Question

Given below are two statements
Statement I: The permanent negative charges result from the isomorphous substitution
Statement II: The permanent negative charges result from exposed crystal edges
In light of the above statements, choose the correct answer from the options given below

• A. Both Statement I and Statement II are true
• B. Both Statement I and Statement II are false
• C. Statement I is true but Statement II is false
• D. Statement I is false but Statement II is true

Hint:

Isomorphous Substitution = Permanent Charge (Inside the crystal).
Broken Edges/OH Groups = pH-Dependent Charge (Outside the crystal).
Permanent charge doesn't care about the pH!

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question tests the fundamental understanding of how electrical charges are generated on the surfaces of soil clay minerals.
Detailed Explanation:

• Permanent Charge (Statement I):
Permanent negative charge is "fixed" in the mineral structure during its formation.
It occurs due to isomorphous substitution, where a central cation of higher valence is replaced by one of lower valence without changing the physical structure of the crystal.
For example: $Al^{3+}$ replacing $Si^{4+}$ in the tetrahedral sheet, or $Mg^{2+}$ replacing $Al^{3+}$ in the octahedral sheet. This leaves a net negative charge that is independent of soil pH. Statement I is True.

• pH-Dependent Charge (Statement II):
Charges that result from exposed crystal edges or broken bonds are variable or pH-dependent.
On the edges, hydroxyl groups ($OH$) can either lose a proton (becoming negative at high pH) or gain a proton (becoming positive at low pH).
These are not "permanent" because they change with the acidity of the surrounding solution. Statement II is False.

• Significance:
$2:1$ clays like montmorillonite have mostly permanent charge. $1:1$ clays like kaolinite and humus have mostly pH-dependent charge.

Step 2: Final Answer:
Statement I correctly identifies isomorphous substitution as the source of permanent charge, whereas Statement II incorrectly attributes it to crystal edges.