Is glass a solid or a liquid?
From Dr. Robert Brill, Research Scientist Emeritus:
In my own view, glass is not a liquid -- nor is it a solid -- in our conventional uses of the words. I like to think of glass as a "fourth state of matter", one that combines some of the properties of both liquids and solids (in particular, crystalline solids), but which differs from each in some important way.
The main property that sets glass aside from other materials -- and allows it to be formed by some means that cannot be used for other materials -- is its viscosity-temperature curve. It is not easy to visualize from just a word description; please refer to the attached article from the Journal of Glass Studies.
You will notice from the curve shown somewhere toward the end of that article that as the temperature of a glass is raised, the viscosity slowly decreases. This is a marked difference from most crystalline materials. With the other materials, the viscosity stays constant as the temperature is increased and then, when the material reaches its melting point, the viscosity drops abruptly and the solid collapses into a liquid state. This is because in a crystalline material all the chemical bonds holding the material together tightly in a crystal lattice are identical and equally strong. Therefore, they all manage to hold together until the melting point is reached and the input of a little more heat energy causes them to break simultaneously. This explains why crystalline materials have sharp melting points.
On the other hand (referring to the diagrams in the same text) you will notice that in a glass the chemical bonds are not identical, because the atoms have become fixed in space before they have had a chance to arrange themselves into the regular crystal lattice that the laws of
thermodynamics say they should form. Consequently, some of the chemical bonds are distorted from what they should be and are weaker than their neighbors. As a glass is heated, the bonds that are most distorted (the weakest ones) break first. Then the next weakest bonds break, and so on. The result is that the glass softens gradually from a rigid state and its viscosity gradually decreases as its temperature increases.
Hence, glasses do not have any sharp melting point such as crystals have. In fact, in the strict sense of the word, glasses do not have any melting point at all.