Editing Entropy (section)

=== Measurement ===
The entropy of a substance can be measured, although in an indirect way. The measurement, known as entropymetry,<ref>{{Cite journal|last1=Kim|first1=Hye Jin|last2=Park|first2=Youngkyu|last3=Kwon|first3=Yoonjin|last4=Shin|first4=Jaeho|last5=Kim|first5=Young-Han|last6=Ahn|first6=Hyun-Seok|last7=Yazami|first7=Rachid|last8=Choi|first8=Jang Wook|year=2020|title=Entropymetry for non-destructive structural analysis of LiCoO 2 cathodes|url=http://xlink.rsc.org/?DOI=C9EE02964H|journal=Energy & Environmental Science|language=en|volume=13|issue=1|pages=286–296|doi=10.1039/C9EE02964H|bibcode=2020EnEnS..13..286K |s2cid=212779004|issn=1754-5692}}</ref> is done on a closed system with constant number of particles <math display="inline">N</math> and constant volume <math display="inline">V</math>, and it uses the definition of temperature<ref>{{cite book|last1=Schroeder|first1=Daniel V.|title=An introduction to thermal physics|url=https://archive.org/details/introductiontoth00schr_817|url-access=limited|date=2000|publisher=Addison Wesley|location=San Francisco, CA [u.a.]|isbn=978-0-201-38027-9|page=[https://archive.org/details/introductiontoth00schr_817/page/n99 88]|edition=[Nachdr.]}}</ref> in terms of entropy, while limiting energy exchange to heat <math display="inline">\mathrm{d} U \rightarrow \mathrm{d} Q</math>:<math display="block">T := {\left( \frac{\partial U}{\partial S} \right)}_{V, N}\ \Rightarrow\ \cdots\ \Rightarrow\ \mathrm{d} S = \frac{\mathrm{d} Q}{T}</math>The resulting relation describes how entropy changes <math display="inline">\mathrm{d} S</math> when a small amount of energy <math display="inline">\mathrm{d} Q</math> is introduced into the system at a certain temperature <math display="inline">T</math>.

The process of measurement goes as follows. First, a sample of the substance is cooled as close to absolute zero as possible. At such temperatures, the entropy approaches zero{{snd}}due to the definition of temperature. Then, small amounts of heat are introduced into the sample and the change in temperature is recorded, until the temperature reaches a desired value (usually 25&nbsp;°C). The obtained data allows the user to integrate the equation above, yielding the absolute value of entropy of the substance at the final temperature. This value of entropy is called calorimetric entropy.<ref>{{cite web|title=Measuring Entropy|url=https://www.chem.wisc.edu/deptfiles/genchem/netorial/modules/thermodynamics/entropy/entropy04.htm|website=chem.wisc.edu}}</ref>