A Simple Argument that is Simply Wrong
Why is a page about thermodynamics in a website about origins? Because many young-earth creationists claim that evolution is thermodynamically impossible.
For example, Henry Morris claims (in 1973) that because "evolution and entropy are opposing and mutually exclusive concepts,... evolution must be impossible" and (in 1976) that "the most devastating and conclusive argument against evolution [of any type, astronomical, chemical, or biological] is the entropy principle,... also known as the Second Law of Thermodynamics... which describes a situation of universally deteriorating order." After more than a decade of hearing scientific explanations of why this claim is not logically justified, Morris declares (in 1985) that "some have tried to imagine exceptions to the Second Law at some time or times in the past, which allowed evolution to proceed in spite of entropy, but such ideas are nothing but wishful thinking." Later, his son (John Morris, 1992) is carrying on the legacy: "the universal Second Law of Thermodynamics shows that things become more disordered through time, not more complex, as evolution insists."

Here is the simple argument: A biological evolution that converts bacteria into humans, with an obvious increase of order and complexity, would violate the Second Law which says "things become more disordered through time, not more complex, as evolution insists."
And here is why it's wrong: The Second Law of Thermodynamics is not violated by either mutation or natural selection, which are the major actions in neo-Darwinian evolution. If an overall process of evolution is split into many small steps involving mutation followed by selection, each step is permitted by the Second Law, and so is the overall process.

Well, that's the short answer. The rest of this page is a longer answer, explaining "why it's wrong" in more detail, clearly and intuitively, in a way you can understand. We'll begin by looking at thermodynamics to see what entropy is and isn't, what The Second Law of Thermodynamics does and doesn't say. (hint: It does not say "things become more disordered through time," as in the distorted misunderstanding of Henry Morris.) Then you'll see the myth of entropy and intuition and two ways to increase entropy and why things happen and why some things don't happen and why our bodies make unusual things happen, before we look at three types of evolution and how to fix a mess.

colors and links: concept-terms are red, important ideas are purple, and quotations are dark blue; an italicized link keeps you inside this page, but a non-italicized link opens another page in its own new window.

Thermodynamics — First Law and Second Law

The First Law of Thermodynamics states that during any reaction the total energy in the universe remains constant. The Second Law of Thermodynamics states that during any reaction the total useful energy in the universe (the energy that is useful for doing work) will decrease.
For example, consider a ball rolling downhill, moving faster and faster. This reaction — which occurs by just letting gravitational force make the ball "do what comes naturally" — can be viewed as potential energy being converted into the kinetic energy of motion. On top of the hill, the ball had potential energy (because it potentially could do useful work, as when water at the top of a dam falls through a generator to produce electricity which runs a motor) but at the bottom this potential energy already has been converted into kinetic energy, and it therefore is not "useful for doing work," as described by the Second Law.

Entropy is Probability (not disorder)

The Second Law can also be described in other ways, at the macroscopic level of everyday objects, or the microscopic level of tiny particles (like atoms and molecules) that is the most important level for most of our questions about origins.
At the micro-level, a system's entropy is a property that depends on the number of ways that energy can be distributed among the particles in the system. Entropy is a measure of probability, because if energy can be distributed in more ways in a certain state, that state is more probable. A useful analogy is to think about the number of ways that two dice can produce sum-states of 7 (this can occur in six ways, 16 61 25 52 34 43) and 2 (this occurs only one way, with 11), and why this causes 7 to be more probable than 2. For similar reasons, because of probability, the chemicals in a system tend to eventually end up in the particular state (their equilibrium state) that can occur in the greatest number of ways when energy is distributed among the zillions of molecules. (Chemical Thermodynamics)
Basically, the Second Law is just a description of probability, simply recognizing that in every naturally occurring reaction, whatever is most probable (when all things are considered) is most likely to happen. But probability is related to entropy and, in a more precise form, the Second Law states that during any reaction the entropy of the universe will increase. / But what about real systems, which are always smaller than the universe? An isolated system (that is "closed" so it cannot exchange energy or matter with its surroundings) is thermodynamically equivalent to a miniature universe, so during a reaction its entropy will increase. But the entropy of an open system (able to exchange energy with its surroundings) can increase, decrease, or stay constant.

