What are the 5 key differences between physisorption and chemisorption?

Here are the five differences NEET tests most often. (1) Force of attraction: physisorption uses weak van der Waals forces; chemisorption uses strong covalent or ionic bonds. (2) Enthalpy of adsorption: physisorption has low enthalpy (20-40 kJ/mol); chemisorption has high enthalpy (40-400 kJ/mol). (3) Reversibility: physisorption is reversible (adsorbate can be removed by lowering pressure or raising temperature); chemisorption is irreversible or very difficult to reverse. (4) Layers formed: physisorption forms multi-molecular layers (multilayer); chemisorption forms only a monomolecular layer (monolayer). (5) Effect of temperature: physisorption decreases with increasing temperature (because adsorption is exothermic); chemisorption first increases (to overcome activation energy) then decreases at very high temperature. These five differences appear directly in NEET match-the-following and true/false questions every few years.

What is the Freundlich adsorption isotherm? What are its limitations?

The Freundlich adsorption isotherm is an empirical equation that relates the extent of adsorption (x/m) to the pressure (P) of the adsorbate gas at a fixed temperature. The equation is: x/m = k x P^(1/n), where x = mass of adsorbate, m = mass of adsorbent, k and n are constants that depend on the nature of the adsorbate and adsorbent, and n is always greater than 1. The log form is: log(x/m) = log k + (1/n) x log P. This gives a straight line when you plot log(x/m) on the y-axis and log P on the x-axis. Slope = 1/n, intercept = log k. Limitations: (1) It is purely empirical, with no theoretical basis. (2) It works only at intermediate pressures. At very high pressures, x/m approaches a maximum and the equation fails (the graph should plateau, not keep rising). (3) It is not valid for gases where the pressure is near saturation. Despite these limitations, the Freundlich isotherm graph and the log-form straight line are very frequently tested in NEET.

What are colloidal solutions? How do they differ from true solutions and suspensions?

The key difference is particle size. True solutions have particles smaller than 1 nm (individual ions or molecules). You cannot see these particles and light passes through without scattering. Colloidal solutions (also called colloids or sols) have particle sizes between 1 nm and 1000 nm (1 micrometer). These particles are too small to see individually or to filter, but large enough to scatter light (Tyndall effect). Suspensions have particles larger than 1000 nm. These particles settle on standing and can be filtered. Summary: true solution is homogeneous at the molecular level; colloid is apparently homogeneous but heterogeneous at the microscopic level; suspension is heterogeneous and particles settle. The Tyndall effect is the easiest way to distinguish a colloid from a true solution in NEET: shine a beam of light through a colloid and you see the beam; shine it through a true solution and the beam is invisible.

Explain the Tyndall effect with an example.

When a beam of light passes through a colloidal solution, the colloidal particles scatter the light in all directions. This makes the beam of light visible when you look at it from the side, even though the solution looks clear from above. This scattering of light by colloidal particles is called the Tyndall effect, named after the scientist John Tyndall who discovered it in 1869. The colloidal particles act as tiny scattering centres because their size (1-1000 nm) is comparable to the wavelength of visible light (400-700 nm). True solutions do NOT show the Tyndall effect because the dissolved particles are too small (sub-nanometre) to scatter visible light significantly. Everyday examples: (1) A beam of sunlight through a dusty room shows a visible shaft of light. (2) Car headlights in fog create a visible cone of light in the fog (water droplets in air = aerosol colloid). (3) The blue colour of the sky is due to scattering of sunlight by colloidal-sized particles in the atmosphere. NEET always tests: "Which of the following shows the Tyndall effect?" Answer: colloid; not true solution.

What is the Hardy-Schulze rule? How does the charge of a coagulating ion affect coagulation?

