Colloids — Concepts, Formulas & Examples

A colloid is a mixture in which one substance is dispersed in another at particle size between 1 and 1000 nanometres. CBSE Class 12 and NEET cover colloids in the surface chemistry chapter. Expect one question a year on types, properties or protective colloids.

Core Concepts

Difference from solution and suspension

Solution — particle size less than 1 nm, transparent, homogeneous (salt water). Suspension — over 1000 nm, settles on standing (muddy water). Colloid — between the two, does not settle, scatters light.

Property	True solution	Colloid	Suspension
Particle size	<1 nm	1-1000 nm	>1000 nm
Visibility	Invisible	Ultramicroscope	Visible
Settles?	No	No	Yes
Filterable?	No	Not by ordinary filter (passes through filter paper but not ultrafilter)	Yes
Tyndall effect	No	Yes	May show
Example	Salt in water	Milk, fog, blood	Muddy water

Classification by phase

Dispersed phase + dispersion medium. Sol (solid in liquid — paint), gel (liquid in solid — jelly), aerosol (solid or liquid in gas — smoke, fog), emulsion (liquid in liquid — milk), foam (gas in liquid — whipped cream).

Complete classification table:

Dispersed phase	Dispersion medium	Type	Example
Solid	Gas	Solid aerosol	Smoke, dust
Solid	Liquid	Sol	Paints, ink, blood
Solid	Solid	Solid sol	Coloured glass, gems
Liquid	Gas	Liquid aerosol	Fog, mist, clouds
Liquid	Liquid	Emulsion	Milk, mayonnaise
Liquid	Solid	Gel	Jelly, butter, cheese
Gas	Liquid	Foam	Whipped cream, shaving foam
Gas	Solid	Solid foam	Pumice stone, bread

Note: Gas in gas is NOT a colloid — gases are always fully miscible, forming true solutions.

Lyophilic and lyophobic colloids

Lyophilic (solvent-loving) — stable, reversible, does not need protection (starch in water). Lyophobic (solvent-hating) — unstable, irreversible, needs protection (metal sols like As $_2$ S $_3$ ).

Property	Lyophilic	Lyophobic
Affinity for solvent	Strong	Weak
Preparation	Easy (just dissolve)	Needs special methods
Stability	Very stable	Unstable, needs stabiliser
Reversibility	Reversible	Irreversible
Viscosity	Higher than medium	Same as medium
Examples	Starch, gelatin, gum arabic	Gold sol, As $_2$ S $_3$ sol

Protective colloids: Lyophilic colloids can protect lyophobic colloids from coagulation. Gelatin added to a gold sol prevents it from coagulating when an electrolyte is added. The gold number measures protective power — the mg of protective colloid needed to just prevent coagulation of 10 mL of gold sol by 1 mL of 10% NaCl. Lower gold number = better protection.

Gold numbers: Gelatin (0.005-0.01) > albumin (0.1) > starch (25). Gelatin is the best protective colloid because it has the lowest gold number.

Properties

Tyndall effect — colloidal particles scatter light, making the beam visible. Brownian motion — random zigzag motion due to collisions with solvent molecules. Electrophoresis — colloidal particles move in an electric field because they carry charge.

Tyndall effect: When a beam of light passes through a colloid, the particles scatter light in all directions (Rayleigh scattering). The path of light becomes visible — like a beam of sunlight in a dusty room. True solutions do not show this because their particles are too small to scatter light.

Brownian motion: Colloidal particles are constantly bombarded by solvent molecules from all sides. The uneven bombardment causes the particles to move in a random zigzag path. First observed by Robert Brown in 1827 (pollen grains in water). Brownian motion prevents sedimentation and keeps the colloid stable.

Electrophoresis: Most colloidal particles carry an electrical charge (from adsorbed ions). In an electric field, positive colloids (e.g., Fe(OH) $_3$ sol) move towards the cathode; negative colloids (e.g., As $_2$ S $_3$ sol) move towards the anode. This charge is also responsible for colloid stability — like-charged particles repel each other and do not aggregate.

Charge on common colloids:

Positive: Fe(OH) $_3$ , Al(OH) $_3$ , basic dyes
Negative: As $_2$ S $_3$ , Au, Ag, starch, clay, acid dyes

Coagulation (flocculation)

Addition of oppositely charged ions neutralises the charge on colloidal particles, causing them to aggregate and settle. The Hardy-Schulze rule — the greater the charge, the more effective the coagulant.

Hardy-Schulze rule: The coagulating power of an ion increases with the magnitude of its charge. For a negatively charged colloid (like As $_2$ S $_3$ ):

\text{Al}^{3+} > \text{Ba}^{2+} > \text{Na}^+

The coagulating power of Al $^{3+}$ is about 500 times that of Na $^+$ .

Methods of coagulation:

Adding an electrolyte (most common)
Mixing oppositely charged colloids (mutual coagulation)
Prolonged dialysis (removes stabilising ions)
Heating (increases kinetic energy, particles collide and aggregate)

Practical applications of coagulation:

Alum (KAl(SO $_4$ ) $_2$ ) purifies water by coagulating suspended particles (Al $^{3+}$ coagulates negatively charged clay)
Cottrell precipitator removes smoke particles (charged plates coagulate aerosol)
Delta formation at river mouths — seawater ions coagulate colloidal clay in river water

Emulsions

A colloid of two immiscible liquids. Oil-in-water (O/W): Oil droplets dispersed in water (milk, face cream). Water-in-oil (W/O): Water droplets dispersed in oil (butter, cold cream). An emulsifier (like soap) stabilises emulsions by sitting at the oil-water interface.

