Which colloid points are asked most in NEET previous-year papers?

NEET questions on colloids usually test the Hardy-Schulze order of coagulating ions, positive and negative sols, lyophilic versus lyophobic sols, the Tyndall effect, the type of emulsion in milk or butter, gold number, micelles and the CMC, and everyday uses such as alum in water purification.

Colloids

ChemistrySurface ChemistryFor JEE aspirants

A colloid is a heterogeneous system in which one substance, the dispersed phase, is spread as particles of 1 to 1000 nm through another, the dispersion medium. Colloids sit between true solutions and suspensions, and their huge surface area and surface charge explain their special behaviour. This page covers how colloids are classified, micelles and soap action, preparation and purification, the Tyndall effect, Brownian movement, charge, electrophoresis, coagulation and the Hardy-Schulze rule, protection, emulsions and uses. Colloids are now a JEE Advanced-only topic: Surface Chemistry has been removed from the JEE Main and NEET syllabi.

On this page1Size range2Classification3Micelles and soap4Preparation5Purification6Properties7Coagulation8Emulsions9Uses

Key Formulas - Quick Reference

★ Must learn Particle size: true solution $< 1$ nm; colloid $1$ - $1000$ nm ( $1 0^{- 9}$ - $1 0^{- 6}$ m); suspension $> 1000$ nm.
Eight types by physical state (gas in gas is not a colloid). Sol = solid in liquid, gel = liquid in solid, emulsion = liquid in liquid.
★ Must learn Micelles form only above the CMC (soaps: $1 0^{- 4}$ - $1 0^{- 3}$ mol L $^{- 1}$ ) and above the Krafft temperature $T_{k}$ .
Charge by preferential adsorption: $AgN O_{3}$ into excess KI gives $AgI/ I^{-}$ (negative); KI into excess $AgN O_{3}$ gives $AgI/A g^{+}$ (positive).
★ Must learn Hardy-Schulze: the ion opposite in charge coagulates; higher charge, more power. Negative sol: $A l^{3 +}$ $>$ $B a^{2 +}$ $>$ $N a^{+}$ . Positive sol: $[Fe (CN)_{6}]^{4 -} >$ $P O_{4}^{3 -}$ $>$ $S O_{4}^{2 -}$ $>$ $C l^{-}$ .
★ Must learn Coagulating value (mmol L $^{- 1}$ to coagulate in 2 h): smaller value, greater coagulating power; power $\propto \frac{1}{coagulating value}$ .
★ Must learn Gold number: the smaller it is, the better the protective colloid (gelatin best, starch worst).
Emulsions: O/W (milk, vanishing cream) and W/O (butter, cold cream).
Surface area of $N$ cubes of edge $a$ cut from a cube of edge $L$ : $6 L^{3} / a$ , so area grows as $1/ a$ .

1. What a Colloid Is

Sugar in water gives a true solution; sand stirred in water gives a suspension that settles. In between lies a large family of colloidal dispersions, or colloids.

Colloid: a heterogeneous system in which one substance, the dispersed phase, is present as very fine particles (1-1000 nm) in another substance, the dispersion medium.

The difference from a true solution is only particle size. A colloidal particle may be one giant molecule (a protein or polymer) or an aggregate of many atoms, ions or molecules. It is too big to behave like a dissolved molecule but too small to settle.

Figure 1: Colloids are defined by size, 1-1000 nm (

1 0^{- 9}

1 0^{- 6}

m). That single fact explains the filter paper, membrane, settling and Tyndall results in the rows below the scale.

Property	True solution	Colloid	Suspension
Particle size	$< 1$ nm	$1$ - $1000$ nm	$> 1000$ nm
Nature	homogeneous	heterogeneous (looks uniform)	heterogeneous
Filter paper	passes	passes	retained
Parchment / animal membrane	passes	retained	retained
Settling	never	does not settle (only in an ultracentrifuge)	settles on standing
Visibility of particles	not visible	scattered light seen in an ultramicroscope	visible (microscope or eye)
Tyndall effect	no	yes	not applicable (opaque)

Small size means an enormous surface area per gram. A 1 cm cube has 6 cm $^{2}$ of surface; cut into $1 0^{12}$ cubes of colloidal size ( $1 μ$ m edge), the total surface becomes 60,000 cm $^{2}$ , or 6 m $^{2}$ . Most colloidal properties come from this surface.

