Question

Assertion (A): Free silica occurs in soil's quartz and opal.
Reason (R): Quartz consists of a continuous framework of silica tetrahedra.

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

Free silica is commonly found as quartz and opal; quartz has a strong tetrahedral SiO$_4$ network.

Answer 1

The Correct Option is A

Quartz and opal are the main sources of free silica in soil. The crystalline structure of quartz is indeed composed of continuous SiO$_4$ tetrahedra, validating the reason.

Answer 2

Structural explanation: Free silica refers to silicon dioxide not chemically combined with other cations in a silicate mineral lattice, and soil quartz and opal are its two common natural forms.
Quartz is fully crystalline, every \( \text{SiO}_4 \) tetrahedron shares all four of its corner oxygen atoms with neighbouring tetrahedra, building an uninterrupted, electrically neutral three-dimensional framework, this is precisely the structural feature the reason describes.
Opal, by contrast, is the hydrated, amorphous form of the same compound, \( \text{SiO}_2 \cdot n\text{H}_2\text{O} \), lacking quartz's long-range order but still counted as free silica.
Because the continuous tetrahedral framework is the structural basis that makes quartz free silica, the reason correctly explains the assertion, so both statements are true and R is the correct explanation of A.