Match List-I with List-II

List-I (Characteristic)

(A) Si is replaced by Al³⁺ ions

(B) Attachment of Ca²⁺ on clay surface

(C) Na⁺ replacing other cations on clay surface

(D) H dissociated from hydroxyl ions on clay surface

List-II (Terminology)

(I) Ionic double layer

(II) pH dependent charge

(III) Dispersion of clay

(IV) Isomorphous substitution

Choose the correct answer from the options given below:

Correct option is D

Explanation of the Correct Option (d): (A)-(IV), (B)-(I), (C)-(III), (D)-(II)
(A) Si is replaced by Al³⁺ ions → (IV) Isomorphous substitution
· Occurs during clay mineral formation.
· One ion replaces another of similar size without changing structure.
· Replacement of Si⁴⁺ by Al³⁺ creates permanent negative charge.
(B) Attachment of Ca²⁺ on clay surface → (I) Ionic double layer
· Ca²⁺ is a divalent cation.
· Strongly held near negatively charged clay surfaces.
· Forms a compressed ionic double layer, improving aggregation.
(C) Na⁺ replacing other cations on clay surface → (III) Dispersion of clay
· Na⁺ is monovalent with large hydration radius.
· Causes expansion of diffuse double layer.
· Leads to clay dispersion and poor soil structure.
(D) H dissociated from hydroxyl ions on clay surface → (II) pH dependent charge
· Charge develops due to protonation or deprotonation.
· Depends on soil pH.
· Common in oxide and organic matter surfaces.
Explanation of Other Options
Option (a)
· Incorrect matching of isomorphous substitution and dispersion.
Option (b)
· Ca²⁺ does not cause dispersion; it improves aggregation.
Option (c)
· Si–Al replacement is not related to ionic double layer.
Key Points Related to the Question
· Isomorphous substitution → permanent charge
· pH-dependent charge → variable charge
· Na⁺ saturation → clay dispersion
· Ca²⁺ saturation → flocculation