At some point during your science studies, you would have been introduced to the idea of The Three States of Matter: solid, liquid, and gas. As you may have realised when thinking about, for example, the melting of glass, or contemplating the nature of a flame, this three state model doesn’t tell the whole story. Solid, liquid, and gas are more like three categories into which more specific states of matter fit. This series explores some of these states which perhaps don’t seem to fit neatly into the three states model as you may have learned it.
There is a pervasive, persistent, and entirely incorrect idea that glass is an extremely high viscosity liquid. The reason this comes about makes a certain amount of sense: glass has the same chemical composition as crystalline quartz, but has a liquid-like lack of long-range order.
The associated claim that glass does flow over time is incorrect. “Evidence” is given by pointing out that early glass windows tend to be thicker at the bottom. This “evidence” is actually incorrectly attributed: glass panes with anything resembling a truly uniform thickness are a very recent advance. Until the early twentieth century, the only way to make a nearly flat sheet of glass was to start by blowing a globe or cylinder, then squashing it flat while it is still hot, either directly down onto itself or after cutting the shape open. The result is a glass pane with significant thickness variations, including typically one edge being thicker than the rest. Logically, these windows were usually installed with the thicker, heavier edge at the bottom. The idea that the glass of the windows has flowed down over time is completely contradicted by the fact that there are occasional examples of these glass panes being installed with the thick edge at the top or on one of the sides.
Glass does not flow until you heat it up past its glass–liquid transition temperature (almost the same as a melting point, more on that shortly.) Glass has both a fixed volume and a fixed shape. The only thing that glass has in common with a liquid is that disorder of its atoms. Even then, the atoms vibrate around fixed positions, as is typical of a solid, rather than moving relatively freely as they would in a liquid.
Glass is a solid.
It is, however, part of a class of solids that often get neglected. It is a perfect example of an amorphous solid.
Amorphous solids differ from their crystalline counterparts in one fundamental way: in a crystalline solid, the constituent atoms are arranged in a highly regular, ordered crystal lattice. In an amorphous solid, there is still an average distance and possibly a characteristic bond angle between atoms, but there’s an awful lot of deviation from those averages, leading to a really very disordered microscopic structure. The result of this for the properties of an amorphous solid, as compared to a crystalline one, is seen in the way it melts. A crystalline solid has a sharp transition from solid to liquid, as you can see when ice melts. A layer of liquid forms on the surface of the solid, and the solid and liquid phases are quite separate and distinct. And amorophous solid doesn’t show the same sharp and absolute phase transition. Rather, at some temperature, its atoms gain enough energy to become mobile enough for the properties of the solid to change to those of a viscous liquid. For glass, that temperature is around 500 C at atmospheric pressure (most glass has impurities, usually deliberately added because of how they change the properties of the glass, so this temperature varies by a lot for different types of glass). The atoms are not suddenly liberated from a crystal structure, so the nature of the transition is slightly different. It is therefore not actually called a melting point, but a glass-liquid transition temperature. Because glass is the archetypal amorphous solid, the same term is used for all amorphous solids, not just silicate glass.Note how the glass appears to soften, then flows, and contrast this with the sharp change shown by the ice and water.
You are familiar with another group of amorphous solids: the vast majority of polymer materials (better known as plastics) are amorphous, sometimes with small ordered crystalline regions, but with a lack of order over all.
Perhaps you have also heard of amorphous silicon in the context of solar cells. If you have a solar powered calculator, you even own your own little piece of amorphous silicon. It is a much cheaper option for making solar cells than its crystalline counterpart, but until recently it was impractical for large solar cells because the technology was less efficient at converting light to electricity. Recent advances in how amorphous silicon solar cells are constructed have improved their efficiency enough that they are starting to appear more often in rooftop solar power systems.
Many things that form amorphous solids are able to form crystalline solids too, when cooled from their liquid phase under different conditions. Amorphous solids can be though of as a bit like snap-frozen liquids. They have been cooled too quickly for the constituent atoms or molecules to have time to neatly line up into a crystalline lattice, like they would if the liquid had been cooled more slowly. How quickly “too quickly” actually is depends on the particular material.
So don’t be fooled; the traditional junior high school definition of a solid as “a state with a fixed shape and volume, where the microscopic particles are tightly packed in a regular pattern” completely excludes an entire, and very important, class of solid materials: the amorphous solids. These aren’t the only type of matter neglected by the traditional three states description. Also in this series: plasmas and liquid crystals.
- States of Matter: Plasma (physicamechanica.wordpress.com)