Explain the reason for formation of delta at the mouth of the river where it meets the sea. [2] **OR** Explain

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Q. Explain the reason for formation of delta at the mouth of the river where it meets the sea. [2] OR Explain why the solid catalyst is used in a finely divided form in case of heterogeneous catalysis. [2]

West Bengal WbchseWest Bengal HS (WBCHSE) Board 2025Subjective· 2mImportance★★★★★

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Delta formation is colloid coagulation by electrolytes; catalysts are used finely divided to maximise surface area for adsorption.

River water carries suspended clay and silt as a negatively-charged colloidal sol, kept stable (dispersed) by the mutual repulsion between the similarly-charged particles as long as the electrolyte concentration is low, as in fresh river water. When the river meets the sea, the colloidal particles suddenly encounter the much higher concentration of dissolved electrolytes (ions like $Na^+$ , $Mg^{2+}$ , $Ca^{2+}$ ) present in seawater. These ions neutralise the charge stabilising the sol, causing the colloidal clay/silt particles to coagulate (flocculate) into larger aggregates that are no longer able to stay suspended, and they settle out at the river mouth — building up the delta over time.

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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).

…

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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. …
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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. …
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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 …
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Q.Explain the reason for formation of delta at the mouth of the river where it meets the sea. [2] OR Explain why the solid catalyst is used in a finely divided form in case of heterogeneous catalysis. [2]

Concept understanding — Coagulation and Emulsions

Q.Explain the reason for formation of delta at the mouth of the river where it meets the sea. [2] OR Explain why the solid catalyst is used in a finely divided form in case of heterogeneous catalysis. [2]

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