The Myth of Entropy-and-Intuition: The popular myth is that the Second Law is easy to understand and apply, so nonscientists can simply think about a process and conclude that "this is (or isn't) consistent with the Second Law." The everyday analogies used by some young-earth creationists — like "a tidy room becoming messy" due to increasing entropy — are not used in scientific applications of the Second Law, because entropy is about energy distributions and associated probabilities, not macroscopic disorder, and our psychological intuitions about "entropy as disorder" are often wrong. Scientists can develop science-based intuitions about entropy, but this requires understanding and careful thinking.
Because this idea is so important, and misunderstanding is so common, I'll share excerpts from Section 1 in my longer page which emphasize that entropy is not about disorder, and it's not always intuitive:
The correct formulations by scientists (not by writers who are science popularizers) never say "with time, things become more disordered." And the everyday analogies used by some young-earth creationists — like "a tidy room becoming messy" due to increasing entropy — are not used by experts in thermodynamics, because thermodynamics is not about macroscopic disorder. These everyday analogies, which depend on human psychological intuitions about disorder and complexity, are often wrong. .....
In his excellent website about thermodynamics, Frank Lambert, a Ph.D. chemist and a teacher whose ideas about entropy have been published in The Journal of Chemical Education, says: "Discarding the archaic idea of ‘disorder’ in regard to entropy is essential. It just doesn't make scientific sense in the 21st century... [because] it can be so grievously misleading. ... Judging from comments of leading textbook authors who have written me, ‘disorder’ will not be mentioned in general chemistry texts after the present cycle of editions. {source}" .....
If disorder is not a central concept in thermodynamics, why is it used in some descriptions of the Second Law? The reasons can be historical (due to the inertia of tradition), dramatic (in sloppy writing by science popularizers who either don't understand the Second Law, or have decided that entertaining readers with colorful analogies is more important than scientific accuracy), epistemological, or heuristic: ..... { These ideas are explained in more detail in my longer page. }
Even though "disorder" is not a central concept in thermodynamics, young-earth creationists imply that disorder is THE central focus of the Second Law. For example, Henry Morris states that the Second Law "describes a situation of universally deteriorating order." [ Morris and others also illustrate entropy increase with everyday analogies. ]

Two Ways to Increase Entropy

The Second Law is simple in principle, but applying it to real systems can be difficult due to the challenge of determining probabilities at the microscopic level. During a reaction, the entropy in an isolated system can increase in two main ways, due to constraint change or temperature change:
• Entropy will increase if the amount of kinetic energy is constant (because the system's mass and temperature are constant) but energy is being distributed in more ways in the final state of a system after the particles (among which energy is being distributed) have changed due to the reaction. One useful principle is that more constraint and thus less freedom of motion produces a decrease of entropy. For example, entropy decreases when the number of particles decreases (as when two particles that can move independently combine to form one larger particle that must move together as a unit), or volume decreases (as in compressing a gas), or in a phase change when particles condense into a more organized form (as when a gas condenses into a liquid and then into a solid).
• But entropy will also increase if there is more kinetic energy, which can be distributed in more ways, even if the particles don't change. In this way, a simple increase in temperature (which is a measure of average kinetic energy) leads to an increase in entropy.
If we think of a system's total entropy as "constraint-entropy plus temperature-entropy" an entropy change can be due to constraint-change or temperature-change, or both. These two factors often produce opposite effects so they are conflicting factors, with constraint saying "entropy decreases" and temperature saying "entropy increases." When this happens the temperature-change is usually a larger factor, so there is an increase in the overall entropy of the universe even though "disorder" seems to decrease, as you'll see in the examples below.

