Is Glass A Solid Or An Extremely Slow Moving Liquid?
When most people look at a window, they see solid panes of glass. But for decades, physicists, who view window glass at the molecular level, have pondered the question of whether or not glass is a solid or merely an extremely slow-moving liquid due to older windows being thicker at the bottom.
The consensus these days is that glass is not a super-cooled liquid but in fact an amorphous solid. The confusion comes about for two reasons:
Structurally, glasses are similar to liquids. Many solids have a crystalline structure on microscopic scales with molecules arranged in a regular lattice. The molecules in glass have a disordered arrangement, but sufficient cohesion to maintain some rigidity. In this state it is often called an amorphous solid or glass (see reference).
Because of the molecular structure of glass there is no first order phase transition as it cools (in other words it doesn’t appear to have a distinct melting point). In a solid there is a sharp distinction between the solid and the liquid state, that is separated by a first order phase transition. Some people claim that glass is actually a super-cooled liquid because there is no first order phase transition as it cools. In fact, there is a second order transition between the super-cooled liquid state and the glass state, so a distinction can still be made.