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absolute zero
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{{short description|coldest possible temperature}}{{About|the minimum temperature possible|other uses|Absolute Zero (disambiguation)}}{{Use dmy dates|date=March 2012}}(File:CelsiusKelvin.svg|thumb|right|150px|Zero kelvin (−273.15 Â°C) is defined as absolute zero.)Absolute zero is the lowest limit of the thermodynamic temperature scale, a state at which the enthalpy and entropy of a cooled ideal gas reach their minimum value, taken as 0°. The fundamental particles of nature have minimum vibrational motion, retaining only quantum mechanical, zero-point energy-induced particle motion. The theoretical temperature is determined by extrapolating the ideal gas law; by international agreement, absolute zero is taken as −273.15° on the Celsius scale (International System of Units),WEB, Unit of thermodynamic temperature (kelvin), SI Brochure, 8th edition, Section 2.1.1.5,weblink Bureau International des Poids et Mesures, 13 March 2010, 1967, 20 June 2017, dead,weblink" title="web.archive.org/web/20141007053944weblink">weblink 7 October 2014, Note: The triple point of water is 0.01 °C, not 0 °C; thus 0 K is −273.15 °C, not −273.16 °C.BOOK, Thermodynamics, C. P., Arora, Tata McGraw-Hill, 2001, 978-0-07-462014-4, Table 2.4 page 43,weblink which equals −459.67° on the Fahrenheit scale (United States customary units or Imperial units).WEB,weblink Zielinski, Sarah, 1 January 2008, Absolute Zero, Smithsonian Institution, 2012-01-26,weblink" title="web.archive.org/web/20130401180715weblink">weblink 1 April 2013, dead, dmy-all, The corresponding Kelvin and Rankine temperature scales set their zero points at absolute zero by definition.It is commonly thought of as the lowest temperature possible, but it is not the lowest enthalpy state possible, because all real substances begin to depart from the ideal gas when cooled as they approach the change of state to liquid, and then to solid; and the sum of the enthalpy of vaporization (gas to liquid) and enthalpy of fusion (liquid to solid) exceeds the ideal gas's change in enthalpy to absolute zero. In the quantum-mechanical description, matter (solid) at absolute zero is in its ground state, the point of lowest internal energy.The laws of thermodynamics indicate that absolute zero cannot be reached using only thermodynamic means, because the temperature of the substance being cooled approaches the temperature of the cooling agent asymptotically, and a system at absolute zero still possesses quantum mechanical zero-point energy, the energy of its ground state at absolute zero. The kinetic energy of the ground state cannot be removed.
Scientists and technologists routinely achieve temperatures close to absolute zero, where matter exhibits quantum effects such as superconductivity and superfluidity.

Thermodynamics near absolute zero

At temperatures near {{convert|0|K|C F}}, nearly all molecular motion ceases and ΔS = 0 for any adiabatic process, where S is the entropy. In such a circumstance, pure substances can (ideally) form perfect crystals as T → 0. Max Planck's strong form of the third law of thermodynamics states the entropy of a perfect crystal vanishes at absolute zero in which a perfect crystal is gone. The original Nernst heat theorem makes the weaker and less controversial claim that the entropy change for any isothermal process approaches zero as T → 0:
lim_{T to 0} Delta S = 0
The implication is that the entropy of a perfect crystal approaches a constant value.The Nerst postulate identifies the isotherm T = 0 as coincident with the adiabat S = 0, although other isotherms and adiabats are distinct. As no two adiabats intersect, no other adiabat can intersect the T = 0 isotherm. Consequently no adiabatic process initiated at nonzero temperature can lead to zero temperature. (≈ Callen, pp. 189–190)A perfect crystal is one in which the internal lattice structure extends uninterrupted in all directions. The perfect order can be represented by translational symmetry along three (not usually orthogonal) axes. Every lattice element of the structure is in its proper place, whether it is a single atom or a molecular grouping. For substances that exist in two (or more) stable crystalline forms, such as diamond and graphite for carbon, there is a kind of chemical degeneracy. The question remains whether both can have zero entropy at T = 0 even though each is perfectly ordered.Perfect crystals never occur in practice; imperfections, and even entire amorphous material inclusions, can and do get "frozen in" at low temperatures, so transitions to more stable states do not occur.Using the Debye model, the specific heat and entropy of a pure crystal are proportional to T 3, while the enthalpy and chemical potential are proportional to T 4. (Guggenheim, p. 111) These quantities drop toward their T = 0 limiting values and approach with zero slopes. For the specific heats at least, the limiting value itself is definitely zero, as borne out by experiments to below 10 K. Even the less detailed Einstein model shows this curious drop in specific heats. In fact, all specific heats vanish at absolute zero, not just those of crystals. Likewise for the coefficient of thermal expansion. Maxwell's relations show that various other quantities also vanish. These phenomena were unanticipated.Since the relation between changes in Gibbs free energy (G), the enthalpy (H) and the entropy is
Delta G = Delta H - T Delta S ,
thus, as T decreases, ΔG and ΔH approach each other (so long as ΔS is bounded). Experimentally, it is found that all spontaneous processes (including chemical reactions) result in a decrease in G as they proceed toward equilibrium. If ΔS and/or T are small, the condition ΔG 

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