Glass is a solid, not a liquid, despite old myths about church windows and a misreading of a physics book. It is an amorphous solid, which means it does not crystallize like most other solids. Glass can have different properties depending on how it is cooled and the presence of impurities. The labels “solid” and “liquid” are idealistic and do not fully capture the range of possible atomic arrangements and properties of physical substances.
One of the most persistent physics questions popular and associated with urban legends is whether glass is a liquid or a solid. The first order answer is that it is a solid and does not flow, even over the centuries. The idea that glass is a liquid comes from two sources: that old church windows are thicker at the bottom than at the top, and a misreading of an old physics book by German physicist Gustav Tammann (1861-1938 ), who called it a “supercooled frozen liquid”. The myth omits the “frozen” part.
A little more subtly, glass is an unconventional solid, known as an amorphous solid. For most liquid substances, cooling causes crystallization and a first-order transition to the solid state. For amorphous solids, instead of crystallizing and undergoing a first order transition, the viscosity continues to increase and no crystallization occurs. This is one reason it is transparent: materials with irregular atomic arrangements transmit light better. While there is a second-order transition where the material properties of a glass change as it solidifies, this is not as substantial as the first-order transition found in most other compounds.
Glass can have a range of different material properties depending on how quickly it is cooled and the presence or absence of trace impurities, which can provide nuclei around which crystallization occurs. This is different from classical solids, which have the same properties as the base material no matter what. Glass is sometimes defined as a system not in a point of equilibrium: technically, it could crystallize at any moment, and this sometimes occurs in material with impurities. Only a crystalline solid is considered to be at equilibrium.
Basically, what the argument boils down to is that “solid” and “liquid” are simply idealistic labels that people apply to various physical substances, even though there is a continuum of possible atomic arrangements with properties that mix between the two. For example, a non-Newtonian fluid looks like a liquid, but under the sudden application of pressure it becomes like a solid. Crucially, to truly understand the world, people need to familiarize themselves with the many possible states of matter beyond the simplistic first-order approximation of “solid,” “liquid,” and “gas.”
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