Exam Trick One to a thousand. Colloids are 1-1000 nm: below 1 nm is a true solution, above 1000 nm a suspension. Filter paper stops only suspensions; parchment stops colloids too.

2. Classification of Colloids

Colloids are classified in three ways: by the physical state of the two phases, by how strongly the dispersed phase interacts with the medium, and by the type of particle.

2.1 By physical state of dispersed phase and medium

Either phase can be a solid, liquid or gas, giving nine combinations. Gas in gas is excluded because gases always mix completely, so eight types of colloid exist.

Figure 2: Eight colloid types from the physical states (gas in gas is a true solution). The three most common, highlighted, are sols (solid in liquid), gels (liquid in solid) and emulsions (liquid in liquid).

The most common are sols (solid in liquid), gels (liquid in solid) and emulsions (liquid in liquid). A sol in water is a hydrosol or aquasol; in alcohol an alcosol; in benzene a benzosol; a colloid in a gas is an aerosol.

2.2 Lyophilic and lyophobic sols

Lyophilic means solvent-loving and lyophobic solvent-hating (with water: hydrophilic, hydrophobic).

Lyophilic sols

Form by simply mixing the substance (gum, gelatin, starch, rubber) with the liquid. Reversible: evaporate the medium and remix to get the sol back. Stable; not easily coagulated.

Lyophobic sols

Metals, their sulphides and hydroxides do not form sols on mixing; special methods are needed. Irreversible once precipitated. Unstable: coagulated by a little electrolyte, heating or shaking; need stabilisers.

Property	Lyophilic	Lyophobic
Preparation	direct mixing	special methods (chemical, Bredig's arc, peptization)
Reversibility	reversible	irreversible
Stability comes from	charge and extensive solvation	charge only
Effect of a little electrolyte	little effect	coagulates
Viscosity	much higher than the medium	about the same as the medium
Surface tension	lower than the medium	about the same as the medium
Tyndall effect	weak	strong
Examples	gum, gelatin, starch, rubber, proteins	gold, sulphur, $A s_{2} S_{3}$ , $Fe (OH)_{3}$ sols

2.3 Multimolecular, macromolecular and associated colloids

Figure 3: Three kinds of colloidal particle: an aggregate of many small units (multimolecular), one giant molecule (macromolecular), or an aggregate of ions that forms only above the CMC and

T_{k}

(associated, micelles).

Feature	Multimolecular	Macromolecular	Associated (micelles)
Particle	aggregate of many atoms or small molecules ( $< 1$ nm each)	one giant molecule of colloidal size	aggregate of ions formed above CMC and $T_{k}$
Usual nature	lyophobic	lyophilic, quite stable	both parts: lyophobic tail, lyophilic head
On dilution	stays a sol	stays a sol	breaks back into ions below CMC
Examples	gold sol, sulphur sol (1000 or more $S_{8}$ units)	starch, cellulose, proteins, enzymes; polythene, nylon, polystyrene, synthetic rubber	soaps, synthetic detergents

Key idea

Classify a colloid by state (8 types), by affinity for the medium (lyophilic or lyophobic) and by particle type (multimolecular, macromolecular, associated).

Quick Recall: tap to check

Why is there no gas-in-gas colloid?

Gases mix completely to give a homogeneous mixture, so there is no separate dispersed phase.

Which sol is reversible: starch or

A s_{2} S_{3}

Starch (lyophilic).

A s_{2} S_{3}

sol is lyophobic and irreversible.

Classify: gold sol, starch, soap above CMC.

Multimolecular, macromolecular, associated.

3. Micelles and the Cleansing Action of Soap

Some substances behave as normal strong electrolytes at low concentration but form colloid-sized aggregates at higher concentration. These aggregates are micelles (associated colloids). Micelles form only above a particular temperature, the Krafft temperature ( $T_{k}$ ), and above a particular concentration, the critical micelle concentration (CMC). For soaps the CMC is $1 0^{- 4}$ to $1 0^{- 3}$ mol L $^{- 1}$ , and a micelle may contain 100 or more ions. Surface-active agents (surfactants) such as soaps and synthetic detergents belong to this class.

Soap is the sodium or potassium salt of a long-chain fatty acid, for example sodium stearate, $C H_{3} (C H_{2})_{16} CO O^{-} N a^{+}$ . The stearate ion has a non-polar hydrocarbon tail (hydrophobic) and a polar $- CO O^{-}$ head (hydrophilic). At low concentration the ions sit at the surface; at the CMC they are pulled into the bulk and cluster into a spherical ionic micelle with the tails inside and the heads outside. Detergents such as sodium lauryl sulphate, $C H_{3} (C H_{2})_{11} S O_{4}^{-} N a^{+}$ , behave the same way with $- S O_{4}^{-}$ as the head.

