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

The correct answer is (b): (ii) and (iv) are correct.

Step 1: Identifying Properties of Amorphous Substances

Amorphous substances are those that do not have a well-defined crystalline structure. Their molecular arrangement lacks a long-range order, but they can exhibit a degree of short-range order, similar to the arrangement observed in liquids.

Step 2: Understanding Melting Point Characteristics

Amorphous substances do not have a sharp melting point. Instead, they transition over a range of temperatures, softening gradually as they are heated, unlike crystalline solids which have a precise melting temperature.

Step 3: Evaluating Each Option

Option (a): (i) is incorrect because amorphous substances do not have long-range order. Option (b): (ii) is correct (short range order) and (iv) is correct (no sharp melting point). Option (c): (iii) is incorrect because they do not have long-range order. Option (d): (i) is incorrect because of long-range order.

Step 4: Selecting the Correct Answer

Based on the evaluations, option (b) is correct since amorphous substances have (ii) short range order and (iv) no sharp melting point.

Key Concepts

Short Range Order

In the fascinating world of chemistry and material science, "short range order" plays a crucial role, especially in amorphous substances. Imagine a group of atoms joined together, where each atom only influences or interacts with its immediate neighbors. This limited interaction forms small clusters where the atoms are organized in a predictable manner.

• Amorphous substances, like glasses and gels, exhibit this type of order, mirroring the arrangement found in liquids.
• Although these substances appear structureless at first glance, the short-range order means that right next to each atom, one can observe a consistent pattern.
• This differs from crystalline solids, where atoms are arranged with long-range periodicity.

It's this short range order that gives amorphous substances particular properties like flexibility and durability, found in everyday objects such as eyeglasses and mobile phone screens.

Long Range Order

Long range order, often absent in amorphous substances, defines how atoms or molecules are arranged over large distances. In crystalline materials, atoms are positioned in repeating patterns that extend all through the material. This regularity over long distances speaks volumes about their stable and defined structures.

• This quality gives crystals distinct characteristics, such as a fixed geometric shape and well-defined edges from planes of atoms.
• Due to this order, crystals contribute significantly to industries that rely on specific structural attributes, like semiconductor manufacturing.
• Amorphous materials, on the other hand, lack this long-range periodicity, resulting in their more irregular and flexible nature.

Without the long range order, amorphous substances do not have the sharply defined properties observed in their crystalline counterparts.

Melting Point

The melting point of substances is a defining characteristic, but for amorphous materials, it presents an intriguing variation. Unlike crystalline substances, which have a precise melting point, amorphous substances melt over a range of temperatures.

• Instead of transitioning from solid to liquid abruptly, these materials gradually soften, offering a smoother transition.
• This is due to their lack of long-range order, which in turn affects how heat is absorbed and distributed through the material.
• This unique feature is why materials like thermoplastic polymers can be heated and reshaped multiple times without losing integrity.

This property is advantageous in various applications, such as in manufacturing processes where gradual melting characteristics are needed.