Why do things happen?
A wide variety of common reactions occur when an attractive force pulls particles closer together, which constrains them (producing a small decrease of entropy) but increases their kinetic energy (*) and temperature (producing a larger increase of entropy), so total entropy increases even though "disorder" seems to decrease. / * When a force makes particles "do what comes naturally" as when balls roll downhill, the speeds increase for the particles, and thus their kinetic energy increases.
We'll look at five examples — from among the many reactions that have occurred during the history of astronomical evolution — involving two particles (electrons, protons) and three forces (electrical, gravitational, nuclear):
A1. An electron (with negative charge) and proton (with positive charge) are attracted toward each other due to electrical force, and eventually (700,000 years after the Big Bang) the temperature is cool enough for them to remain together and form a hydrogen atom, electron + proton → H
A2. Later, after gravitation has pulled lots of H-atoms (H) toward each other, so they are close enough to "find each other" and interact, electrical forces produce an attraction that causes H-atoms to form HH-molecules, H + H → HH
B1. In outer space, HH-molecules are pulled toward each other by gravitational force so they move faster and (like a ball rolling downhill) their kinetic energy & temperature increase. When the temperature is high enough, the HH-molecules are "jiggled apart" into H-atoms and then protons & electrons, in the reverse of Reactions A2 and A1.
B2. As the compression caused by gravity continues, the temperature keeps rising and eventually the protons are slamming into each other so hard that nuclear force (which is extremely strong but operates only at very short distances) overcomes the electrical repulsion between protons and pulls them together, which starts a series of powerful nuclear reactions that convert four protons into a helium nucleus, and a star is born.
B3. Later in the life of some stars, further nucleosynthesis occurs in a series of nuclear reactions that produce the heavier elements (lithium,..., carbon, nitrogen, oxygen,..., iron) from which our planet and our bodies are formed.
C. Some stars later become supernovas containing these heavy atoms, from which atoms heavier than iron can then be nucleosynthesized. When a supernova explodes it ejects heavy-nucleus atoms into space where gravitational forces can make the atoms condense (along with H) into stars, or into planets in solar systems.

What happened, and why?
For these five reactions, let's look at the changes in entropy caused by changes in constraint and temperature:
entropy changes due to constraint-changes: In Reactions A1-and-A2 the number of particles decreases when four particles (2 electrons and 2 protons) become two (H-atom and H-atom) and then one (HH-molecule) so constraint increases and this tends to cause a decrease of entropy. In B1 and C, gravity pulls particles closer together, thus increasing constraints (especially in C where gas particles become solid planets!) and this decreases entropy. In B2 and B3, nuclear forces convert many small particles into a few larger particles — in B2, four protons become one helium-nucleus; and in B3, many protons and helium-nuclei combine into a smaller number of heavier nuclei — which decreases their entropy.
entropy changes due to temperature-changes: In some of these reactions (A1, A2, B2) when particles are pulled closer by an attractive force they move faster so their kinetic energy and temperature and entropy increase; in other reactions (B2, B3) attractive nuclear forces lead to a nuclear reaction in which some mass is converted into energy (e = mc²) which causes an increase in temperature, and thus an increase of entropy.
• change of universe-entropy: In each reaction, a small entropy decrease (due to constraint-change) is overcome by a larger entropy increase (due to temperature-change) so the total entropy of the universe increases, consistent with the Second Law.
• change of system-entropy: This change depends on how a system is defined and how "open" it is to a transfer of energy. If a large amount of energy moves (as heat, photons,...) from the system to its surroundings, the entropy increase due to temperature-change (which occurs in each of the five reactions) will occur in the surroundings, not in the system, and the system's entropy can decrease due to its constraint-changes. But in each reaction there is still an entropy increase for the universe, as described by The Second Law of Thermodynamics.
• change of apparent disorder: In each reaction the particles become more constrained when they are organized into a form that is more ordered, organized, and complex. The overall result (in a sequence from A through C) is to convert electrons & protons into planets in solar systems — during a process of astronomical evolution that produced a change from simplicity to complexity, and an increase in ordered structures at the microscopic level and also at the macroscopic level (so there is a decrease in perceived "disorder") — due to the simple operation of attractive forces.
two kinds of intuition: An everyday intuition (based on the incorrect idea that "entropy is disorder") will reach wrong conclusions because in each reaction the apparent disorder decreases, so (based on psychological intuition about disorder) entropy should decrease, but in reality the entropy increases. By contrast, thermodynamic intuition (based on a correct understanding of entropy) leads to the correct conclusion, that entropy increases in each reaction. { During the past few decades there has been a decline in silly ideas about astronomical evolution among young-earth scientists, but ideas about the Second Law are often too vague and intuitive, so creationist and scientific concepts about relationships between "disorder" and entropy are often different. }
why reactions occur: At normal temperatures, most reactions are "driven forward" by the formation of stronger attractive-force interactions between particles (which is manifested as a temperature increase), not by a decrease of constraints. But at higher temperatures a decrease of constraints becomes more important, so in B1 "when the temperature is high enough the HH-molecules are ‘jiggled apart’ into protons and electrons, in the reverse of Reaction A."

