because: the dispersed particles are solvated (surrounded by solvent molecules), which prevents aggregation. Lyophobic sols need electrostatic stabilisation (surface charge) and are easily coagulated.

Tyndall Effect

When a beam of light passes through a colloidal solution in a dark room, the path of light becomes visible as a bright cone. This is the Tyndall effect, discovered by John Tyndall (1869).

Cause: Colloidal particles (1–1000 nm) are of the same order of magnitude as the wavelength of visible light (400–700 nm). They scatter light in all directions (Rayleigh scattering). True solutions (< 1 nm) do not scatter light because particles are too small.

The blue colour of sky is also a Tyndall effect: dust and air particles in the atmosphere scatter shorter wavelengths (blue, violet) more than longer wavelengths (red). Blue is scattered most in all directions → sky appears blue.

At sunrise/sunset: light travels a longer path through atmosphere → blue is scattered away; longer wavelengths (orange, red) reach our eyes → sun appears red/orange.

Tyndall effect is used to:

1. Distinguish colloids from true solutions
2. Confirm colloidal nature of a substance

Brownian Motion

Colloidal particles exhibit continuous, random, zig-zag motion called Brownian motion (first observed by Robert Brown in 1827 with pollen in water).

Cause: Colloidal particles are bombarded unequally from all directions by the molecules of the dispersion medium (kinetic molecular motion). At any instant, the net force is not zero, causing a random displacement.

Significance:

• Brownian motion opposes sedimentation — keeps colloidal particles suspended
• It is evidence for the kinetic molecular theory of matter
• Larger particles show less Brownian motion (more symmetrical bombardment)

True solutions do not show Tyndall effect; all particles show Brownian motion (though for molecular-scale solute it is not distinct enough to observe).

Coagulation / Flocculation

Coagulation (or flocculation) is the process of aggregation of colloidal particles followed by their settling. The colloidal sol is destroyed.

Methods of coagulation:

1. Addition of electrolyte — the most important method (see Hardy-Schulze rule)
2. Boiling — increases kinetic energy, particles collide and aggregate; also removes adsorbed charge
3. Electrophoresis — particles migrate to oppositely charged electrode, lose charge, and settle
4. Mixing oppositely charged sols — mutual coagulation (e.g., As₂S₃ sol + Fe(OH)₃ sol)

Hardy-Schulze Rule

The coagulating power of an electrolyte depends on the valency (charge) of the ion with opposite sign to the colloidal particle:

Higher the valency of the oppositely charged ion → greater the coagulating power

For a negatively charged sol (e.g., As₂S₃, which is −ve):

• Coagulating ions are cations
• Order of coagulating power: Al³⁺ > Ba²⁺ > Na⁺ (trivalent > divalent > monovalent)

For a positively charged sol (e.g., Fe(OH)₃, which is +ve):

• Coagulating ions are anions
• Order: [Fe(CN)₆]⁴⁻ > PO₄³⁻ > SO₄²⁻ > Cl⁻

Coagulating value (flocculation value): minimum concentration of electrolyte (in millimoles/litre) needed to coagulate a sol in 2 hours. Lower coagulating value = higher coagulating power.

Peptisation

Peptisation is the process of dispersing a freshly prepared precipitate into a colloidal sol by adding a small amount of electrolyte called a peptising agent.

Mechanism: The peptising agent's ions are adsorbed on the surface of the precipitate → particles acquire surface charge → electrostatic repulsion prevents aggregation → precipitate disperses into colloidal particles.

Example:

• AgCl precipitate + AgNO₃ (small amount) → AgCl sol (Ag⁺ adsorbed on AgCl surface)
• Fe(OH)₃ precipitate + FeCl₃ (small amount) → Fe(OH)₃ sol (Fe³⁺ adsorbed, +ve charge)

Peptisation is the reverse of coagulation. Note that only a small amount of electrolyte peptises; large amounts coagulate.

Electrophoresis

Electrophoresis (cataphoresis) is the migration of colloidal particles towards the electrode of opposite charge under the influence of an applied electric field.

