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I'm not trying to apply the 2LOT to only part of a system. I'm asking if there is a related law or kind of a sub-law.

My highschool physics teacher said that if I take an ice cube and shine a bright light at it, it melts, thus increases entropy. If I do the same to a plant it grows, thus decreases entropy. He said living plants and man-made machines alone can do that. Kill the plant then shine a light on it, it'll just heat up. Heat up, shine a light on, or add energy to any naturally occurring object that is not alive and just breaks, melts, withers, etc.

(Don't tell me to look at the whole system, that's not what I'm asking.)

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    $\begingroup$ Would a typhoon be a exception? Moving over warmer waters favors intensification. I may be confusing a state that appears better organized with the concept of entropy, though. $\endgroup$ – Bones Jan 13 at 7:35
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    $\begingroup$ see physics.stackexchange.com/questions/193677/… especially the last two sentences in the longish quote. $\endgroup$ – hyportnex Jan 13 at 15:35
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    $\begingroup$ Where's the line between "naturally occurring" and "man-made"? Is it some arbitrary line in complexity? Where does that line put, say, nuclear reactors - surely they, like photovoltaic cells, are always man-made, right? (Or, to be more explicit: don't be too quick to dismiss any given system as "necessarily" man-made.) $\endgroup$ – Emilio Pisanty Jan 14 at 21:00
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    $\begingroup$ I get your point that theoretically there anything a living organism can make should be able to be brought together by pure luck. And what about if a man-made robot manufactured a photovoltaic cell, is it still man-made? Nonetheless, I don't see comets, or rivers decreasing entropy. $\endgroup$ – Joe C Jan 15 at 5:21
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    $\begingroup$ I just thought of something. When a piece of coal gets compressed over millions of years to form a diamond is that an endothermic process that decrease entropy? $\endgroup$ – Joe C Jan 15 at 5:26
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No, this is not true.

Entropy depend on the number of possible ways $Ω$ the energy $E$ in a given system can be arranged among its $N$ number of particles.

When one adds energy to a system, 3 things may occur:

The number of particles increases by breaking the bounds between more complex molecules. In this case, entropy will increase because there will be a bigger number of particles to share the energy, and a higher energy range to be shared as kinetic energy of the particles. This is a similar process of burning organic materials.

The number of particles remains the same. In this case entropy will increase just because there will be a higher energy range for those particles to share as kinetic energy. This process is similar to heat up something without destroying it.

The energy added to the system will be used to bound the particles in the system. In this case entropy will decrease because there will be less particles in the system, and the energy will be stored as a well-organized potential energy. This is what happens when plants bound water and carbon dioxide as glycose during photosynthesis.

Now your teacher seems to believe that only man-made or living things are capable of process number 3. However, this is a common process in nature. Organic content are created after lighting or volcanoes.

PS: Historically, every time a person attributed special characteristics to living things as the only ones capable of this or that, have been proven wrong.

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One of the most common processes which locally decreases entropy in the subsystem which does not involve living or man-made objects is spontaneous crystalization, e.g. snow flakes or salt crystallizing from a drying bowl. A crystal is an ordered lattice, often extremely chemically pure, which has a much lower entropy than the same atoms, e.g., dissolved in a solution.

One might argue that since crystallization usually is an exothermic process, the premise that we input energy (shine light) into the subsystem is broken. There are however some endothermic crystallization processes, which would satisfy that criteria.

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I was preparing an answer and I would like to keep it below between the lines. While writing it and in need of an example I came to a point that removes the ground for the question. That is at the bottom at the very point it naturally came into discussion. Nevertheless, the answer in preparation appears of some relevance, too. That's why I post it.


It seems to me that is possible for open (energy & mass exchange) systems in general.

How to compare the entropy of the plant "before and after growing"? It is difficult to define living things as (sub)systems without including flows or even dividing them further until equilibrium thermodynamics can be applied. For instance, in your example, the growing plant has fixated oxygen and carbon dioxide from the surrounding. So it is not easy to answer while satisfying your last requisite. Or if carbon dioxide and oxygen were already accounted within the system at least the plants itself has changed in itself. Perhaps a specialist of non-equilibrium thermodynamics can, but I am not. I try to make the discussion more sensible, at least according to my thinking.

