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ChemistrySurface Chemistry
Which of the following statements about colloidal solutions are correct?
A. Colloidal particles show Tyndall effect.
B. Colloidal particles do not pass through ordinary filter paper.
C. Colloidal particles show Brownian motion.
D. Addition of electrolyte causes coagulation of lyophilic sols more easily than lyophobic sols.
Options
1
A and B only
2
A, B and C only
3
A and C only
4
A, C and D only
Correct Answer
Option 3 : A and C only
Solution — Each Statement
A

Statement A — TRUE ✓

Colloidal particles (1–1000 nm) scatter light — this is the Tyndall effect. When a beam of light passes through a colloid, the path becomes visible as a bright cone. True solutions don't show this (particles too small). Suspensions show it too (particles too large, cause turbidity).

B

Statement B — FALSE ✗

Colloidal particles DO pass through ordinary filter paper (pore size ~1000 nm, colloidal particles 1–1000 nm — most pass through). They are retained only by special membranes like semipermeable membranes used in dialysis. This is how dialysis purifies colloidal sols.

C

Statement C — TRUE ✓

Colloidal particles show Brownian motion — random, zigzag motion due to continuous bombardment by solvent molecules. This keeps the colloid from settling (prevents sedimentation). The motion was first observed by botanist Robert Brown for pollen grains in water.

D

Statement D — FALSE ✗

It's the OPPOSITE — lyophobic sols coagulate much MORE easily than lyophilic sols. Lyophilic sols (e.g., starch, gelatin) are stabilised by the thick hydration shell and strong solvent interaction, making them resistant to coagulation. Lyophobic sols (e.g., Fe(OH)₃, As₂S₃) have no such protection and coagulate easily with small amounts of electrolyte.

Theory: Colloids & Surface Chemistry
1. Classification of Dispersions by Particle Size

📌 True solution: particle size <1 nm, transparent, no Tyndall, passes semipermeable membrane

📌 Colloidal solution: particle size 1–1000 nm, translucent, Tyndall effect, doesn't pass semipermeable membrane

📌 Suspension: particle size >1000 nm, opaque, settles on standing, retained by filter paper

📌 Colloidal particles pass through ordinary filter paper but NOT through semipermeable membrane

2. Tyndall Effect

When a beam of light passes through a colloidal solution, colloidal particles scatter the light in all directions, making the path of light visible (bright cone). This is called the Tyndall effect or Tyndall cone. It is NOT shown by true solutions (particles too small to scatter) but IS shown by colloids and suspensions. The effect distinguishes colloids from true solutions. Real-world examples: beam of sunlight through dust, car headlights in fog, light through milk.

3. Brownian Motion

Colloidal particles are in constant, random, zigzag motion called Brownian motion. This is caused by unequal bombardment of colloidal particles by solvent molecules from different directions. Brownian motion prevents sedimentation of colloidal particles — it counteracts the effect of gravity. It was explained by Einstein in 1905 and provided direct evidence for the existence and motion of atoms and molecules.

4. Electrophoresis and Charge on Colloids

Colloidal particles carry an electric charge (either positive or negative). When an electric field is applied, charged colloidal particles migrate toward the oppositely charged electrode — this is electrophoresis. Positive sols: Fe(OH)₃, Al(OH)₃, TiO₂. Negative sols: As₂S₃, clay, starch, gold sol. The charge on colloids helps stabilise them by causing electrostatic repulsion between particles.

5. Coagulation — Lyophilic vs Lyophobic

Coagulation is the process of precipitating colloidal particles by destroying their stability. Lyophobic sols (water-hating, e.g., metal hydroxides, sulphides) are stabilised only by the surface charge — adding electrolyte neutralises the charge, allowing particles to aggregate and settle. They coagulate easily. Lyophilic sols (water-loving, e.g., starch, gelatin, proteins) are stabilised by both charge AND strong solvation (thick hydration shell) — they require far more electrolyte to coagulate. The Hardy-Schulze rule applies to lyophobic sols: higher valence of oppositely charged ion → greater coagulating power.

6. Hardy-Schulze Rule

Coagulating power ∝ valence of oppositely charged ion

For positive sol: coagulating power: PO₄³⁻ > SO₄²⁻ > Cl⁻

For negative sol: coagulating power: Al³⁺ > Ba²⁺ > Na⁺

This rule explains why alum (KAl(SO₄)₂·12H₂O) is effective in water purification — Al³⁺ ions efficiently coagulate negatively charged clay and mud particles in river water.

