Short Answer

Amorphous solids are called supercooled liquids because their particles are not arranged in a regular pattern like crystals. Instead, they have a random arrangement similar to liquids. When a liquid cools very quickly, its particles do not get enough time to arrange in an orderly structure, so it forms an amorphous solid.

These solids behave partly like liquids because they soften gradually when heated and may slowly flow over long periods. This liquid-like behaviour, along with their irregular structure, is why they are known as supercooled liquids.

Detailed Explanation

Why Amorphous Solids Are Called Supercooled Liquids

Amorphous solids are an important class of solids that differ from crystalline solids in their internal arrangement and physical behaviour. One of the main reasons they are special is that they are often referred to as supercooled liquids. This name reflects their structure, formation, and properties that resemble those of liquids more than of true solids.

To understand why amorphous solids are called supercooled liquids, we must examine how they form, how their particles are arranged, and how they behave under heat and pressure.

1. Irregular and Liquid-Like Structure

The internal structure of amorphous solids is irregular and random, similar to the arrangement of particles in a liquid.
In liquids:

• Particles do not follow a repeating pattern.
• There is no long-range order.

In amorphous solids:

• Particles also lack long-range order.
• They show only short-range order.

This similarity in structure makes amorphous solids look more like frozen liquids than true solids.

2. Formed by Rapid Cooling of Liquids

Amorphous solids are often formed when a liquid is cooled quickly.
When cooling happens too fast:

• Particles do not get enough time to arrange into a regular crystal lattice.
• The liquid becomes rigid suddenly without forming a repeating structure.

This process is called supercooling, and the resulting material is known as a supercooled liquid.

Examples:

• Glass is produced by rapidly cooling molten silica.
• Plastic becomes amorphous when cooled quickly.

This rapid cooling traps the liquid-like arrangement inside the solid.

3. No Sharp Melting Point (Like Liquids)

Crystalline solids melt sharply at one temperature.
But amorphous solids:

• Do not have a definite melting point.
• They soften gradually over a range of temperatures.

This behaviour is exactly like that of liquids, which gradually become thinner (less viscous) as temperature increases.

Therefore, their melting behaviour supports the idea that they are supercooled liquids.

4. Ability to Flow Slowly Over Time

Although amorphous solids appear solid, they can flow extremely slowly under gravity.
This flow is very tiny but noticeable over long periods.

Example:

• Old glass windows are often found thicker at the bottom than at the top.
  This happens because glass has slowly flowed over many years.

This slow movement is a liquid-like property, reinforcing that amorphous solids behave as supercooled liquids.

5. Lack of Long-Range Order Like Liquids

Both liquids and amorphous solids:

• Have no long-range order.
• Have atoms arranged in a disordered manner.

The only difference is that in amorphous solids, particle motion is mostly restricted.
This restricted movement gives them a solid appearance even though structurally they resemble liquids.

6. Thermodynamic Similarities with Liquids

Amorphous solids and liquids share similar thermodynamic characteristics, such as:

• Irregular arrangement of molecules
• Non-uniform bonding
• Gradual softening on heating

Because their behaviour and energy pattern resemble liquids more than solids, they are often placed between solids and liquids.

7. Examples of Supercooled Liquids (Amorphous Solids)

Common examples include:

• Glass
• Rubber
• Plastics
• Gels
• Pitch
• Paraffin wax

Each of these materials has an irregular structure and softens gradually like a liquid.

8. Why They Are Not True Solids

Amorphous solids:

• Do not have a fixed geometric shape
• Do not show anisotropy
• Do not have a fixed melting point
• Do not have long-range order

These characteristics are opposite to crystalline solids, making them closer to liquids in structure and behaviour.

9. Behaviour Under Heat Supports the Name

When heated:

• Amorphous solids do not break suddenly.
• They soften slowly into a more liquid-like form.

This behaviour is similar to how thick liquids behave when heated gradually.

Thus, they behave more like a very thick liquid trapped in a solid form.

Conclusion

Amorphous solids are called supercooled liquids because their internal structure is random, just like liquids, and they are formed by rapidly cooling a liquid before its particles can become ordered. They soften gradually, may flow slowly, and lack a sharp melting point—all of which are typical liquid-like behaviours. These similarities explain why amorphous solids are considered supercooled liquids rather than true crystalline solids.