Is glass a liquid? The short answer is no, it’s not. Glass is an amorphous solid. But why is it frequently misperceived as a liquid? Understanding this involves a journey through the states of matter and the unique properties of materials.
- Glass is a solid. Specifically, it is an amorphous solid because the chemical bonds between atoms are not arranged in an ordered, crystalline structure.
- The reason some people think it’s a liquid is because of an observation that some old window panes are thicker at the bottom than the top, leading them to think the glass slowly flowed.
- Glass can be a liquid, when you melt it. So can other solids, like ice, iron, and plastic.
What Is Glass?
When we think of glass, the silicate-based material used in windows, jars, and bottles comes to mind. But, the term “glass” actually refers to any material that undergoes a transition into an amorphous solid state. For example, obsidian (a natural silicate) is a type of glass. So are tire rubber and polypropylene. There are even glassy metals, such as Metglas, which forms from iron, nickel, phosphorus, and boron. Amorphous carbon sometimes is considered a glass. Crystallized carbon is a diamond; one form of crystallized silicon dioxide (SiO2) is quartz.
Liquid vs Solid
What “amorphous” means is that the atoms within the structure are not as ordered as in a crystalline or polycrystalline solid. Yet, there is some structure in the bonding between atoms. The atoms or molecules of an amorphous solid form a rigid structure. Meanwhile, the particles in a liquid are very loosely connected and can slide against each other.
- Liquids: Liquids flow easily, conforming to the shape of their container. In liquids, molecules move freely, which gives them their flowing property. Glass, although appearing smooth and fluid-like, does not have this molecular freedom, distinguishing it from liquids.
- Supercooled Liquids: A material in this state, like some types of glass when they first form, is cooled below its freezing point but still remains liquid. It hasn’t transformed into a solid because it cooled so quickly that a regular crystalline structure didn’t form. At this stage, the glass-making process captures the chaotic arrangement of a liquid, but it quickly transitions to an amorphous solid as it cools further. Some people argue that glass is a supercooled liquid, but it only takes this state when the molten glass is still very hot.
- Solids: Solids include crystalline, polycrystalline, and amorphous forms. Glass, like other solids, maintains a fixed volume and shape. But unlike crystalline solids, the molecules in glass are not arranged in a regular pattern. This lack of a regular pattern is what leads some to mistakenly think of glass as a liquid.
Viscosity and Deformation
How do we know glass (or any material) is a solid? Basically, it’s because its viscosity is too high for it to flow. Also, solids reach a point where they break. You can deform a solid, such as by indenting it or twisting it, but only up to a point. A liquid, in contrast, flows. When you deform it, a liquid flows back and “repairs” itself.
- Viscosity: This is a measure of a fluid’s resistance to flow. Molten (liquid) has a high viscosity, making it thick and resistant to flow, somewhat like honey. As it cools, its viscosity increases even further until it behaves like a solid. Similarly, molten steel and wax are liquids, but become solids as they cool.
- Deformation: When solids deform, they change shape. Crystalline solids break along well-defined planes, but amorphous solids, like glass, do not have these planes. Hence, glass can shatter in unpredictable patterns, reinforcing its mysterious nature and further leading to misconceptions about its liquid-like qualities.
The Myth of Flowing Glass
So, it seems obvious that glass is a solid. Why do some people claim it is a liquid?
There is a myth that old European window glass “flows” over time, becoming thicker at the bottom. While it is true that old window glass is thicker at the bottom, this is because of how the glass was made and installed, not because it flowed. One way of making old glass was blowing the glass and spinning it into a large flat disc. The disc tended to have uneven thickness. When using the glass for windows, installers set the thicker, heavier side as the bottom. There is no evidence of glass flowing over its frame at the base or out of its frame at the top. Also, there is no dulling of ancient obsidian arrowheads, distortion of ancient glass beads or bottles, or “flowing” of natural volcanic glass that cooled ages ago.
Difference Between Quartz and Glass
Silicon dioxide (SiO₂) is the primary constituent of both glass (man-made glass like window glass or natural glass like obsidian) and quartz. The transition from molten silica to either an amorphous structure (glass) or a crystalline structure (quartz or other crystalline forms) depends on the cooling rate and conditions under which the material forms. Here’s a breakdown of how this works:
- Rapid Cooling (Quenching): When molten silica cools rapidly, there isn’t sufficient time for the silicon and oxygen atoms to arrange themselves into a regular, repeating crystal lattice. Instead, they solidify in a disordered arrangement and form an amorphous solid, which is glass. This rapid cooling process effectively “traps” the material in a state that’s structurally similar to a liquid but is mechanically a solid.
- Slow Cooling: If molten silica cools very slowly, the atoms have more time to organize themselves into a crystalline structure. This organized, repeating arrangement of atoms is characteristic of minerals like quartz. But, temperature is only part of the story. Getting silicon and oxygen atoms in a crystalline arrangement typically involves a lot of pressure, too.
References
- Dyre, Jeppe C. (2006). “Colloquium : The glass transition and elastic models of glass-forming liquids”. Reviews of Modern Physics. 78 (3): 953–972. doi:10.1103/RevModPhys.78.953
- Henderson, Julian (2013). Ancient Glass. Cambridge University Press. doi:10.1017/CBO9781139021883
- Ojovan, M. I. (2004). “Glass formation in amorphous SiO2 as a percolation phase transition in a system of network defects”. Journal of Experimental and Theoretical Physics Letters. 79 (12): 632–634. doi:10.1134/1.1790021
- Shelby, J.E. (2017). Introduction to Glass Science and Technology. Royal Society of Chemistry. ISBN 978-0-85404-639-3.
- Zarzycki, J. (1991). Glasses and the Vitreous State. Cambridge University Press. ISBN 978-0521355827.