Figure 4: A soap ion has a water-hating tail and a water-loving head. Below the CMC the ions line the surface; at the CMC (

1 0^{- 4}

1 0^{- 3}

mol L

^{- 1}

for soaps) about 100 or more ions cluster into an ionic micelle with the tails hidden inside.

Cleansing action: soap ions surround an oil or grease droplet with their tails inside the grease and their heads in the water. The droplet becomes a micelle that is pulled into the water and washed off. The negatively charged sheath around each droplet keeps droplets from joining again. So soap works by both micelle formation and emulsification.

Figure 5: Soap cleans by forming micelles around grease: tails dissolve in the oil, charged heads face the water, and the negatively charged sheath keeps the droplets apart so they wash away (emulsification).

4. Preparation of Colloids

4.1 Chemical methods

Molecules formed by double decomposition, oxidation, reduction or hydrolysis aggregate into sol particles:

A s_{2} O_{3} + 3 H_{2} S double decomposition A s_{2} S_{3} (sol) + 3 H_{2} O

S O_{2} + 2 H_{2} S oxidation 3S (sol) + 2 H_{2} O

2AuC l_{3} + 3HCHO + 3 H_{2} O reduction 2Au (sol) + 3HCOOH + 6HCl

FeC l_{3} + 3 H_{2} O hydrolysis Fe (OH)_{3} (sol) + 3HCl

4.2 Bredig's arc method and peptization

Bredig's arc (electrical disintegration): for sols of metals such as gold, silver and platinum. An arc is struck between electrodes of the metal under the dispersion medium, which is kept cold in ice. The metal vaporises and the vapour condenses into colloidal particles: dispersion and condensation together.
Peptization: converting a fresh precipitate into a sol by shaking it with the dispersion medium and a small amount of electrolyte (the peptizing agent). The precipitate adsorbs one ion of the electrolyte, becomes charged, and the like charges break it into colloidal particles. Fresh $Fe (OH)_{3}$ with a little $FeC l_{3}$ gives a positive sol.

Figure 6: Two physical routes to a sol. Bredig's arc: vaporise a metal in an arc under the liquid and let it condense (dispersion + condensation), cooled by ice. Peptization: a little electrolyte charges a fresh precipitate so it breaks into colloidal particles.

5. Purification of Colloidal Solutions

Freshly made sols carry excess electrolyte and other soluble impurities. A trace of electrolyte stabilises a sol, but too much coagulates it, so the impurities are reduced to a small, necessary level.

Dialysis: removal of dissolved substances from a sol by diffusion through a suitable membrane (parchment, cellophane, animal bladder). Ions and small molecules pass through; colloidal particles do not. The sol is held in a bag (dialyser) in running water.
Electrodialysis: dialysis is slow; an electric field across the membrane speeds it up. It works only when the impurity is an electrolyte.
Ultrafiltration: ordinary filter paper lets colloidal particles through. Soaking it in collodion (4% nitrocellulose in alcohol-ether), hardening with formaldehyde and drying makes an ultrafilter that holds back colloidal particles. Pressure or suction speeds it up; the residue is stirred with fresh medium to get a pure sol.

Figure 7: Dialysis: the membrane passes ions and small molecules (crystalloids) but not colloidal particles. Electrodialysis adds an electric field to pull the ions out faster; it works only when the impurity is an electrolyte.

Key idea

Prepare by building up (chemical reactions, condensation) or breaking down (Bredig's arc, peptization); purify by letting only the small particles out (dialysis, ultrafiltration).

6. Properties of Colloidal Solutions

6.1 Colligative properties

Colloidal particles are big aggregates, so a sol has far fewer particles than a true solution of the same mass concentration. Osmotic pressure, lowering of vapour pressure, depression in freezing point and elevation in boiling point are therefore very small.

6.2 Tyndall effect

A beam of light passed through a sol and viewed at right angles lights up the path as a bluish cone (the Tyndall cone), because colloidal particles scatter light in all directions. A true solution looks dark from the side. It was first seen by Faraday and studied by Tyndall; dust and smoke make the projector beam visible in a cinema hall.

