In solid state chemistry, which of the following substances is an amorphous solid rather than a well ordered crystalline solid?

Introduction / Context:
Solids can be broadly classified as crystalline or amorphous. Crystalline solids have a long range, regularly repeating arrangement of particles, while amorphous solids lack such long range order and behave more like very viscous liquids frozen in place. Distinguishing between these types of solids is important in materials science, glass technology and basic chemistry. This question asks you to identify which listed substance is amorphous.

Given Data / Assumptions:

• The substances listed are quartz glass (SiO2), chrome alum, silicon carbide (SiC) and graphite.
• We assume standard forms of these materials as commonly encountered in laboratory and industrial contexts.
• We use the standard definitions of crystalline and amorphous solids from solid state chemistry.
• The focus is on structural order, not on conductivity or other properties.

Concept / Approach:
Quartz in its crystalline form is a well ordered crystal of silicon dioxide, but when SiO2 is rapidly cooled from the melt without time to crystallise, it forms glass. This glassy form of SiO2 is an amorphous solid, known as fused silica or quartz glass. Chrome alum is a crystalline double salt of chromium and potassium, silicon carbide is a covalent network crystal with a well defined lattice, and graphite is a crystalline allotrope of carbon with layered hexagonal sheets. Therefore, among the options, quartz glass is the amorphous solid.

Step-by-Step Solution:
Step 1: Recall that amorphous solids lack long range order, and glass is a classic example of an amorphous material.Step 2: Quartz glass (SiO2) refers specifically to the glassy, non crystalline form of silicon dioxide obtained by cooling the melt quickly.Step 3: Chrome alum is a double salt that crystallises from aqueous solution into well defined crystals, indicating a crystalline structure.Step 4: Silicon carbide (SiC) forms a highly ordered covalent network, used as an abrasive and in high temperature applications, and is crystalline.Step 5: Graphite, though soft, consists of carbon atoms arranged in well ordered hexagonal layers and is considered crystalline.Step 6: Thus, among the choices, only quartz glass (SiO2) is an amorphous solid.

Verification / Alternative check:
Another way to confirm is to think of how these materials behave with respect to melting and cleavage. Glass does not have a sharp melting point; instead, it softens over a range of temperatures and breaks with irregular fracture surfaces, which are typical properties of amorphous solids. Crystalline substances like chrome alum, silicon carbide and graphite have more definite melting points and cleave or fracture along specific planes related to their crystal lattices. These observations support the classification of quartz glass as amorphous and the others as crystalline.

Why Other Options Are Wrong:
Option b, chrome alum, is known to crystallise, often forming violet crystals used in laboratories. Option c, silicon carbide, occurs in well defined crystalline forms such as moissanite and is used as a hard, crystalline abrasive. Option d, graphite, has a layered crystal structure responsible for its lubricating properties and anisotropic electrical conductivity. None of these have the disordered, glassy structure characteristic of amorphous solids.

Common Pitfalls:
A common misunderstanding is to confuse softness or brittleness with being amorphous. For example, graphite is soft and easily marks paper, but it is still crystalline. Another pitfall is to assume that any material containing the word "quartz" must be crystalline, ignoring the fact that quartz glass refers specifically to a glassy, amorphous form of SiO2. To avoid such errors, focus on whether the solid has a regular long range ordering of its particle positions, not just on its mechanical strength or name.

Final Answer:
The amorphous solid among the given substances is quartz glass (SiO2).