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

Option (b) is correct: (ii) and (iv) are correct.

Step 1: Understanding Amorphous Substances

Amorphous substances are materials that lack a well-defined, long-range crystalline structure. This means they do not exhibit a regular pattern over large distances, although they may display some form of short-range order or disorganization at a smaller scale.

Step 2: Analyzing Key Characteristics

To determine which statements are correct, recognize that amorphous substances have a short range order due to localized arrangements but do not exhibit a long range order. Additionally, they do not have a sharp melting point, tending to soften over a range of temperatures instead.

Step 3: Matching Characteristics with Options

Given the characteristics of amorphous substances, statement (ii) 'short range order' and statement (iv) 'have no sharp melting point' are correct. Therefore, we can eliminate other options that do not include both of these qualities.

Step 4: Identifying Correct Answer

From the available options, option (b) '(ii) and (iv) are correct' matches with our understanding of amorphous substances having short range order and no sharp melting point.

Key Concepts

Short Range Order

Imagine you have a box full of marbles that are stacked in a neat, orderly fashion. Each marble is surrounded by others in a predictable pattern, forming layers that continue indefinitely. Crystalline materials resemble this structure, having a repeating pattern over long distances. In contrast, amorphous substances only show order on a tiny scale—more like a small cluster of marbles grouped together at random. This is known as short range order. Short range order means that, even though there's no orderly pattern over large stretches of the material, some local organization still exists. For example:

• If you look at sugar crystals, they have both short and long range order, making them a crystal.
• Amorphous substances like glass, however, only have short range order.

Recognizing this helps us understand why amorphous materials behave differently. They have ordered regions that are too small to form a solid structure that is crystalline, affecting their properties significantly.

Long Range Order

Crystalline solids are characterized by long range order, which extends the neat and repeatable arrangement of atoms over large distances throughout the material. Think of it like a perfect repeating wallpaper pattern that covers an entire room seamlessly. This is very different from amorphous substances where such extensive order does not exist. In crystalline substances:

• Every atom is positioned in a predictable manner relative to others far away.
• This is akin to creating a grid structure that extends infinitely.

In amorphous substances:

• The lack of long range order prevents them from forming true crystal lattices.
• This absence is a reason for their unique properties such as varying strength and transparency.

Understanding the absence of long range order is crucial. It explains why amorphous materials do not exhibit the same mechanical and optical properties as their crystalline counterparts.

Sharp Melting Point

A sharp melting point is a characteristic feature of crystalline solids. It implies that the substance changes from solid to liquid at a specific temperature. Imagine butter melting in a pan—it happens quickly as it hits a certain heat threshold because butter has a sharper melting range. For crystalline materials:

• The uniform arrangement allows them to transform all at once at a single temperature point.
• This results in a clear phase transition.

Amorphous substances, on the other hand, do not have a sharp melting point. For amorphous materials:

• They tend to melt over a range of temperatures.
• This is due to their irregular structure, which makes different parts start to soften at different temperatures.

Without a sharp melting point, it can be difficult to define the precise transition temperature for amorphous substances, making them unique in how they respond to heat.