Figure 8: Tyndall effect. Colloidal particles scatter light, so the beam's path glows (Tyndall cone) when viewed at right angles; a true solution stays dark. This is the quickest test to tell a colloid from a true solution.

Zsigmondy (1903) used the effect in the ultramicroscope: particles appear as bright spots against a dark background. It shows the light scattered by the particles, not the particles themselves, so it gives no information about their size or shape.

6.3 Colour

The colour of a sol depends on the wavelength of light scattered, which depends on particle size and nature, and on how the light is viewed. Milk and water looks blue by reflected light and red by transmitted light. Finest gold sol is red; as the particles grow it turns purple, then blue and finally golden.

6.4 Brownian movement

Seen under an ultramicroscope, colloidal particles move in a continuous zig-zag path (Robert Brown). The motion comes from the unequal bombardment of the particles by molecules of the medium. It does not depend on the nature of the colloid; it is faster for smaller particles and a less viscous medium. Its stirring effect stops the particles from settling, so it helps keep a sol stable.

6.5 Charge on colloidal particles

All particles in a given sol carry the same charge, positive or negative. Like charges repel and keep the particles apart, which is the main reason a lyophobic sol is stable.

Positively charged sols	Negatively charged sols
hydrated metal oxides: $A l_{2} O_{3} \cdot x H_{2} O$ , $Cr O_{3} \cdot x H_{2} O$ , $F e_{2} O_{3} \cdot x H_{2} O$	metals: copper, silver, gold sols
basic dyes: methylene blue sol	metal sulphides: $A s_{2} S_{3}$ , $S b_{2} S_{3}$ , CdS sols
haemoglobin (blood)	acid dyes: eosin, congo red sols
oxides: $Ti O_{2}$ sol	starch, gum, gelatin, clay, charcoal sols

The charge comes from electron capture during Bredig's arc, from preferential adsorption of ions from the solution, or from formation of an electrical double layer. A particle prefers to adsorb the ion that is common to it and present in excess:

Dilute $AgN O_{3}$ added to dilute KI (KI in excess): AgI adsorbs $I^{-}$ , giving a negative sol, $AgI/ I^{-}$ . KI added to $AgN O_{3}$ ( $AgN O_{3}$ in excess): AgI adsorbs $A g^{+}$ , giving $AgI/A g^{+}$ , positive.
$FeC l_{3}$ added to excess hot water: hydrated ferric oxide adsorbs $F e^{3 +}$ , a positive sol, $F e_{2} O_{3} \cdot x H_{2} O/F e^{3 +}$ . $FeC l_{3}$ added to NaOH solution: it adsorbs $O H^{-}$ , a negative sol, $F e_{2} O_{3} \cdot x H_{2} O/O H^{-}$ .

Exam Trick Positive sols are a short list: hydrated metal oxides (

F e_{2} O_{3}

A l_{2} O_{3}

Cr O_{3}

Ti O_{2}

), basic dyes (methylene blue) and haemoglobin. Metals, sulphides, acid dyes, starch, gum, clay and charcoal are negative.

6.6 Electrical double layer and zeta potential

The charged layer of adsorbed ions attracts counter ions from the medium, forming a second layer. The two together are the Helmholtz electrical double layer. The first layer is held firmly (fixed layer); the second is mobile (diffused layer). The potential difference between them is the electrokinetic or zeta potential. Particles with double layers repel each other at a distance and stay dispersed; adding electrolyte squeezes the diffused layer, lowers the zeta potential and lets the particles come together.

Figure 9: The electrical double layer: adsorbed

I^{-}

(fixed layer) and mobile counter ions (diffuse layer). The potential difference between them is the zeta (

ζ

) or electrokinetic potential. Adding electrolyte squeezes the diffuse layer, lowers

ζ

and lets particles coagulate.

6.7 Electrophoresis and electroosmosis

When a potential is applied across two platinum electrodes dipping in a sol, the particles move towards one electrode. This is electrophoresis, and it proves that the particles are charged: positive particles move to the cathode and negative ones to the anode. If the particles are prevented from moving (for example by a membrane), the dispersion medium moves instead in the field: this is electroosmosis.

Figure 10: Charged sol particles move in an electric field (electrophoresis): a negative sol rises on the anode side. The Cottrell precipitator uses the same idea to discharge smoke particles, which settle as ash.

Key idea

Scattering (Tyndall), zig-zag motion (Brownian) and charge (double layer, electrophoresis) are the three properties that set colloids apart from true solutions.

Quick Recall: tap to check

Why does a true solution not show the Tyndall effect?

