Question

Match List-I with List-II

List-I (Characteristic) & List-II (Terminology)
(A) $\text{Si}^{4+}$ is replaced by $\text{Al}^{3+}$ ions & (I) Ionic double layer
(B) Attachment of $\text{Ca}^{2+}$ on clay surface & (II) pH dependent charge
(C) $\text{Na}^+$ replacing other cations on clay surface & (III) Dispersion of clay
(D) H dissociated from hydroxyl ions on clay surface & (IV) Isomorphous substitution

• (A) - (I), (B) - (II), (C) - (IV), (D) - (III)
• (A) - (III), (B) - (IV), (C) - (I), (D) - (II)
• (A) - (I), (B) - (III), (C) - (IV), (D) - (II)
• (A) - (IV), (B) - (I), (C) - (III), (D) - (II)

Hint:

Isomorphous substitution in the tetrahedral sheet ($\text{Al}^{3+}$ replacing $\text{Si}^{4+}$) or octahedral sheet ($\text{Mg}^{2+}$ replacing $\text{Al}^{3+}$) is the primary source of permanent, pH-independent negative charge in 2:1 clay minerals like montmorillonite.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Soil colloids (clays and organic matter) possess electric charges on their surfaces that regulate nutrient retention, soil structure, and chemical interactions.

Step 3: Detailed Explanation:
Let us analyze and match each characteristic with its correct term:
- (A) $\text{Si}^{4+}$ is replaced by $\text{Al}^{3+}$ ions: This is the replacement of one metal cation by another of similar size within the crystal lattice of clay minerals without changing the mineral's crystal structure.
This process creates a permanent negative charge on the clay and is known as Isomorphous substitution. Thus, (A) matches with (IV).
- (B) Attachment of $\text{Ca}^{2+}$ on clay surface: The electrostatic attraction of hydrated cations (like calcium) to the negatively charged clay surface forms an Ionic double layer. This layer consists of the negatively charged solid surface and a corresponding layer of attracted cations in solution. Thus, (B) matches with (I).
- (C) $\text{Na}^+$ replacing other cations on clay surface: When sodium ($\text{Na}^+$) replaces divalent cations like calcium on the exchange complex, its large hydrated radius weakens the attractive forces between clay platelets.
This causes the soil aggregates to break apart, leading to the Dispersion of clay and soil structure collapse. Thus, (C) matches with (III).
- (D) H dissociated from hydroxyl ions on clay surface: At high soil pH levels, hydrogen ions ($\text{H}^+$) dissociate from the exposed hydroxyl ($\text{OH}$) groups at clay mineral edges or organic matter surfaces, creating a negative charge. This is known as pH dependent charge. Thus, (D) matches with (II).
Combining these matches gives: (A) - (IV), (B) - (I), (C) - (III), (D) - (II).

Step 4: Final Answer:
The correct matching sequence corresponds to option (D).