How soap acts as an emulsifier: The hydrophobic tail dissolves in oil, the hydrophilic head points into water. This creates a stable film around oil droplets, preventing them from merging. The type of emulsion (O/W or W/O) depends on the relative amounts and the emulsifier type.

Worked Examples

Tiny molecules in the atmosphere scatter short (blue) wavelengths more than long (red) wavelengths. The effect is related to but stronger than Tyndall scattering. Sunsets are red because the long path through atmosphere scatters out all the blue.

Milk is an emulsion. Lactobacillus converts lactose to lactic acid, lowering pH. The lower pH denatures milk proteins, causing them to coagulate into solid curd.

Muddy water contains negatively charged clay particles (colloid). Alum dissolves to release Al $^{3+}$ ions. By Hardy-Schulze rule, Al $^{3+}$ (high charge) is very effective at neutralising the charge on clay particles. The clay coagulates and settles, leaving clear water.

Milk is O/W (oil in water) — dilutable with water, dyed by water-soluble dyes. Butter is W/O (water in oil) — dilutable with oil, dyed by oil-soluble dyes. The simple dilution test: if adding water mixes easily, it is O/W.

Common Mistakes

Confusing solution and colloid. Solutions are clear; colloids scatter light.

Saying lyophilic colloids need protection. Lyophobic do; lyophilic do not.

Writing that colloids always settle. They do not — that is a suspension.

Interpreting gold number backwards. Lower gold number means BETTER protective power (less material needed). Gelatin (0.005) is a much better protective colloid than starch (25).

Confusing the Tyndall effect with fluorescence. Tyndall effect is scattering of light by particles. Fluorescence is absorption and re-emission at a different wavelength. Tyndall shows the beam path; fluorescence makes the solution glow.

Exam Weightage and Revision

NEET 2023 asked about the Hardy-Schulze rule. JEE Main 2024 tested the gold number. CBSE boards ask about types of colloids and Tyndall effect. Surface chemistry gives 1-2 questions per exam.

When a question gives a scenario, identify the core mechanism first, then match it to the concepts above. Most wrong answers come from reading the scenario too quickly.

Three properties — Tyndall, Brownian, electrophoresis. NEET asks one of these almost every year.

Practice Questions

Q1. Why does a beam of light become visible when passing through milk but not through salt water?

Milk is a colloid — its fat globules (1-1000 nm) scatter light (Tyndall effect), making the beam visible. Salt water is a true solution — its ions are less than 1 nm, too small to scatter light. The beam passes through invisibly.

Q2. Arrange in order of coagulating power for a negatively charged colloid: NaCl, BaCl $_2$ , AlCl $_3$ .

By Hardy-Schulze rule, higher cation charge = greater coagulating power: AlCl $_3$ > BaCl $_2$ > NaCl (Al $^{3+}$ > Ba $^{2+}$ > Na $^+$ ).

Q3. What is dialysis and how is it used to purify colloids?

Dialysis is the separation of crystalloid impurities from a colloid using a semipermeable membrane (like cellophane). Colloidal particles are too large to pass through the membrane, while dissolved ions and small molecules pass through. The colloid is placed in a bag of the membrane, suspended in pure water. Impurities diffuse out, purifying the colloid. This principle is used in kidney dialysis machines.

Q4. Give one example each of a solid aerosol and a liquid aerosol.

Solid aerosol (solid in gas): Smoke — tiny carbon particles dispersed in air. Liquid aerosol (liquid in gas): Fog — tiny water droplets dispersed in air. Both scatter light, which is why smoke and fog reduce visibility.

Q5. Why is gelatin a better protective colloid than starch?

Gelatin has a lower gold number (0.005-0.01) than starch (25). This means much less gelatin is needed to protect a gold sol from coagulation. Gelatin molecules are highly hydrophilic and form a thick protective layer around lyophobic particles, effectively shielding them from electrolyte-induced coagulation.

FAQs

Why do rivers form deltas where they meet the sea? River water carries colloidal clay particles (negatively charged). When it meets seawater (rich in Na $^+$ , Mg $^{2+}$ , Ca $^{2+}$ ions), these ions coagulate the clay colloid. The coagulated clay settles at the river mouth, forming a delta over centuries.

Is blood a colloid? Yes. Blood is a complex colloid — it contains plasma proteins (albumin, globulins) and blood cells dispersed in the liquid plasma. Blood shows the Tyndall effect and other colloidal properties. The colloidal nature of blood is medically important — changes in blood colloid osmotic pressure can cause oedema.

What is the Cottrell precipitator? An industrial device that removes colloidal smoke particles from factory exhaust. It works by electrophoresis — smoke passes between charged plates, the charged particles are attracted to the opposite plate, and they deposit there. Clean gas exits. This is how power plants control particulate pollution.

Colloids are everywhere — milk, fog, blood, paint, smoke, whipped cream. Learn one example for each type and the chapter becomes intuitive.