Is Glass a Liquid or a Solid?

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  • Written By: Michael Anissimov
  • Edited By: Bronwyn Harris
  • Last Modified Date: 12 July 2018
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One of the most persistent popular and urban legend-associated physics questions is whether glass is a liquid or a solid. The first-order answer is that it's a solid, and does not flow, even over centuries. The notion that glass is a liquid comes from two sources: that old church windows are thicker at the bottom than the top, and an erroneous reading of an old physics book by German physicist Gustav Tammann (1861-1938), which referred to it as a "frozen supercooled liquid." The myth omits the "frozen" part.

Somewhat more subtly, glass is an unconventional solid, known as an amorphous solid. For most liquid substances, cooling results in crystallization and a first-order transition to a solid state. For amorphous solids, instead of crystallizing and undergoing a first-order transition, the viscosity continues to increase and no crystallization occurs. That is part of why it is transparent — materials with irregular atomic arrangements transmit light better. Though there is a second-order transition in which the material properties of a glass change when it solidifies, this is not as substantial as the first-order transition found among 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 than classical solids, which have the same basic material properties no matter what. Glass is sometimes defined as a system not at an equilibrium point — technically, it could crystallize at any time, and this sometimes does occur 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 merely 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 instance, a non-Newtonian fluid seems like a liquid, but under the sudden application of pressure, becomes like a solid. Fundamentally, to truly understand the world, peoples need to become familiar with the numerous possible states of matter beyond the simplistic first-order approximation of "solid," "liquid," and "gas."


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Post 11

I think one of the things that confuses people is the idea of "liquid" and "solid". When most people hear the word "liquid", they think of something like water. Water is a liquid because it flows quickly and isn't very substantial. Concrete, on the other hand, is a "solid", because it's thick and sturdy and still looks like a brick 200 years later.

When you say glass is a liquid, it gets confusing. It's not going to leak out of the frame like water, but it's still oozing and morphing like a thick liquid. It may take centuries for that glass to flow, but it's still flowing. Mercury is a metal, which means it should be solid, but it flows like a liquid. In scientific terms, liquid and solid states don't always match what most people would call a liquid or a solid.

Post 10

i agree with #9. i think that glass is a solid because how can glass stay in your window if its a liquid? it will just leak, so i agree that it is a solid.

Post 9

Is anyone really confused by the fact that glass (like the glass in a window) is classified as a liquid? Really? Have you ever seen a window in your car flow out over the dash? That would make driving difficult.

Ever seen a glass shelf stay bent in the middle when you store heavy objects on the shelf for a long time? Ever heard of old glasses or telescopes failing to work because the lenses "flowed into a different shape"? Nope, and that's because they still work! For all observable purposes, glass acts as a solid. Not sure why folks are talking about light being matter here though. this is about glass.

Post 7

wait. so let me get this straight: glass is neither a solid nor a gas? or is it both? RAR! this is so confusing!

Post 5

why try to understand if understanding takes out some of the mystery and beauty of the universe? What if we only see what light wants us too? Ignorance is bliss.

Post 3

Why is it a solid?

Post 2

Anon: There has been much controversy about light and matter. Light is energy, not matter. Correct me if I'm wrong here, it's not matter in the usual sense, anyway. Light has some characteristics of waves and of particles. The particles, photons, have zero mass. Therefore, I think that means it's not a legit matter? As it has zero mass, and the exact definition of matter is "anything that takes up space and has mass," light is not matter. Nor is heat, thoughts, emotions, etc. Go by the rule of thumb. If you cannot hold it in your hand, it is not matter. For example. Technically, you can hold air in your hand, so it is a matter. You cannot hold an emotion in your hand, so it is not matter. This is just by my knowledge. I would do further research if it is for a paper or something like that. --Ashley.

Post 1

Is light a matter?

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