Sublimation is thetransition of a substance directly from thesolid to thegas state, without passing through theliquid state.[1] The verb form of sublimation issublime, or less preferably,sublimate.[2]Sublimate also refers to the product obtained by sublimation.[2][3] The point at which sublimation occurs rapidly (for further details, seebelow) is called critical sublimation point, or simply sublimation point. Notable examples include sublimation ofdry ice at room temperature and atmospheric pressure, and that of solidiodine with heating.
The reverse process of sublimation isdeposition (also calleddesublimation), in which a substance passes directly from a gas to a solid phase, without passing through the liquid state.[4]
Technically, all solids may sublime, though most sublime at extremely low rates that are hardly detectable under usual conditions. Atnormal pressures, mostchemical compounds andelements possess three different states at differenttemperatures. In these cases, the transition from thesolid to thegas state requires an intermediate liquid state. The pressure referred to is thepartial pressure of the substance, not thetotal (e.g. atmospheric) pressure of the entire system. Thus, any solid can sublime if itsvapour pressure is higher than the surrounding partial pressure of the same substance, and in some cases, sublimation occurs at an appreciable rate (e.g. water ice just below 0 °C).
For some substances, such ascarbon andarsenic, sublimation from solid state is much more achievable thanevaporation from liquid state and it is difficult to obtain them as liquids. This is because the pressure of theirtriple point in itsphase diagram (which corresponds to the lowest pressure at which the substance can exist as a liquid) is very high.
Sublimation is caused by the absorption of heat which provides enough energy for some molecules to overcome theattractive forces of their neighbors and escape into the vapor phase. Since the process requires additional energy, sublimation is anendothermic change. Theenthalpy of sublimation (also called heat of sublimation) can be calculated by adding theenthalpy of fusion and theenthalpy of vaporization.
Confusions
While the definition of sublimation is simple, there is often confusion as to what counts as a sublimation.
False correspondence with vaporization
Vaporization (from liquid to gas) is divided into two types: vaporization on the surface of the liquid is calledevaporation, and vaporization at the boiling point with formation of bubbles in the interior of the liquid is calledboiling. However there isno such distinction for the solid-to-gas transition, which is always called sublimation in both corresponding cases.
Potential distinction
For clarification, a distinction between the two corresponding cases is needed. With reference to aphase diagram, the sublimation that occurs left of the solid-gas boundary, the triple point or the solid-liquid boundary (corresponding to evaporation in vaporization) may be calledgradual sublimation; and the substancesublimes gradually, regardless of rate. The sublimation that occurs at the solid-gas boundary (critical sublimation point) (corresponding to boiling in vaporization) may be calledrapid sublimation, and the substancesublimes rapidly. The words "gradual" and "rapid" have acquired special meanings in this context and no longer describe the rate of sublimation.[citation needed]
Misuse for chemical reaction
The termsublimation refers specifically to aphysical change ofstate 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 solidammonium chloride into hydrogen chloride and ammonia isnot sublimation but a chemical reaction. Similarly the combustion of candles, containingparaffin wax, tocarbon dioxide andwater vapor isnot sublimation but a chemical reaction with oxygen.
Historical definition
Sublimation is historically used as a generic term to describe a two-step phase transition ― a solid-to-gas transition (sublimation in a more precise definition) followed by a gas-to-solid transition (deposition).[5][6] (Seebelow)
Examples
Comparison of phase diagrams of carbon dioxide (red) and water (blue) showing the carbon dioxide sublimation point (middle-left) at 1 atmosphere. As dry ice is heated, it crosses this point along the bold horizontal line from the solid phase directly into the gaseous phase. Water, on the other hand, passes through a liquid phase at 1 atmosphere.
The examples shown are substances that noticeably sublime under certain conditions.
Solidcarbon dioxide (dry ice) sublimes rapidly along the solid-gas boundary (sublimation point) below the triple point (e.g., at the temperature of −78.5 °C, atatmospheric pressure), whereas its melting into liquid CO2 can occur along the solid-liquid boundary (melting point) at pressures and temperatures above the triple point (i.e., 5.1 atm, −56.6 °C).