The Hardy-Schulze rule states: the higher the charge (valence) of the coagulating ion, the greater its power to coagulate a colloid. In other words, you need much less of a high-valence ion than a low-valence ion to coagulate the same colloid. This is because colloidal particles are charged. To coagulate them, you need to neutralise their charge by adding an oppositely charged electrolyte ion. A trivalent ion (like Al3+) is much more effective at neutralising the surface charge than a monovalent ion (like Na+). Coagulating power of cations: Al3+ > Ca2+ > Na+ (for a negatively charged colloid like As2S3 sol). Coagulating power of anions: PO43- > SO42- > Cl- (for a positively charged colloid like Fe(OH)3 sol). Key NEET point: the coagulating ion is always the one opposite in charge to the colloid. Negatively charged As2S3 sol is coagulated by cations (Al3+ is most effective). Positively charged Fe(OH)3 sol is coagulated by anions (PO43- is most effective). The coagulating value (concentration needed) is the inverse of coagulating power: lower coagulating value = more effective.

What is the difference between lyophilic and lyophobic colloids?

Lyophilic means "solvent-loving." Lyophobic means "solvent-fearing." Lyophilic colloids: the dispersed phase has a strong affinity for the dispersion medium. Examples: gum, starch, gelatin, proteins, rubber. These colloids are reversible (if you evaporate the solvent and add it back, the colloid re-forms). They are stable and resistant to coagulation because the particles are heavily solvated (surrounded by solvent molecules that form a protective layer). They do NOT need a stabiliser. Their viscosity is much higher than the pure solvent. Lyophobic colloids: the dispersed phase has no affinity for the medium. Examples: gold sol, arsenic sulfide sol, sulfur sol, silver sol. These colloids are irreversible. They are less stable and coagulate readily when electrolyte is added. They require a stabiliser (like a protective colloid or an emulsifier) to remain stable. They do not significantly increase the viscosity of the medium. NEET uses this distinction in coagulation questions: lyophobic colloids coagulate much more easily than lyophilic ones.

What is gold number? Why is it important?

Gold number is the minimum amount (in milligrams) of a protective colloid that must be added to 10 mL of a standard gold sol to prevent coagulation when 1 mL of 10% NaCl solution is added. It was introduced by Zsigmondy. The key thing to remember: a lower gold number means a better (more efficient) protective colloid, because you need less of it to protect the gold sol. Examples of gold numbers: gelatin = 0.005-0.01 mg (very good protective colloid), haemoglobin = 0.03-0.07 mg, gum arabic = 0.15-0.25 mg, potato starch = 25 mg (poor protective colloid). Lyophilic colloids like gelatin and starch act as protective colloids for lyophobic colloids. They coat the lyophobic particles and prevent them from being coagulated by electrolytes. This protective action is used in practical applications: gelatin stabilises ice cream, starch stabilises food emulsions. NEET questions test the concept: "Which has the lowest gold number? Answer: gelatin (best protective colloid)."

How is catalysis different in homogeneous vs heterogeneous systems?

In homogeneous catalysis, the catalyst and the reactants are in the same phase (all gases, or all liquids). The catalyst participates by forming intermediate compounds with the reactants, which then break down to give products and regenerate the catalyst. Examples: (1) Lead chamber process: 2SO2 + O2 + [NO catalyst] → 2SO3, where all components are gaseous. (2) Friedel-Crafts reaction in organic chemistry uses AlCl3 dissolved in the same solvent as the reactants. In heterogeneous catalysis, the catalyst is in a different phase from the reactants. The catalyst is almost always a solid and the reactants are gases or liquids. The mechanism involves adsorption of reactants on the catalyst surface, weakening of bonds, reaction, and desorption of products. Examples: (1) Haber process: N2(g) + 3H2(g) → 2NH3(g) with solid Fe catalyst and Mo/K2O promoters. (2) Contact process: 2SO2(g) + O2(g) → 2SO3(g) with solid V2O5 catalyst. (3) Hydrogenation of alkenes with Ni or Pt catalyst. Heterogeneous catalysis is more commonly tested in NEET than homogeneous catalysis.

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Surface ChemistryNEET Chemistry · Class 12 · NCERT Chapter 14

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9 NEET previous-year questions on Surface Chemistry, each with the correct answer and a step-by-step solution. Sourced directly from official NEET papers across every booklet code.

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Surface Chemistry (9)

Coagulation

Electrophoresis

Electro-osmosis

Tyndall effect

Solution

The Tyndall effect, which is the scattering of light by colloidal particles, does not depend on the charge of the particles. Coagulation, electrophoresis, and electro-osmosis all involve the movement or aggregation of particles due to charge, so option (d) is correct.

