Glass: Liquid or Solid?

Is glass a liquid or a solid? Actually, the answer is not straightforward.

Solids, made of atoms or molecules that lack the thermal energy to bump past one another and so get locked into place by electric attraction, are generally classified as crystalline or amorphous. In a crystalline solid, the atoms or molecules lie in an orderly array (i.e. they have an orderly internal structure). Not so in amorphous solids, in which the atoms or molecules lie in a random jumble.

Glass is an amorphous solid. However, some chemists prefer to label glass as a supercooled liquid. Normally, the cooling of a substance from a very hot liquid state results in the crystallization of the substance. Its atoms or molecules line up in an orderly manner as they settle into place. Glass is produced by cooling a molten liquid fast enough that crystallization doesn't occur. As the glass cools, the time needed for it to exhibit liquid behavior, such as flowing, increases and reaches extremes. It becomes like a very thick syrup, so viscous (or resistant to flow) that liquid behavior becomes noticeable only on a geologic timescale (as in billions of years).

Another defining characteristic of glasses, which differentiates them from normal solids, is that they lack a latent heat of fusion. The latent heat of fusion is the amount of thermal energy which must be absorbed or lost for a substance to change states (from solid to liquid or vice versa). In other words, if you were to put a container of liquid in a room set to a temperature well below the liquid's freezing point, the liquid would begin giving off heat and decrease in temperature. When it reached its freezing point, however, it would remain at a constant temperature (the freezing point) as it continued to give off heat. This would be the actual phase transition, and the heat given off during this transition is termed the "latent heat of fusion". After the liquid solidified, it would start dropping in temperature again as it continued releasing heat. It would stop releasing heat when its temperature matched the temperature of the room. Water, for example, has a specific amount of heat it must release, per gram, as it transitions from liquid water to ice. While it's releasing this heat of fusion, its temperature remains steady at 0 degrees C. Glass, on the other hand, never enters a stage during its production in which its temperature stops decreasing as it continues to give off heat. There is no defined phase transition between molten glass and "solid" glass.

As a side note, some people claim to have seen evidence of flow in very old glass (perhaps very old windows in a historic building). These claims are not valid. Because of the amount of time needed for glass to start exhibiting liquid behavior, no human will ever witness such a thing.