I can't recall any non-living chemical system for which entropy decreases during an endothermic reaction whatever coupled to another process. But chemistry flasks are way less organized than organisms.

On the other hand, the comment mentioning whether physics let me think that indeed something similar can happen. A system in meteorology is certainly open, as such it might operate a similar process at least on its temporal scale.

What is sure is that living things apparently do that (processes that are endothermic and reduces entropy) but is definitively clear as well that in parallel they operate processes leading to heat and wastes. This way of seeing does not require the surrounding as you requested - because we could refer to a mature organism that does not grow nor it "does waste away": the correct balance now does occur within itself.

If it suffices for the definition of living things I do not know (manufactured things by living entities not considered).

I think it is a complicated argument that goes normally untreated or leads to debate even among specialists. For instance, there was (is?) a huge debate on photosynthesis, namely if unitary elemental steps of photosynthesis obey the second principle of thermodynamic (not considering efficiency limitations!). By the way,...


.... considering the efficiency of the process remove the ground for the question, at least using the plant as an example !!!

Not all photons contribute to the growth of the plant. Actually, a very tiny fraction of them leads to organized molecules that store energy. Most of the photons just undergo much degradation into heat that warms the plant. So we cannot say that the process reduces entropy.

A non-living analog is a photovoltaic cell. If we look at the outcome of interest it also apparently reduces entropy while absorbing energy. However, also in this case and without considering the surrounding, most of the photons absorbed just eat up the device. Again the low entropy is just apparent if we do not look carefully enough.

Moreover, I noticed the following. Most likely a solar cell is a manufactured object. However, in principle, the right combination of just a few naturally occurring materials might layer in the right way and gives a whatever inefficient photovoltaic effect. This would be a system leading to an apparently endergonic and entropy reducing process when we omit the wasted electrons and as such it independently negatively answer to the question. At least inanimated systems can behave as the living ones, in this specific respect. No (sub)system anyway does what described in the question if carefully inspected.

edit "entropy decreases during an endothermic reaction whatever coupled to another process" was before saying "exothermic" that was a trivial quid pro quo, not the meaning.

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  • $\begingroup$ You're saying that the plant scattering light and getting heated up by light is so entropic that it makes the entire process of photosynthesis net entropic? Now keep in mind that O2 has much less entropy than CO2, so even the plant's waste is a decrease in entropy. $\endgroup$ – Joe C Jan 15 at 5:14
  • $\begingroup$ @Joe C it we not account efficiency. The photosynthesis is marvellous and perfect for nature but has efficiency of a few percent! For hundred photons impinging just about 3 are getting stored. The rest is degraded to heat. The debate is when considering the effectively converted photons, an aspect that I ve tried to address at start. But I do not want/cannot do more on this non ending debate in PV community. It seems to me that considering small enough subset the second principle might not hold as it is clear from statistical treatment. Back to your comment: yes entropy gets higher $\endgroup$ – Alchimista Jan 15 at 8:17
  • $\begingroup$ @Joe C. For the second part: we do the same! You are even reducing the system to just individual reactants! Where does the carbon in the emitted CO2 comes from? Hopes it helps. $\endgroup$ – Alchimista Jan 15 at 8:21
  • $\begingroup$ Almost unrelated. We all certainly agree that the striving against thermodynamical equilibrium is a prerogative of living things. It is fascinating and certainly related to non-equilibrium thermodynamics. I would say that their complexity allow for many flasks that speak to each other. Reactions as the clock one, or the spontaneous appearance of "figures" in initially homogeneous solutions rised much optimism and interested but I am not aware of further progress. My book dedicated a couple of pages to such topics and I think general physical chem book are still doing the same. $\endgroup$ – Alchimista Jan 15 at 8:34

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