7. Adsorption vs Absorption

Adsorption: accumulation of molecules (adsorbate) on the surface of a solid (adsorbent). It is a surface phenomenon. Physical adsorption (physisorption): weak van der Waals forces, reversible, low temperature, multilayer. Chemical adsorption (chemisorption): strong covalent/ionic bonds, irreversible, high temperature, monolayer. Freundlich adsorption isotherm: x/m = k·p^(1/n), where x/m is amount adsorbed per gram and p is pressure. Factors affecting adsorption: nature of adsorbent, surface area, temperature, nature of adsorbate, pressure.

8. Applications of Colloids

📌 Milk is an emulsion (fat droplets in water) — colloidal system

📌 Fog/mist: liquid droplets in gas (aerosol)

📌 Smoke: solid particles in gas (aerosol)

📌 Blood: colloidal dispersion of proteins, cells in plasma

📌 Medicines: many drugs delivered as colloidal dispersions

📌 Dialysis: purification of colloidal solutions using semipermeable membrane

📌 Electrostatic precipitator: removes colloidal smoke particles using electric field (Cottrell precipitator)

📌 Water purification: alum coagulates mud (negatively charged) particles

Frequently Asked Questions
1. Why do colloidal particles pass through ordinary filter paper but not semipermeable membranes? ⌄
Ordinary filter paper has pores of ~1000–10000 nm — large enough for colloidal particles (1–1000 nm) to pass through. Semipermeable membranes (parchment, cellophane) have pores of ~1 nm — only true solution particles (ionic/molecular, <1 nm) can pass. This is why dialysis (using semipermeable membranes) can separate colloidal particles from crystalloids (small molecules/ions).
2. How does Tyndall effect differ between sol and true solution? ⌄
True solution (e.g., NaCl in water): no Tyndall effect because ions/molecules (less than 1 nm) are too small to scatter visible light appreciably. Colloidal sol (e.g., Fe(OH)₃ sol): shows bright Tyndall cone because colloidal particles (1–1000 nm) are comparable in size to wavelengths of visible light and scatter it effectively.
3. What is dialysis and when is it used? ⌄
Dialysis removes crystalloids (small ions and molecules) from a colloidal solution using a semipermeable membrane. The colloid is placed in a bag made of semipermeable membrane (e.g., cellophane) and placed in flowing water. Small ions pass out through the membrane; large colloidal particles are retained. Used medically in kidney dialysis — removes waste small molecules from blood while retaining large protein molecules.
4. What is the Hardy-Schulze rule? ⌄
The coagulating power of an electrolyte for a lyophobic sol depends on the valence (charge) of the oppositely charged ion — higher valence = greater coagulating power. For positive sol (Fe(OH)₃): PO₄³⁻ > SO₄²⁻ > Cl⁻ (trivalent > divalent > monovalent). For negative sol (As₂S₃): Al³⁺ > Ba²⁺ > Na⁺. The minimum concentration of electrolyte required to coagulate 1L of sol in 2 hours is called flocculation value.
5. Why is gelatin added to ice cream? ⌄
Gelatin is a lyophilic colloid (protein) that forms a protective layer around ice crystal (lyophobic) particles. This prevents ice crystals from growing (protecting against coagulation), maintains smooth texture, and prevents formation of large ice crystals during storage. It acts as a protective colloid — the lyophilic gelatin increases the stability of the lyophobic ice crystal dispersion.
6. What is electrophoresis used for practically? ⌄
Gel electrophoresis separates DNA, RNA, and proteins by size using electric field — backbone of molecular biology. Blood typing and protein analysis use electrophoresis. Industrial: painting car bodies (electrodeposition — charged paint particles migrate to oppositely charged metal surface). Medical: detecting sickle cell anaemia (abnormal haemoglobin migrates differently). Forensics: DNA fingerprinting.
7. What is the Freundlich adsorption isotherm? ⌄
x/m = k·p^(1/n), where x = mass adsorbed, m = mass of adsorbent, p = pressure, k and n are constants. Log(x/m) = log k + (1/n)log p — a straight line when log(x/m) plotted vs log p. Slope = 1/n (between 0 and 1), intercept = log k. It's empirical (not derived from theory), so it holds only over limited pressure ranges. Langmuir isotherm is more theoretically rigorous.
8. How does soap clean oily surfaces? ⌄
Soap (RCOONa) has a hydrophilic head (COO⁻Na⁺) and hydrophobic tail (R — long carbon chain). In water, soap molecules arrange around oil droplets with tails pointing into the oil and heads pointing outward into water — forming micelles. The oil droplet is now enclosed in a negatively charged shell (COO⁻ groups), keeping it dispersed in water and easily washed away. This emulsification is the basis of cleansing action.
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