∆ S is negative and, therefore, ∆ H should be highly positive.

∆ S is negative and therefore, ∆ H should be highly negative

∆ S is positive and, therefore, ∆ H should be negative

∆ S is positive and, therefore, ∆ H should also be highly positive.

Solution

For spontaneous adsorption of a gas, $Δ S$ is negative due to the decrease in entropy as gas molecules are confined to the surface. For the process to be spontaneous, $Δ G$ must be negative, which requires $Δ H$ to be highly negative to overcome the negative entropy change, so option (b) is correct.

∆G is negative but ∆H and ∆S are positive

∆G,∆H and ∆S all are negative

∆G and ∆H are negative but ∆S is positive

∆G and ∆S are negative but ∆H is positive

Solution

∆H < 0 Adsorption process is exothermic ∆G < 0 Adsorption process is Feasible ∆S < 0 Adsorption process is accompanied of decrease in entropy.

Liquid in gas

Gas in liquid

Solid in gas www.vedantu.com 40

Gas in gas

Solution

Fog is a colloidal solution in which liquid droplets are dispersed in gas.

(iv) (iii) (ii) (i)

(i) (ii) (iii) (iv)

(ii) (iv) (i) (iii)

(iii) (iv) (ii) (i)

Solution

Fact

Viscosity

Solubility

Stability of the colloidal particles

Size of the colloidal particles

Solution

Measuring Zeta potential is useful in determining the stability of colloidal particles, as it indicates the degree of repulsion between adjacent particles. Higher Zeta potential values lead to greater stability by preventing particle aggregation, so option (c) is correct.

Sodium lauryl sulphate

Sodium stearate

Cetyltrimethyl ammonium bromide

Sodium dodecylbenzene sulphonate

Solution

Cetyltrimethyl ammonium bromide is a cationic detergent because it has a positively charged quaternary ammonium ion. Sodium lauryl sulphate, sodium stearate, and sodium dodecylbenzene sulphonate are anionic detergents, so option (c) is correct.

NaCl solution

Glucose solution

Starch solution

Urea solution

Solution

The Tyndall effect is exhibited by colloidal solutions, where the particles scatter light. Starch solution is a colloidal solution, while NaCl, glucose, and urea solutions are true solutions, so option (c) is correct.

Both Statement I and Statement II are correct.

Both Statement I and Statement II are incorrect.

Statement I is correct but Statement II is incorrect.

Statement I is incorrect but Statement II is correct.

Solution

According to hardy Schulze rule • Flocculating power of cation increases with increases in charge on cation of electrolyte in case of negatively charge colloid, hence order is 32Al Ba Na+ + +>> • Flocculating power of anion increases with increases in charge on anion of electrolyte in case o f positively charge colloids Hence order is NaCl < Na2SO4 < Na3PO4

Column - I	Column - II
(a) Pure nitrogen	(i) Chlorine
(b) Haber process	(ii) Sulphuric acid
(c) Contact process	(iii) Ammonia
(d) Deacon’s process	(iv) Sodium azide or Barium azide

Surface ChemistryNEET Chemistry · Class 12 · NCERT Chapter 14

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Surface ChemistryNEET Chemistry · Class 12 · NCERT Chapter 14

1NEET 2014Medium4 bookletsWhich property of colloids is not dependent on the charge on colloidal particles ?

2NEET 2014Medium4 bookletsWhich of the following statements is correct for the spontaneous adsorption of a gas ?

3NEET 2016Medium4 bookletsWhich one of the following characteristics is associated with adsorption?

4NEET 2016Medium4 bookletsFog is a colloidal solution of:

5NEET 2019Medium4 bookletsMatch the following:Column - IColumn - II(a) Pure nitrogen(i) Chlorine(b) Haber process(ii) Sulphuric acid(c) Contact process(iii) Ammonia(d) Deacon’s process(iv) Sodium azide or Barium azideWhich of the following is the correct option?

6NEET 2020Medium24 bookletsMeasuring Zeta potential is useful in determining which property of colloidal solution ?

7NEET 2020Medium24 bookletsWhich of the following is a cationic detergent ?

8NEET 2021Medium24 bookletsThe right option for the statement “Tyndall effect is exhibited by”, is :

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