Problem 94 - A Complete Resource Book in Chemistry for JEE Main

Question

Amorphous substances show (i) short and long range order (ii) short range order (iii) long range order (iv) have no sharp melting point (a) (i) and (ii) are correct (b) (ii) and (iv) are correct (c) (ii), (iii) and (iv) are correct (d) (i) and (iv) are correct

Step-by-Step Solution

Answer

(b) (ii) and (iv) are correct.

Step 1: Understanding Amorphous Substances

Amorphous substances, unlike crystalline substances, do not have a well-defined, repeating lattice structure. This means they do not possess long-range order, which is a characteristic of crystalline materials.

Step 2: Determining Short Range Order

Amorphous substances typically exhibit short-range order. This means that while they lack the long-range repeating pattern of crystals, there is still some degree of local organization in the atomic or molecular structure.

Step 3: Analyzing Melting Points

Amorphous substances do not have sharp melting points. Instead, they soften over a range of temperatures because their structural organization is less orderly compared to crystalline solids.

Step 4: Selecting Correct Statements

From the understanding of amorphous substances, we can determine that statements (ii) 'short range order' and (iv) 'have no sharp melting point' are true for amorphous materials. Therefore, the correct answer is choice (b).

Key Concepts

Short Range Order

Amorphous substances, unlike their crystalline counterparts, exhibit what is known as "short range order." This concept refers to the local organization and arrangement of atoms or molecules within the material. In these substances, while the atoms maintain a degree of regularity and orderliness over a small distance or cluster, this pattern does not extend over significant distances in the material. So, in a very localized region—at the atomic scale—the structure appears somewhat ordered. This is similar to how each room in a house may be organized, but the overall arrangement or architecture of the entire house lacks a coherent pattern.

This characteristic distinguishes amorphous substances from crystalline materials, which exhibit both short and long range order. The local order gives rise to certain physical properties that can still resemble those of crystals, even though the overall structure is not crystalline.

Long Range Order

Long range order is a term used to describe the extended, repeating, periodic arrangement of atoms or molecules across an entire structure, which is a hallmark of crystalline materials. In crystalline solids, such as quartz or diamond, the orderly structure repeats over large distances, virtually throughout the entire material. This is like a well-orchestrated marching band where every step is uniform and synchronized over a long distance.

Amorphous substances, in contrast, do not exhibit such long-range order. Their atomic or molecular arrangements are irregular, with no repeating pattern beyond very short distances. This lack of a broad-scale ordered structure results in unique properties, differentiating them from crystalline solids which have more predictable and consistent properties over larger scales.

Melting Point of Amorphous Substances

One of the most notable characteristics of amorphous substances is their behavior when heated, specifically regarding their melting points. Traditional crystalline solids have sharp melting points—defined temperatures at which they transition from solid to liquid. This sharp transition occurs because the long range order breaks down at a specific energy level, enabling the ordered lattice to collapse into a liquid state.

In contrast, amorphous substances do not have such a sharp melting point. Instead, they experience a gradual softening over a range of temperatures. This gradual transition is due to their lack of long range order. Without a precise lattice structure that governs the melting and boiling points, the substance starts to soften and change state more gradually as the thermal energy disrupts the short range order. This softening effect is often utilized in industries, particularly in glass and certain plastics production, where materials need to be molded and shaped over a temperature range rather than a single melting point.