ABSTRACT
Liquids play an essential role in human activities and in life itself, but the liquid state of matter leads a rather precarious existence. Each covalent compound is stable as a liquid over a relatively narrow range of physical conditions, although metals and salts tend to be liquid over wider ranges. Liquids exist only at temperatures where the thermal energy is of the same order as the energy of interaction between molecules. Once the thermal energy increases significantly beyond that the liquid evaporates, and if the thermal energy decreases much below that the liquid solidifies to a crystal or in a metastable state to a glass. All glasses have a tendency to crystallise or devitrify, and this process involves shrinkage because ordered packing is more efficient than disordered packing. This causes some of the older glass artefacts to have weakened, although it is possible to make special glasses which devitrify in a reasonably short time and are useful as glass ceramics.