• Negatively charged sol → particles migrate to anode (positive electrode)
• Positively charged sol → particles migrate to cathode (negative electrode)

Use:

• Determines the sign of charge on colloidal particles
• Industrial purification (e.g., removal of carbon black in smokestacks — Cottrell precipitator)
• Analysis of proteins and DNA (gel electrophoresis)

Dialysis

Dialysis is the process of removing dissolved crystalloids (small molecules/ions) from a colloidal solution using a semi-permeable membrane.

Principle: Colloidal particles (1–1000 nm) cannot pass through a semi-permeable membrane (e.g., parchment paper, cellophane); crystalloids (ions, small molecules) can.

Application: Purification of colloids (blood dialysis in kidney failure — an artificial kidney uses dialysis membranes to remove urea and salts from blood).

Electrodialysis: Dialysis is speeded up by applying an electric field across the membrane — ions migrate faster under electrical force.

Gold Number

Gold number (Zsigmondy, 1901) is defined as the minimum mass (in mg) of a protective colloid required to just prevent the coagulation of 10 mL of gold sol when 1 mL of 10% NaCl solution is added.

Protective colloid: A lyophilic colloid (gelatin, starch, egg albumin) that is added to a lyophobic sol to stabilise it by adsorbing on the particles' surface.

Lower gold number → better protective ability:

Gelatin has the lowest gold number → most effective protective colloid.

Preparation of Colloids

Condensation methods (building up from smaller particles):

1. Chemical reduction: HAuCl₄ + HCHO → gold sol (Au nanoparticles)
2. Double decomposition: As₂O₃ + H₂S → As₂S₃ sol
3. Oxidation: SO₂ + H₂S → Sulphur sol
4. Hydrolysis: FeCl₃ + 3H₂O → Fe(OH)₃ sol + 3HCl (boiling FeCl₃ solution)

Dispersion methods (breaking down from larger particles):

1. Mechanical grinding (colloidal mill, ball mill): rubber, graphite sols
2. Electrical disintegration (Bredig's arc): Metal (Pt, Au) electrodes in water + electric arc → metal sol
3. Peptisation (already described above)
4. Ultrasonic dispersion: High-frequency sound waves break particles into colloidal size

Why Lyophobic Sols are Less Stable

Lyophobic sols depend on the electric double layer (Helmholtz/Gouy-Chapman layer) around particles for stability. The surface of the particle adsorbs ions of one type; the diffuse outer layer has ions of opposite charge. The resulting zeta potential (electrokinetic potential) keeps particles apart by electrostatic repulsion.

When electrolyte is added:

• Counter-ions enter the double layer, compressing it
• Zeta potential falls
• Particles approach each other → van der Waals attraction dominates → coagulation

This is the basis of the DLVO theory (Derjaguin–Landau–Verwey–Overbeek).

Emulsions — Types and Emulsifying Agents

An emulsion is a colloidal dispersion of two immiscible liquids:

• O/W (oil in water): Oil droplets in water — e.g., milk, vanishing cream. Stabilised by soaps (sodium stearate).
• W/O (water in oil): Water droplets in oil — e.g., butter, cold cream. Stabilised by heavy metal soaps (calcium stearate).

Emulsifying agents (emulsifiers) work by adsorbing at the liquid-liquid interface and reducing interfacial tension: soaps, detergents, gum, egg yolk (lecithin), proteins.

Demulsification (breaking emulsion): heating, centrifugation, adding electrolyte.

Charge on Common Colloidal Sols

The charge on AgI depends on which ion is in excess — the ion common to the colloid that is adsorbed first. This illustrates preferential adsorption.

Example 1: Applying Hardy-Schulze Rule

Problem: Arrange the following electrolytes in increasing order of their
coagulating power for an As₂S₃ sol:
NaCl, BaCl₂, AlCl₃, Na₃PO₄

Step 1: Identify charge on As₂S₃ sol.
        As₂S₃ is negatively charged (adsorbs S²⁻ from H₂S during preparation).

