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The term sublimation refers specifically to a physical change of state and is not used to describe the transformation of a solid to a gas in a chemical reaction. For example, the dissociation on heating of solid ammonium chloride into hydrogen chloride and ammonia is not sublimation but a chemical reaction.
In physics, chemistry, and other related fields like biology, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states of matter : solid , liquid , and gas , and in rare cases, plasma .
Again, the molecules do not go through an intermediate liquid state when going from the gas to the solid. See also physical vapor deposition, which is a class of processes used to deposit thin films of various materials onto various surfaces. Deposition releases energy and is an exothermic phase change.
A state of matter is also characterized by phase transitions. A phase transition indicates a change in structure and can be recognized by an abrupt change in properties. A distinct state of matter can be defined as any set of states distinguished from any other set of states by a phase transition.
The red lines show the sublimation temperature and the blue line the boiling point, showing how they vary with pressure. The simplest phase diagrams are pressure–temperature diagrams of a single simple substance, such as water. The axes correspond to the pressure and temperature.
At present, there is no single equation of state that accurately predicts the properties of all substances under all conditions. An example of an equation of state correlates densities of gases and liquids to temperatures and pressures, known as the ideal gas law, which is roughly accurate for weakly polar gases at low pressures and moderate temperatures.
In thermodynamics, the enthalpy of sublimation, or heat of sublimation, is the heat required to sublimate (change from solid to gas) one mole of a substance at a given combination of temperature and pressure, usually standard temperature and pressure (STP). It is equal to the cohesive energy of the solid.
The Lindemann criterion states that melting occurs because of "vibrational instability", e.g. crystals melt; when the average amplitude of thermal vibrations of atoms is relatively high compared with interatomic distances, e.g. <δu 2 > 1/2 > δ L R s, where δu is the atomic displacement, the Lindemann parameter δ L ≈ 0.20...0.25 and R s is ...