Young Brunette Girl Posing In A Transparent Vitreous Glass Swing Chair

Some people consider glass to be a liquid due to its lack of a first-order phase transition where certain thermodynamic variables such as volume, entropy and enthalpy are discontinuous through the glass transition range. However, the glass transition may be described as analogous to a second-order phase transition where the intensive thermodynamic variables such as the thermal expansivity and heat capacity are discontinuous. Despite this, the equilibrium theory of phase transformations does not entirely hold for glass, and hence the glass transition cannot be classed as one of the classical equilibrium phase transformations in solids.
- Unsolved problems in physics
What is the nature of the transition between a fluid or regular solid and a glassy phase?
"The deepest and most interesting unsolved problem in solid state theory is probably the theory of the nature of glass and the glass transition." P.W. Anderson