Water
Snow andice sublime gradually at temperatures below the solid-liquid boundary (melting point) (generally 0 °C), and at partial pressures below the triple point pressure of 612 Pa (0.00604 atm), at a low rate.[7] Infreeze-drying, the material to be dehydrated is frozen and its water is allowed to sublime under reduced pressure or vacuum. The loss of snow from asnowfield during a cold spell is often caused by sunshine acting directly on the upper layers of the snow. Sublimation of ice is a factor to the erosive wear ofglacier ice, known asablation inglaciology.[8]
Naphthalene
Experimental set up for the sublimation reaction ofnaphthalene, solid naphthalene sublimes and form the crystal-like structure at the bottom of thewatch glassSolid compound ofnaphthalene sublimed to form a crystal-like structure on the cool surface.
Naphthalene, an organic compound commonly found in pesticides such asmothballs, sublimes easily because it is made of non-polar molecules that are held together only byvan der Waals intermolecular forces. Naphthalene is a solid that sublimes gradually atstandard temperature and pressure,[9] at a high rate, with the critical sublimation point at around 80 °C (176 °F).[10] At low temperature, its vapour pressure is high enough, 1mmHg at 53°C,[11] to make the solid form of naphthalene evaporate into gas. On cool surfaces, the naphthalene vapours will solidify to form needle-like crystals.
Iodine
Iodine sublimes gradually and produces visible fumes on gentle heating atstandard atmospheric temperature. It is possible to obtain liquid iodine at atmospheric pressure by controlling the temperature at just between the melting point and the boiling point of iodine. Inforensic science, iodine vapor can reveal latentfingerprints on paper.[12]
Other substances
Camphor subliming in acold finger. The crude product in the bottom is dark brown; the white purified product on the bottom of the cold finger above is hard to see against the light background.
Crystals offerrocene after purification by vacuum sublimation
Sublimation is a technique used bychemists to purifycompounds. A solid is typically placed in asublimation apparatus and heated undervacuum. Under this reducedpressure, the solidvolatilizes and condenses as a purified compound on a cooled surface (cold finger), leaving a non-volatile residue ofimpurities behind. Once heating ceases and the vacuum is removed, the purified compound may be collected from the cooling surface.[14][15]For even higher purification efficiencies, atemperature gradient is applied, which also allows for the separation of different fractions. Typical setups use an evacuated glass tube that is heated gradually in a controlled manner. The material flow is from the hot end, where the initial material is placed, to the cold end that is connected to a pump stand. By controlling temperatures along the length of the tube, the operator can control the zones of re-condensation, with very volatile compounds being pumped out of the system completely (or caught by a separatecold trap), moderately volatile compounds re-condensing along the tube according to their different volatilities, and non-volatile compounds remaining in the hot end.Vacuum sublimation of this type is also the method of choice for purification of organic compounds for use in theorganic electronics industry, where very high purities (often > 99.99%) are needed to satisfy the standards for consumer electronics and other applications.[citation needed]
Historical usage
In ancientalchemy, aprotoscience that contributed to the development of modern chemistry and medicine, alchemists developed a structure of basic laboratory techniques, theory, terminology, and experimental methods.Sublimation was used to refer to the process in which a substance is heated to a vapor, then immediately collects as sediment on the upper portion and neck of the heating medium (typically aretort oralembic), but can also be used to describe other similar non-laboratory transitions. It was mentioned by alchemical authors such asBasil Valentine andGeorge Ripley, and in theRosarium philosophorum, as a process necessary for the completion of themagnum opus. Here, the wordsublimation was used to describe an exchange of "bodies" and "spirits" similar to laboratory phase transition between solids and gases. Valentine, in hisLe char triomphal de l'antimoine (Triumphal Chariot of Antimony, published 1646) made a comparison tospagyrics in which a vegetable sublimation can be used to separate the spirits in wine and beer.[16] Ripley used language more indicative of the mystical implications of sublimation, indicating that the process has a double aspect in the spiritualization of the body and the corporalizing of the spirit.[17] He writes:[18]
And Sublimations we make for three causes, The first cause is to make the body spiritual. The second is that the spirit may be corporeal, And become fixed with it and consubstantial. The third cause is that from its filthy original. It may be cleansed, and its saltiness sulphurious May be diminished in it, which is infectious.
