Sublimation

FIELDS OF STUDY: Physical Chemistry; Chemical Engineering; Environmental Chemistry

ABSTRACT

The property of sublimation is described, and the thermodynamics of the process are discussed. Sublimation is the change of phase from solid to gas without passing through the intermediate liquid phase. Sublimation is a relatively common feature of volatile materials and plays a significant role in the natural water cycle.

The Nature of Phase Transitions

Three distinct physical phases, or states of matter, are characteristic of most materials. They can exist as a solid, a liquid or a gas. The physical phase of any material is dependent on temperature, with a phase transition from solid to liquid taking place at the material’s melting point and a similar transition from liquid to gas taking place at its boiling point. These temperatures are intensive characteristics of each material, meaning that they remain the same no matter how much of the material is present.

For pure compounds, the temperatures at which phase transitions take place are sharply defined, while for mixtures of compounds, the phase change generally takes place over a much broader range of temperatures. However, there are unique combinations of materials for which the melting point or boiling point is as sharply defined as for a pure compound. These combinations are said to be "eutectic" for solid-liquid phase changes and "azeotropic" for liquid-gas phase changes. Any phase change in a material that occurs by increasing the atomic or molecular energy of the material is an endothermic process, requiring the input of thermal energy from outside the thermodynamic system of the material.

The Process of Sublimation

Phase changes for most materials progress from solid through liquid to gas. There are many materials that decompose before reaching their melting or boiling point, and for these the progression of phase changes is presumed to follow the same pattern. The process of sublimation represents a break in the pattern, since a material undergoing sublimation passes directly from the solid phase to the gas phase without going through a liquid phase. It is possible that for at least some of these materials, the outermost layer of the solid phase, perhaps only one or two molecules in thickness, is in a liquid phase, but the difference is a technicality.

At any temperature, the atoms in a material are in a state of constant motion (except at a temperature of absolute zero, when by definition no atomic motion is possible). In gases and liquids, atoms and molecules physically move through space and regularly bump into each other; in solids, this degree of motion does not occur. In all three phases, however, atoms and the bonds between them vibrate in a variety of ways, with their kinetic energy proportional to the temperature. Accordingly, collisions occur in all three phases, and in solids below their melting point and liquids below their boiling point, collisions between molecules or atoms can impart sufficient energy to knock some particles free of the forces holding them together and drive them into a gas phase. When this happens with a liquid, it is called evaporation; when it happens with a solid, it is called sublimation.

Sublimation and Vapor Pressure

When water is in a closed container, molecules are constantly escaping the liquid and becoming water vapor (evaporating); at the same time, molecules of water vapor are also reentering the liquid (condensing). The point at which the processes of evaporation and condensation in this closed system are happening at an equal rate is called equilibrium, and the pressure exerted by the gas from a liquid (such as water) or solid (such as water ice) in a closed system at equilibrium is called its "vapor pressure." Every element or compound has a vapor pressure, determined by the energetic motion of the atoms or molecules that constitute the material.

The stronger the intermolecular forces holding a material together, the lower the material’s vapor pressure. Understandably, materials in which the intermolecular forces are weaker, and thus the vapor pressure is higher, are more prone to sublimation. Since the molecules being ejected from the material during sublimation must move against the pressure of the surrounding atmosphere, it requires more energy for sublimation to occur against higher pressures than against lower pressures.

Every material that has a vapor pressure in the solid phase, no matter how small it may be, can undergo sublimation. Water ice readily sublimates, and it is estimated that three-quarters of the snow and rain that falls in northern climates is returned to the atmosphere through sublimation and evaporation. Sublimation therefore plays an important role in the natural water cycle of the planet.

PRINCIPAL TERMS

• endothermic: describes a process that requires the input of energy in the form of heat in order to proceed.
• gas: a state of matter in which material is fluid, has indefinite volume and shape, and is of variable density due to its ability to expand to fill any available space.
• liquid: a state of matter in which material is fluid, has definite volume but indefinite shape, and maintains a relatively constant density.
• phase transition: the change of matter from one state to another, such as from solid to liquid or liquid to gas, due to the transfer of thermal energy.
• solid: a state of matter in which material is non-fluid, has definite volume and shape, and maintains a near-constant density.

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