An amorphous solid is a solid whose atoms are not in a regular crystalline pattern. The word amorphous comes from the Greek word ámorphos, which means "shapeless."
When matter is in a solid form, it may take the form of an amorphous solid depending on its molecular structure and how it was cooled. Examples of amorphous solids include glass, plastic, and gel, though most materials can be either found or made amorphous through processing.
Solids and States of Matter
There are three main states of matter: solid state, liquid state and the gaseous state. Gases take the shape and volume of their container, liquids maintain a volume but take the shape of their container, and solids hold their own shape and volume.
When a solid is heated to its melting point, it becomes a liquid, and when a liquid is heated to its boiling point, it becomes a gas. The process also works in reverse: When a gas is cooled, it condenses to a liquid, and when a liquid is cooled, it freezes into a solid.
Amorphous solids share some similarities with liquids, in that liquids also do not have a regular atomic or molecular structure; in fact, in amorphous solids, the line between solid and liquid is not well-defined, making it impossible for them to have exact melting points. Most amorphous solids still hold their own shape and volume, even with a disordered structure.
Different Types of Solids
Solids can be classified into two types based on their different fundamental structures. Depending on whether their structure is regular or disordered, they can be crystalline solids or noncrystalline amorphous materials.
Almost any material can be made amorphous if it is cooled quickly enough from its liquid phase, but some materials are naturally amorphous because their component atoms or molecules can't fit together in a regular pattern. Other materials are amorphous because they contain defects or impurities that disrupt the creation of a stable lattice.
Crystalline solids have their molecules or atoms arranged in a repeating pattern, called a lattice structure. The smallest repeating unit of that lattice structure is called a unit cell. They are the most common type of solid. When they break, they often do so in flat faces and geometric shapes.
Amorphous solids have no long-range order. This means that the pattern of atoms or molecules in one place in the solid will look totally different from the pattern in another part of the solid. However, most amorphous solids do have short-range order: A picture of a very small part of the solid at the molecular level may actually look ordered!
Properties of Amorphous Solids
As mentioned, amorphous solids do not have specific melting points because there is no clear delineation between their liquid phase and amorphous solid phase. Since the distance between neighboring atoms or molecules varies over the whole material, thermal energy doesn't move through it evenly, which means it slowly softens over a wide temperature range rather than melting at one temperature.
Amorphous solids break into curved or irregular surfaces due to their lack of internal structure: Imagine the difference between the surface of a broken quartz crystal (crystalline) and a broken piece of obsidian (amorphous). This will often make broken amorphous solids, such as obsidian and glass, very sharp.
X-ray diffraction is a common method of identifying crystalline materials. It works by looking at the pattern of reflected or refracted light off of the regular pattern of atoms in the material. It does not work, however, on amorphous solids, which have no regular pattern to identify them with.
Examples of Amorphous Solids
Common amorphous solids include rubber, plastic and glass, although thin films are also often studied for their amorphous phases. Cotton candy is also an example of an amorphous solid, as is obsidian (which can also be considered a glass).
About the Author
Meredith is a science writer and physicist based in Seattle. She received her Bachelor of Science degree in physics from the University of Illinois at Urbana-Champaign and her Master of Science degree in physics from the University of Washington. She has written for Live Science, Physics, Symmetry, and WIRED, and was an AAAS Mass Media Fellow in 2019.