Attainment of Perfect Order at Low Temperatures

Experience indicates that the Third Law of Thermodynamics not only predicts that So — 0, but produces a potential to drive a substance to zero entropy at 0 Kelvin. Cooling a gas causes it to successively become more ordered. Phase changes to liquid and solid increase the order. Cooling through equilibrium solid phase transitions invariably results in evolution of heat and a decrease in entropy. A number of solids are disordered at higher temperatures, but the disorder decreases with cooling until perfect order is obtained. Exceptions are 

The Third Law predicts that isotopic species should separate at very low temperatures. This does not happen because the energy necessary to overcome 

Paramagnetic substances have a magnetic entropy associated with the random orientation of the spin of the unpaired electrons as shown in (a) below  

If a paramagnetic salt is cooled, eventually a temperature is reached where the magnetic moments of the electrons line up. This temperature is known as the Curie point or the Neel point, depending upon whether the spins line up parallel below this temperature as in (b), with the moments pointing in the same direction to reinforce one another and produce ferromagnetism, or the moments line up in opposite directions as in (c) so that they cancel and antiferromagnetism occurs. 

In summary, the Third Law predicts that ordering processes are favored as the temperature is lowered, so that eventually perfect order should be obtained in any solid as its temperature approaches 0 K. But kinetic effects are such that the equilibration times needed to achieve this order are sometimes very long. 

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