Step 2: Coagulating ions must be CATIONS (opposite charge to sol).
        From each electrolyte, identify the cation and its charge:
        - NaCl → Na⁺ (charge +1)
        - BaCl₂ → Ba²⁺ (charge +2)
        - AlCl₃ → Al³⁺ (charge +3)
        - Na₃PO₄ → Na⁺ (charge +1) — PO₄³⁻ is anion, doesn't coagulate −ve sol

Step 3: Apply Hardy-Schulze rule: higher valency → greater coagulating power.
        Al³⁺ > Ba²⁺ > Na⁺ = Na⁺

Step 4: Among NaCl and Na₃PO₄: both give Na⁺ (valency 1), so equal coagulating power.

Answer: Increasing order: NaCl = Na₃PO₄ < BaCl₂ < AlCl₃
        (AlCl₃ is most effective because Al³⁺ has the highest cationic charge)

Example 2: Tyndall Effect — Identifying Colloids

Problem: Three liquids A, B, C are tested in a dark room with a torch beam.
- A shows a bright cone of light (Tyndall cone)
- B shows no visible beam path
- C shows no beam path and particles settle in 5 minutes

Identify A, B, and C as true solution, colloid, or suspension. Explain.

Step 1: Analyse each observation:
        A — Tyndall cone visible → particles are in the colloidal range (1–1000 nm)
             and scatter light → A is a COLLOID

        B — No beam visible → particles are too small to scatter visible light
             (<1 nm) → B is a TRUE SOLUTION

        C — No beam + particles settle → particles are too large (>1000 nm),
             suspension particles settle due to gravity → C is a SUSPENSION

Step 2: Confirm with particle size:
        True solution < 1 nm → B
        Colloid 1–1000 nm → A (Tyndall effect)
        Suspension > 1000 nm → C (settles)

Answer: A = Colloid, B = True solution, C = Suspension
Key: Tyndall effect is the definitive test for colloidal nature.

1. A freshly prepared Fe(OH)₃ precipitate is treated with a small amount of FeCl₃ solution. What happens and why? What is this process called?
2. Explain why mixing As₂S₃ sol and Fe(OH)₃ sol leads to coagulation of both.
3. What is the gold number? If gelatin has a gold number of 0.005 and starch has 25, which is a better protective colloid?
4. Describe the Tyndall effect. How does it explain the blue colour of the sky?
5. (Stretch) A protein exists in a colloidal sol. At pH 4 it migrates towards the cathode in electrophoresis; at pH 8 it migrates to the anode; at pH 6 it does not migrate. Explain these observations in terms of the protein's isoelectric point and charge, and state what pH 6 represents.

• NCERT Class 12 Chemistry, Chapter 5 — Surface Chemistry, Sections 5.3–5.5
• Colloids give 2–3 JEE/NEET questions per year; Hardy-Schulze rule and gold number are very frequently tested
• Common MCQ formats: rank electrolytes by coagulating power, identify Tyndall vs fluorescence, match colloid type with example

Study strategy: Make a table of all 8 types of colloids (dispersed phase × medium) with 2 examples each — memorise them cold. For Hardy-Schulze questions, always determine sol charge first, then rank cation/anion valency. Brownian motion, Tyndall effect, and electrophoresis are conceptual — understand the mechanism, not just the definition.

Practice MCQs on colloidal classification, Tyndall effect, Hardy-Schulze rule, and gold number in the Surface Chemistry — Colloids topic bank. Aim for Medium difficulty after clearing Easy.

Unsure about why mixing two oppositely charged sols causes coagulation? Ask the Drishti AI tutor to explain mutual coagulation with charge neutralisation diagrams, or to clarify the difference between dialysis and electrophoresis.

Complete all 5 Quick Check questions and log your score in your Drishti progress tracker. Clear 4/5 before moving to Catalysis.

• Read: Surface Chemistry — Catalysis — enzyme lock-and-key model, zeolites, activation energy diagrams
• Then: Chapter 6 — General Principles of Isolation of Elements (metallurgy)
• Practice: Full Surface Chemistry Mixed MCQs (Hard difficulty for JEE Foundation)