Its particles (

< 1

nm) are far smaller than the wavelength of light, so they scatter too little.

Charge on the sol when KI is added to excess

AgN O_{3}

Positive,

AgI/A g^{+}

What moves in electroosmosis?

The dispersion medium, when the colloidal particles are held back.

7. Coagulation

Coagulation (precipitation) is the settling of colloidal particles after their charge is removed: the particles come together, grow and settle under gravity. A lyophobic sol can be coagulated by:

Electrophoresis: particles reach the oppositely charged electrode, lose their charge and settle.
Mixing two oppositely charged sols in about equal amounts: charges neutralise (mutual coagulation), for example hydrated ferric oxide (positive) with arsenious sulphide (negative).
Boiling: more collisions with medium molecules disturb the adsorbed layer and reduce the charge.
Persistent dialysis: removes the traces of electrolyte that keep the sol stable.
Adding electrolytes: ions of charge opposite to the sol (the coagulating ions) neutralise it. A negative ion coagulates a positive sol and a positive ion a negative sol.

7.1 Hardy-Schulze rule and coagulating value

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

Negative sol (e.g. $A s_{2} S_{3}$ ): $A l^{3 +}$ $>$ $B a^{2 +}$ $>$ $N a^{+}$ .
Positive sol (e.g. $F e_{2} O_{3} \cdot x H_{2} O$ ): $[Fe (CN)_{6}]^{4 -} >$ $P O_{4}^{3 -}$ $>$ $S O_{4}^{2 -}$ $>$ $C l^{-}$ .

Figure 11: Hardy-Schulze rule. The coagulating ion carries the charge opposite to the sol, and its power rises steeply with that charge. Bars use the theoretical

z^{6}

law (1 : 64 : 729 : 4096); measured values follow the same order.

Coagulating value is the minimum concentration of an electrolyte, in millimoles per litre, needed to precipitate a sol in two hours. The smaller the coagulating value, the higher the coagulating power of the ion.

Exam Trick Opposite and Higher. Pick the ion with the charge opposite to the sol, then the one with the highest charge. The ion with the same sign as the sol has no role.

JEE Advanced Why charge matters so much. The DLVO theory of colloid stability gives a critical coagulation concentration proportional to

1/ z^{6}

for a counter ion of charge

z

. So the coagulating powers of 1+, 2+ and 3+ ions stand roughly as

1 : 2^{6} : 3^{6} = 1 : 64 : 729

. Measured coagulating values follow the same order of magnitude. Coagulation happens when added electrolyte compresses the diffuse layer and the zeta potential falls below a critical value, so van der Waals attraction wins over electrical repulsion.

7.2 Coagulation of lyophilic sols

Lyophilic sols are stabilised by two factors, charge and solvation. To coagulate one, both must be removed: add an electrolyte and a suitable solvent. Alcohol or acetone dehydrates a hydrophilic sol, and then a little electrolyte coagulates it.

7.3 Protection of colloids and gold number

Lyophilic sols are more stable than lyophobic ones because their particles carry a sheath of the solvent. Added to a lyophobic sol, lyophilic particles form a layer around the lyophobic particles and protect them from electrolytes. Such lyophilic colloids are protective colloids.

Their protective power is measured by the gold number (Zsigmondy): the minimum mass in milligrams of a protective colloid that must be added to 10 mL of a standard red gold sol to prevent its coagulation (red to blue change) on adding 1 mL of 10% NaCl solution. The smaller the gold number, the greater the protective power.

Protective colloid	Gold number (mg)
gelatin	0.005-0.01
haemoglobin	0.03-0.07
egg albumin	0.08-0.10
gum arabic	0.15-0.25
potato starch	about 25

7.4 Solving any coagulation question

Figure 12: Two steps for every coagulation question: fix the sign of the sol, then rank the oppositely charged ions by charge (Hardy-Schulze).

Key idea

Coagulation = removing the charge. The oppositely charged ion does it, and the higher its charge, the smaller the amount needed.

8. Emulsions

An emulsion is a liquid-liquid colloid: fine droplets of one liquid dispersed in another. Shaking two immiscible or partly miscible liquids gives a coarse emulsion. One of the liquids is usually water, so there are two types:

Figure 13: Two types of emulsion. The emulsifier forms an interfacial film around each droplet: its water-loving end faces the water and its oil-loving end faces the oil, so droplets cannot merge.