why chemical reactions occur: In the common experiences of everyday life, most reactions are chemical, not astronomical. These chemical reactions occur because stronger bonds ("stronger attractive-force interactions between particles") are formed due to the reaction.
As a non-astronomical example of why things happen, let's look at the entropy changes for a simple chemical reaction that forms water in an explosion, when three gas molecules (HH OO HH) become two liquid molecules (HOH HOH).
In an isolated system, the entropy changes are analogous to the changes in Reactions A-C above: the constraint-changes (with three molecules becoming two, and gas becoming liquid, *) produce a small entropy decrease, but temperature-change (during the explosive reaction) produces a larger entropy increase, so entropy increases inside the isolated system, which is thermodynamically equivalent to a miniature universe.   /   * Other entropy-determining characteristics (molecular energy levels,...) also differ for the initial and final molecules, but the two major constraint-changes are 3-to-2 and gas-to-liquid.
In an open system, if kinetic energy escapes as heat from the system (into the surroundings) and the system's initial & final temperatures are the same, now (in contrast with the isolated system) there is no entropy increase due to temperature-change (because the temperature doesn't change), but entropy is lost due to constraint-changes, so entropy of the system decreases by a small amount. But the surroundings gains kinetic energy, causing its temperature to increases, and its entropy increases by a larger amount, so for the universe (defined as system + surroundings) entropy increases.   {The sizes of entropy changes — the "small amount" and "larger amount" — are discussed in the appendix.}

Why some things don't happen.
If we think of all possible reactions, and ask "Why do some reactions occur, but others don't occur?", we find two reasons: • If a reaction would violate the Second Law, it is thermodynamically unfavorable and will not occur. • Sometimes a reaction that could occur (since it is allowed by the Second Law) does not occur because its rate of reaction is extremely slow, so it is kinetically unfavorable. This possibility is why, earlier, I said "chemicals tend to eventually end up in their [thermodynamically optimal] equilibrium state" but not "chemicals will end up..."
For example, gasoline and oxygen can exist in a car's gas tank for years without reacting, even though their reaction is thermodynamically favorable and in some situations they will react very vigorously. Why do they not react? Because these chemicals must overcome an obstacle — an activation energy — before they can react, and usually (at normal temperatures when there is no spark or flame) the collisions between molecules of gasoline and oxygen don't supply enough energy to let them overcome this obstacle and become "activated," so they don't react and are in a temporary metastable state. {activation energy is explained more thoroughly in my longer page about thermodynamics-and-evolution}
As with most things in nature, the results of activation energies can be either bad or good. We want some reactions to occur but they don't occur due to activation energies, and we think this is bad. But gasoline doesn't burn in a car's gas tank, only in the engine, and we think this is good. Activation energies are also biologically useful because they provide a kinetic obstacle — so undesirable reactions are prevented, and (as explained below) desirable reactions can be controlled — and this allows life.

