Question

Which of the following is the only incorrect statement regarding amorphous solids? (a) On heating, they may become crystalline at some temperature. (b) They may become crystalline on keeping for long time. (c) Amorphous solids can be moulded by heating. (d) They are anisotropic in nature.

Short answer

The only incorrect statement regarding amorphous solids is (d) They are anisotropic in nature.

Step-by-step solution

Understanding Amorphous Solids

Familiarize yourself with the characteristics of amorphous solids. Amorphous solids are known to be isotropic, as they do not have a long-range order or a regular arrangement of their particles. They may change their state upon heating or over time.

Analyzing the Given Statements

Evaluate each of the given statements to determine which describes a property not consistent with amorphous solids. Statements (a), (b), and (c) correctly describe properties of amorphous solids, while statement (d) asserts that they are anisotropic, which contradicts the isotropic nature of amorphous solids.

Identifying the Incorrect Statement

Identify the statement that is not true for amorphous solids. Since amorphous solids are isotropic and not anisotropic, statement (d) 'They are anisotropic in nature' is the only incorrect statement.

Key concepts

Isotropic Materials

Isotropic materials are those that have the same physical properties in all directions. When we refer to solids, this implies that properties such as refractive index, thermal conductivity, and mechanical strength are uniform regardless of which direction you measure them. This characteristic is due to the lack of a long-range, ordered molecular structure; the particles inside isotropic materials are arranged randomly.

Imagine it like a crowd of people, all standing at different angles and distances from one another, compared to an army standing in perfect rows and columns. Materials like glasses and plastics are often isotropic. They contrast with anisotropic materials, such as many crystals and woods, where the properties depend on the direction of measurement. This fundamental difference is critical for applications in optics, electronics, and materials science where uniform behavior is desired in all directions.

Crystallization of Amorphous Solids

The process where amorphous solids transform into crystalline form is known as crystallization. Amorphous solids, like glass or certain plastics, lack a well-defined long-range order found in crystals. However, under certain conditions, such as heating to a specific temperature, these solids can begin to reorganize their structure to form a regular, repeating pattern characteristic of crystals.

For example, when glass is heated, it can reach a point called the glass transition temperature. Above this temperature, it becomes more fluid and if cooled slowly, can crystallize. This transition is not immediate and may vary depending on factors such as the rate of temperature change and the presence of impurities that could catalyze or interfere with the ordering process. The crystallization of amorphous solids has tremendous technological importance in manufacturing devices, where precise material properties are required.

Properties of Amorphous Solids

Amorphous solids have distinct properties that set them apart from their crystalline counterparts. Due to their disordered internal structure, they often exhibit flexibility in their form and can be molded when heated, making them invaluable for manufacturing a wide range of products.

These solids generally show a gradual softening upon heating, rather than a sharp melting point typical of crystalline substances. This softening is due to the lack of a rigid crystal lattice, allowing the molecules to move more freely at higher temperatures. Additionally, amorphous solids can be transparent, as their disordered arrangement does not produce the scattering of light that occurs in crystalline solids.

Their resistance to crystallization over time, sometimes referred to as physical stability, is another key property. While they may crystallize under the right conditions, amorphous solids can remain in a disordered state for very long periods, contributing to their widespread use in items like amorphous metals, polymers, and certain pharmaceuticals.