Explain what is meant by coagulation of a colloidal sol, and state the Hardy–Schulze rule. For the coagulation of a negatively charged $ ext{As}_2 ext{S}_3$ sol, arrange the ions $ ext{Na}^+$, $ ext{Ba}^{2+}$ and $ ext{Al}^{3+}$ in increasing order of their coagulating power

Question

Explain what is meant by coagulation of a colloidal sol, and state the Hardy–Schulze rule. For the coagulation of a negatively charged $	ext{As}_2	ext{S}_3$ sol, arrange the ions $	ext{Na}^+$, $	ext{Ba}^{2+}$ and $	ext{Al}^{3+}$ in increasing order of their coagulating power.

Accepted Answer

Coagulation is the process by which a lyophobic sol is destabilized and made to settle out, typically by adding an electrolyte whose ions carry a charge opposite to that of the sol particles; these ions are adsorbed onto the particle surfaces and neutralize the repulsive charge that had been keeping the particles apart, allowing them to aggregate and grow too heavy to remain dispersed. The Hardy–Schulze rule states that the coagulating power of an ion of opposite charge increases sharply, and disproportionately, with the magnitude of that ion's charge. For the negatively charged $\text{As}_2\text{S}_3$ sol, coagulation is brought about by cations, so a higher cation charge coagulates more effectively: $\text{Al}^{3+}$ (trivalent) is far more effective than $\tex …

Start your 14-day free trial to unlock the full solution →

Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2026Set SEM31 markMCQ
$Q. Lyophobic sol can be protected / stabilised by$
(a) Boiling
(b) Addition of electrolyte
(c) Addition of coagulating agent
(d) Addition of lyophilic sol
›Reveal solutionSolution

Lyophobic sols are unstable and easily coagulated; adding a lyophilic sol (protective colloid) coats the particles and stabilises them. Correct option (d).

Lyophobic (solvent-hating) sols are only weakly stabilised and coagulate readily. Adding a lyophilic (solvent-loving) sol makes the lyophilic particles form a protective layer around the lyophobic particles, preventing them from coming together - this is protective action (measured by the gold number).

…

Start your 14-day free trial to see the full solution →
CBSE 2026Set SEM31 markMCQ
$Q. Which one of the following does not involve coagulation?$
(a) Use of NaCl to stop bleeding
(b) Formation of delta
(c) Peptization
(d) Treatment of pond's water by potash alum
›Reveal solutionSolution
Coagulation is aggregation of colloidal particles; peptization is the opposite (breaking a precipitate into a sol). So the process not involving coagulation is peptization. Correct option (c).

Examining each:

Use of NaCl to stop bleeding: electrolyte coagulates the colloidal blood -> coagulation.

Formation of delta: river (colloidal clay) sol coagulated by electrolytes in sea water -> coagulation.

Treatment of pond water with potash alum: Al3+ ions coagulate suspended colloidal impurities -> coagulation. …
Start your 14-day free trial to see the full solution →
CBSE 2025Set ANNUAL1 mark
$Q. Between butter and milk which one is oil in water type emulsion and which one is water in oil type emulsion?$
›Reveal solutionSolution
Milk = fat droplets dispersed in water (O/W); butter = water droplets dispersed in fat (W/O).

An emulsion is a colloidal dispersion of one liquid in another immiscible liquid. There are two types, distinguished by which liquid forms the dispersed phase and which forms the continuous (dispersion) medium:

Oil-in-water (O/W): oil (or fat) droplets are dispersed throughout a continuous water phase.

Water-in-oil (W/O): water droplets are dispersed throughout a continuous oil/fat phase. …
Start your 14-day free trial to see the full solution →
CBSE 2023Set ANNUAL1 mark
$Q. Which of the following salts is most effective for coagulating ferric hydroxide sol? Na2SO4, FeCl3, K4[Fe(CN)6]$
›Reveal solutionSolution
Fe(OH)3 sol carries a positive charge, so negatively charged ions coagulate it, and the higher the charge on that ion, the more effective it is (Hardy-Schulze rule).

Colloidal Fe(OH)3 particles adsorb Fe3+ ions preferentially and are therefore positively charged. According to the Hardy-Schulze rule, the ion of opposite charge to the sol is responsible for coagulation, and its coagulating power increases sharply with its charge (roughly with the 6th power of the charge, empirically).

Comparing the anions available:

Na2SO4 supplies SO4^2- (charge -2)

FeCl3 supplies Cl- (charge -1) — and its Fe3+ cation would actually stabilise, not coagulate, the positive Fe(OH)3 sol …
Start your 14-day free trial to see the full solution →

Q.Explain what is meant by coagulation of a colloidal sol, and state the Hardy–Schulze rule. For the coagulation of a negatively charged $\text{As}_2\text{S}_3$ sol, arrange the ions $\text{Na}^+$ , $\text{Ba}^{2+}$ and $\text{Al}^{3+}$ in increasing order of their coagulating power.

Concept understanding — Coagulation and Emulsions

🎓Unlock everything free for 14 days

Unlock everything free for 14 days