Oil in water (O/W)

Water is the medium, oil the droplets. Milk (liquid fat in water), vanishing cream. Emulsifiers: proteins, gums, natural and synthetic soaps.

Water in oil (W/O)

Oil is the medium, water the droplets. Butter, cold cream. Emulsifiers: heavy-metal salts of fatty acids, long-chain alcohols, lampblack.

O/W emulsions are unstable and may separate into two layers. An emulsifying agent (emulsifier) is added to stabilise them: it forms an interfacial film between droplet and medium.
An emulsion can be diluted with any amount of its dispersion medium, but the dispersed liquid forms a separate layer. So milk mixes with water (O/W), while butter does not.
Droplets are often negatively charged and can be precipitated by electrolytes. Emulsions show Brownian movement and the Tyndall effect.
Emulsions are broken (demulsified) by heating, freezing or centrifuging. Making an emulsion is emulsification.

9. Colloids Around Us and Their Uses

Example or application	What happens (colloid idea)
Blue colour of the sky	dust and water droplets scatter blue light (Tyndall scattering)
Fog, mist, clouds and rain	air cooled below its dew point condenses on dust particles as colloidal droplets; droplets grow into rain, or oppositely charged clouds meet
Artificial rain	electrified sand or an oppositely charged sol sprayed on clouds coagulates the droplets
Food: milk, butter, halwa, ice cream, fruit juice	all colloids of one kind or another
Blood	colloidal solution of an albuminoid substance; alum or $FeC l_{3}$ stops bleeding by coagulating it (styptic action)
Soils	fertile soil is colloidal; humus acts as a protective colloid and soil holds moisture and nutrients
Delta formation	river water (colloidal clay) meets electrolytes of sea water and coagulates
Cottrell smoke precipitator	charged smoke particles are discharged at electrodes and settle as ash
Purification of drinking water	alum coagulates suspended impurities
Medicines	argyrol (silver sol, eye lotion), colloidal antimony (kala-azar), colloidal gold (injection), milk of magnesia (emulsion, stomach disorders); large surface, easily assimilated
Tanning of leather	positively charged hide and negatively charged tannin coagulate each other; chromium salts are also used
Cleansing action of soaps and detergents	micelle formation and emulsification
Photographic plates and films	emulsion of light-sensitive AgBr in gelatin on glass or celluloid
Rubber industry	latex is a sol of negatively charged rubber particles; rubber is obtained by coagulating it
Industrial products	paints, inks, synthetic plastics, rubber, graphite lubricants and cement are colloids

Quick Recall: tap to check

Which ion is most effective for coagulating

A s_{2} S_{3}

sol:

N a^{+}

B a^{2 +}

A l^{3 +}

A l^{3 +}

: opposite charge to the negative sol and the highest valence.

A smaller gold number means?

A better protective colloid (gelatin is the best common one).

Milk and butter: which type of emulsion each?

Milk is O/W; butter is W/O.

9.1 The whole concept at a glance

Figure 14: The whole concept on one page. Revise from the centre outwards.

10. Solved Examples

Solved Example 1

Why is it essential to wash a precipitate with water before estimating it quantitatively? (NCERT Intext 5.8)

Solution:

A freshly formed precipitate adsorbs ions from the solution on its surface (the ions common to it are held most). These adsorbed impurities would add to the mass and give a wrong result, so they are washed off with water before drying and weighing.

Solved Example 2

A cube of a solid of edge 1 cm is broken into cubes of edge 10 nm. Find the total surface area before and after.

Solution:

Before: $6 \times (1 cm)^{2} = 6$ cm $^{2}$ .

Number of small cubes $= (\frac{1 cm}{10 nm})^{3} = (\frac{1 0 ^{- 2}}{1 0 ^{- 8}})^{3} = 1 0^{18}$ .

A = 1 0^{18} \times 6 \times (1 0^{- 8} m)^{2} = 6 \times 1 0^{2} m^{2} = 600 m^{2}

The area grows a million times, from 6 cm $^{2}$ to 600 m $^{2}$ , which is why colloids are such good adsorbents.

Solved Example 3

Which electrolyte is most effective in coagulating an

A s_{2} S_{3}

sol?
(A) NaCl
(B)

BaC l_{2}

(C)

AlC l_{3}

(D) KCl

Solution:

Answer: (C). $A s_{2} S_{3}$ sol is negative, so cations coagulate it. $A l^{3 +}$ has the highest charge (Hardy-Schulze rule).