How our bodies make unusual things happen.
Inside our bodies, reactions occur that would not occur outside our bodies, and molecules exist that would not exist outside our bodies. How and why can this happen?
kinetics: Many reactions that usually are kinetically unfavorable can occur because some proteins, which are called enzymes, act as catalysts that increase a reaction rate and "make things happen" by providing a way to lower the activation energy and/or bring chemicals together in a spatial orientation that is "just right" for reacting. Enzymes operate in the context of control systems that control which reactions do and don't occur, and when. These control systems are analogous to a thermostat that turns a furnace on and off, when we do and don't need heat, but are much more complex and wonderful.
thermodynamics: Many reactions that usually are thermodynamically unfavorable can occur when they are coupled with another reaction. For example, if a biologically useful reaction that is unfavorable (because it produces a change of -400 in universe-entropy) is combined with a sufficiently favorable reaction (that produces a change of +500 in universe-entropy) the overall coupled reaction is favorable, since it produces an increase of +100 in universe-entropy. Our bodies use external fuel — the chemical potential energy stored in the foods we eat and the oxygen we breathe — to drive these coupled reactions.
Living organisms in the earth's biosystem are maintained in their unusual state (having high chemical potential energy, with low constraint-entropy and normal temperature-entropy) by using external energy from the sun, with solar energy first being directly consumed by plants, which convert it into chemical potential energy that can be consumed by animals. In both plants and animals, coordinated biological systems produce the mechanisms of life — operating in rate increasers, control systems, coupled reactions, and in many other ways — that are necessary to make these unusual things happen.

Entropy and Evolution

Why are young-earth creationists excited about thermodynamics? In 1976, Henry Morris explains his great discovery: "The most devastating and conclusive argument against evolution is the entropy principle. This principle (also known as the Second Law of Thermodynamics) implies that... evolution in the 'vertical' sense (that is, from one degree of order and complexity to a higher degree of order and complexity) is completely impossible. The evolutionary model of origins and development requires some universal principle which increases order... however the only naturalistic scientific principle which is known to effect real changes in order is the Second Law, which describes a situation of universally deteriorating order." In 1985, he summarizes the logic of his thermo-based argument: "The law of increasing entropy is a universal law of decreasing complexity, whereas evolution is supposed to be a universal law of increasing complexity."
Have creationists found a "devastating and conclusive argument against evolution"? When we're thinking about this question, scientific details are important, so we'll look at three very different types of evolution: astronomical, chemical, and biological.
In his 1976 paper, Morris claims that all types of evolution — as indicated in the [square brackets] below — are impossible because evolution "requires some universal principle which increases order, causing random particles eventually to organize themselves into complex chemicals, non-living systems to become living cells [chemical evolution], and populations of worms to evolve into human societies [biological evolution]" and he asks, "What is the information code that tells primeval random particles how to organize themselves into stars and planets [astronomical evolution], and what is the conversion mechanism that transforms amoebas into men [biological evolution]?"

Astronomical Evolution

A careful examination of why things happen shows that, during the history of our universe, a variety of reactions occurred because particles "did what came naturally" when they felt the effect of an attractive force: electrical, gravitational, or nuclear. The overall result of these reactions — which produced hydrogen atoms and molecules, stars, heavy-nucleus atoms, and planets in solar systems — was an "evolution" that produced a change from simplicity to complexity, and an increase in ordered structures at the microscopic and macroscopic levels. This increase of "ordered complexity" does not violate any principles of thermodynamics because, contrary to the claims of Morris, the Second Law is not a "universal law of decreasing complexity." None of these reactions violates the Second Law, and neither does the overall process.
While these localized reactions were happening (with temperature-increase producing an increase of total entropy) an overall mega-reaction of the universe was its expansion from an ultra-dense beginning into a much larger volume (with constraint-decrease producing an increase of total entropy). Yes, the small-scale localized contractions (due to attractive forces) and the larger-scale overall expansion (which occurred for other reasons) both produced an increase of total entropy in the universe.
Most scientists who are Christians are either old-earth evolutionary creationists or old-earth progressive creationists. We think the universe is 14 billion years old, and we accept modern scientific theories about the natural development of stars and galaxies, planets and solar systems, and the atoms that form our earth and our bodies. But most young-earth creationists think this astronomical evolution did not occur because the universe is less than 10 thousand years old, although there has been a decline in one silly young-earth claim about evolution because "except for the claims by Henry Morris, I have not been able to find any ‘thermodynamics in astronomy’ claims in the websites of prominent young-earth creation organizations."