Sublimation predictions
Theenthalpy of sublimation has commonly been predicted using theequipartition theorem. If thelattice energy is assumed to be approximately half the packing energy,[clarification needed] then the following thermodynamic corrections can be applied to predict the enthalpy of sublimation. Assuming a 1molarideal gas gives a correction for the thermodynamic environment (pressure and volume) in which pV = RT, hence a correction of 1RT. Additional corrections for thevibrations,rotations and translation then need to be applied. From theequipartition theorem gaseous rotation and translation contribute 1.5RT each to the final state, therefore a +3RT correction. Crystalline vibrations and rotations contribute 3RT each to the initial state, hence −6RT. Summing the RT corrections; −6RT + 3RT + RT = −2RT.[19] This leads to the following approximate sublimation enthalpy. A similar approximation can be found for theentropy term if rigid bodies are assumed.[20][21]
Dye-sub printing is a digital printing technology using full color artwork that works with polyester and polymer-coated substrates. Also referred to as digital sublimation, the process is commonly used for decorating apparel, signs and banners, as well as novelty items such as cell phone covers, plaques, coffee mugs, and other items with sublimation-friendly surfaces. The process uses the science of sublimation, in which heat and pressure are applied to a solid, turning it into a gas through an endothermic reaction without passing through the liquid phase.[citation needed]
In sublimation printing, unique sublimation dyes are transferred to sheets of “transfer” paper via liquid gel ink through a piezoelectric print head. The ink is deposited on these high-release inkjet papers, which are used for the next step of the sublimation printing process. After the digital design is printed onto sublimation transfer sheets, it is placed on a heat press along with the substrate to be sublimated.[citation needed]
In order to transfer the image from the paper to the substrate, it requires a heat press process that is a combination of time, temperature and pressure. The heat press applies this special combination, which can change depending on the substrate, to “transfer” the sublimation dyes at the molecular level into the substrate. The most common dyes used for sublimation activate at 350 degrees Fahrenheit. However, a range of 380 to 420 degrees Fahrenheit is normally recommended for optimal color.[citation needed]
The result of the sublimation process is a nearly permanent, high resolution, full color print. Because the dyes are infused into the substrate at the molecular level, rather than applied at a topical level (such as with screen printing and direct to garment printing), the prints will not crack, fade or peel from the substrate under normal conditions.[citation needed]
^Fassnacht, S. R. (2004). "Estimating Alter-shielded gauge snowfall undercatch, snowpack sublimation, and blowing snow transport at six sites in the coterminous USA".Hydrol. Process.18 (18):3481–3492.Bibcode:2004HyPr...18.3481F.doi:10.1002/hyp.5806.S2CID129927018.
^Caroll, J. (2014).Natural Gas Hydrates. Gulf Professional. p. 16.ISBN9780128005750.
^Staff writer(s) (2015)."what solids go through sublimation?". National Science Foundation and UCSB School-University partnership. Retrieved13 November 2015.
^Pavia, D. (2005).Introduction to organic laboratory technique. Thomson Brooks/Cole. pp. 781–782.ISBN978-0534408336.
^Girard, James (2011).Criminalistics: Forensic Science, Crime and Terrorism. Jones & Bartlett Learning. pp. 143–144.ISBN978-0-7637-7731-9.
^Holleman, Arnold F.; Wiberg, Egon; Wiberg, Nils (1985). "Arsen".Lehrbuch der Anorganischen Chemie (in German) (91–100 ed.). Walter de Gruyter. pp. 675–681.ISBN978-3-11-007511-3.
^R. B. KingOrganometallic Syntheses. Volume 1 Transition-Metal Compounds; Academic Press: New York, 1965.ISBN0-444-42607-8.
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