Solved Example 4

The correct order of coagulating power of anions for a positively charged hydrated ferric oxide sol is
(A)

C l^{-}

>

S O_{4}^{2 -}

>

P O_{4}^{3 -}

>

[Fe (CN)_{6}]^{4 -}

(B)

[Fe (CN)_{6}]^{4 -}

>

P O_{4}^{3 -}

>

S O_{4}^{2 -}

>

C l^{-}

(C)

S O_{4}^{2 -}

>

C l^{-}

>

P O_{4}^{3 -}

>

[Fe (CN)_{6}]^{4 -}

(D)

P O_{4}^{3 -}

>

[Fe (CN)_{6}]^{4 -}

>

C l^{-}

>

S O_{4}^{2 -}

Solution:

Answer: (B). Anions coagulate a positive sol, and power rises with the size of the charge: ferrocyanide (4−), phosphate (3−), sulphate (2−), chloride (1−).

Solved Example 5

A dilute

AgN O_{3}

solution is added drop by drop to excess dilute KI solution. What is the charge on the AgI sol formed, and to which electrode will its particles move in electrophoresis?

Solution:

KI is in excess, so AgI adsorbs the common ion $I^{-}$ from the medium: $AgI/ I^{-}$ , a negative sol ( $K^{+}$ ions form the diffuse layer). Negative particles move to the anode.

Solved Example 6

The coagulating values of NaCl and

BaC l_{2}

for a sol are 52 and 0.69 mmol L

^{- 1}

. Compare their coagulating powers and state the sign of the charge on the sol.

Solution:

\frac{power of Ba ^{2 +}}{power of Na ^{+}} = \frac{52}{0.69} \approx 75

$B a^{2 +}$ is about 75 times more effective. The two salts share $C l^{-}$ , so the difference comes from the cations: the sol is negatively charged.

Solved Example 7

Gold numbers of four protective colloids are: gelatin 0.005, egg albumin 0.08, gum arabic 0.15, potato starch 25. The best protective colloid is
(A) gelatin
(B) egg albumin
(C) gum arabic
(D) potato starch

Solution:

Answer: (A). The smaller the gold number, the less of the colloid is needed to protect the gold sol, so gelatin protects best and starch worst.

Practice Questions

What modification can you suggest in the Hardy-Schulze law? (NCERT Intext 5.7)Answer: It considers only the valence of the coagulating ion. It could also include the size (hydration) of the ion, and the fact that oppositely charged sols coagulate each other (mutual coagulation).
How are colloids classified on the basis of (i) physical states of components (ii) nature of dispersed phase (iii) interaction between dispersed phase and medium?Answer: (i) Eight types: sol, gel, emulsion, foam, aerosol, solid sol. (ii) Multimolecular, macromolecular, associated. (iii) Lyophilic and lyophobic.
What are lyophilic and lyophobic sols? Give one example each. Why are hydrophobic sols easily coagulated?Answer: Lyophilic: solvent-loving, e.g. starch; lyophobic: solvent-hating, e.g. gold sol. Hydrophobic sols are stabilised only by charge, which a little electrolyte neutralises.
Difference between multimolecular and macromolecular colloids, with one example each. How are associated colloids different?Answer: Multimolecular: aggregates of small units (sulphur sol); macromolecular: single giant molecules (starch). Associated colloids are electrolytes that form micelles only above CMC and $T_{k}$ .
Explain what is observed when (i) a beam of light is passed through a sol (ii) NaCl is added to hydrated ferric oxide sol (iii) an electric current is passed through a sol.Answer: (i) Tyndall effect, path lit up. (ii) The positive sol is coagulated by $C l^{-}$ . (iii) Electrophoresis: particles move to one electrode.
What are emulsions? What are their types? Give an example of each.Answer: Liquid-in-liquid colloids. O/W, e.g. milk; W/O, e.g. butter.
How do emulsifiers stabilise an emulsion? Name two emulsifiers.Answer: They form an interfacial film around the droplets that keeps them apart. Soaps, proteins, gums (O/W); heavy-metal soaps, long-chain alcohols (W/O).
Action of soap is due to emulsification and micelle formation. Comment.Answer: Soap ions form micelles around grease (tails in grease, heads in water), emulsifying it; the charged sheath keeps droplets apart so they are washed away.
Explain: (i) electrophoresis (ii) coagulation (iii) dialysis (iv) Tyndall effect.Answer: (i) Movement of sol particles in an electric field. (ii) Settling of particles when their charge is removed. (iii) Removal of dissolved impurities through a membrane. (iv) Scattering of light by colloidal particles.
Give four uses of emulsions.Answer: Milk and butter in food; cleansing action of soap; photographic films (AgBr in gelatin); medicines such as milk of magnesia; creams in cosmetics.
What are micelles? Give an example of a micellar system.Answer: Aggregates of surfactant ions formed above CMC and $T_{k}$ ; soap (sodium stearate) in water.
Explain the terms: (i) alcosol (ii) aerosol (iii) hydrosol.Answer: Sol in alcohol; colloid in a gas (smoke, fog); sol in water (starch in water).
Comment on the statement that colloid is not a substance but a state of a substance.Answer: The same substance can be a true solution or a colloid depending on particle size: NaCl dissolves in water but gives a sol in benzene; soap is a true solution below CMC and a colloid above it.

Common Mistakes to Avoid

Watch out

Counting gas in gas as a colloid. Gases always form a homogeneous mixture.
Using the ion with the same charge as the sol in Hardy-Schulze questions. Only the oppositely charged ion coagulates.
Mixing up sol charges: $A s_{2} S_{3}$ and metal sols (Au, Ag, Cu) are negative; hydrated ferric oxide, methylene blue and haemoglobin are positive.
Reading a larger gold number as better protection. A smaller gold number means a better protective colloid.
Reading a larger coagulating value as greater coagulating power. It is the reverse.
Saying micelles form at any concentration. They need concentration above the CMC and temperature above the Krafft temperature.
Thinking dialysis removes the colloidal particles. It removes crystalloids (ions, small molecules); the sol stays in the bag.
Calling butter O/W. Butter and cold cream are W/O; milk and vanishing cream are O/W.

Frequently Asked Questions

What is a colloid in chemistry?

A colloid is a heterogeneous system in which particles of one substance, the dispersed phase, with diameters between 1 and 1000 nm, are spread through another substance, the dispersion medium. Colloids lie between true solutions and suspensions; examples are milk, smoke, gold sol and starch solution.

What is the difference between lyophilic and lyophobic sols?

Lyophilic sols such as starch and gelatin form by direct mixing, are reversible and are stabilised by both charge and solvation, so they are stable. Lyophobic sols such as gold and arsenious sulphide need special methods, are irreversible, depend only on charge and are easily coagulated by electrolytes.

What is the Tyndall effect?

The Tyndall effect is the scattering of light by colloidal particles, which makes the path of a beam visible as a bright cone when viewed at right angles. It needs particles not much smaller than the wavelength of light and a large difference in refractive index. True solutions do not show it.

Why do colloidal particles carry an electric charge?

Colloidal particles become charged by preferentially adsorbing ions common to them from the medium, by electron capture in Bredig's arc, or through an electrical double layer. For example, silver iodide adsorbs iodide ions when potassium iodide is in excess and becomes negative. Like charges repel and keep the sol stable.

What is the Hardy-Schulze rule?

The Hardy-Schulze rule says that coagulation is caused by the ion carrying charge opposite to the sol particles, and the greater its valence, the greater its coagulating power. For a negative sol the order is Al3+ greater than Ba2+ greater than Na+; for a positive sol ferrocyanide greater than phosphate greater than sulphate greater than chloride.

What are micelles and what is the critical micelle concentration?

Micelles are aggregates of surfactant ions, such as soap or detergent ions, with hydrophobic tails inside and charged heads outside. They form only above the critical micelle concentration, about 0.0001 to 0.001 mol per litre for soaps, and above the Krafft temperature. Below these limits the ions stay separate.

Are colloids in the JEE Main and NEET syllabus?

No. Surface Chemistry, which includes colloids, emulsions and micelles, has been removed from the JEE Main and NEET syllabi after NCERT dropped the chapter from the rationalised Class 12 textbook. It remains in JEE Advanced, which lists colloid types, preparation, general properties, emulsions, surfactants and micelles.

What does JEE Advanced ask from colloids?

The JEE Advanced syllabus lists colloid types, methods of preparation and general properties, and elementary ideas of emulsions, surfactants and micelles. Questions test Hardy-Schulze and coagulating values, sign of charge from the order of mixing, electrophoresis, zeta potential and multiple-correct statements on colloid properties.

Previous year questions on Colloids

1 question from past papers, each with a step-by-step solution.

JEE Advanced 2023 Paper 2, Chemistry Section 1 Q2

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