Chemical Evolution

As explained above, the "ratchet" mechanism of neo-Darwinian evolution can produce an increase in biocomplexity. But this evolutionary mechanism would not exist before life began, and neither would the coordinated biological systems that make unusual things happen inside our bodies, including the formation of specific biomolecules that are needed to make the systems, in a "chicken and egg" problem. In addition, some chemical reactions that seem important for life (to make essential biomolecules) are energetically unfavorable, like a ball rolling uphill. { These problems are examined in my page about Thermodynamics and the Origin of Life. }

Biological Evolution

Yes, an evolution of increasing biological complexity can occur while total entropy (of the universe) increases. Is the Second Law violated by either mutation or natural selection, which are the major actions in neo-Darwinian evolution? No. And if an overall process of evolution is split into many small steps involving mutation followed by selection, each step is permitted by the Second Law, and so is the overall process.
A neo-Darwinian scenario with a one-way ratchet — with harmful mutations producing no major change in a population because organisms with these mutations are eliminated by selection, and neutral mutations able to survive selection, and rare beneficial mutations (facilitated by mechanisms such as gene duplication) preserved by selection — can produce genetic information and increasingly complex organisms. Therefore, it's wrong to claim that natural evolution cannot produce any increase of biological complexity. We can ask scientifically interesting questions about complexity — how much can be produced, how quickly, by what mechanisms, and can it be irreducible — but we should not waste time on unwarranted criticisms that claim the Second Law as justification.

Entropy and Information may seem similar in some ways because both are related to complexity, but they are different. Credible questions about the development of biological information have been asked, first by Thaxton & Bradley (framed in terms of thermodynamics) and later (mainly using information theory) by William Dembski, Steve Meyer, and other design theorists. Their questions about chemical evolution and biological evolution are worthy of serious consideration, and scientists are currently debating the merits of their claims. (an I.O.U. — Soon, maybe by mid-October 2010, there will be a little more about "information" here, and the homepage for Questions about Biological Evolution will link to pages by other authors about this question.)

Another type of long-term change is Geological Evolution: Almost all scientists think there is extremely strong evidence (*) supporting their conclusion that our earth is billions of years old, with geological features that usually developed slowly (over a long period of time) and occasionally developed quickly (in a short period of time) due to a "catastrophic event" such as a volcano, flood, or earthquake. Young-earth creationists disagree, claiming that most of the earth's geology and fossil record were formed in a catastrophic global flood. But young-earth creationists don't claim that conventional geology violates the Second Law of Thermodynamics, so for geological evolution there are no major questions about thermodynamics and entropy.
* You can examine this evidence in AGE OF THE EARTH - SCIENCE (with links to pages by many authors, both old-earth and young-earth) and (in a page I wrote) Young-Earth Flood Geology and Old-Earth Evidence.

Fixing a Four-Alarm Mess
In The Battle of Beginnings (pages 91-96), Del Ratzsch describes the "four-alarm mess" involving thermodynamic arguments against evolution — which are often unclear about the type of evolution being criticized (is it biological, chemical, astronomical, or just "evolution" in general?) — made by prominent young-earth creationists (Henry Morris,...) and by other creationists who borrow their arguments, plus misunderstandings by their critics, and so on.
As explained throughout this page, a generalized claim — that any evolutionary increase of complexity is impossible due to a "universal law of decreasing complexity" — is based on a misunderstanding of the Second Law, and is just wrong. By contrast, some specific claims are worthy of serious consideration and critical evaluation, but only for some aspects of evolution, mainly for a pre-biological "chemical evolution" to form the first life.
Part of the confusing four-alarm mess is the inconsistency. In anti-evolution writings by young-earth creationists there is a wide range of quality, both inside pages and between pages. Quality can vary within a page, if basic thermodynamic principles (that are accepted by everyone) and justifiable specific claims are mixed with unjustifiable general claims. And quality varies even more between pages, from one writer to another.
If they want to help us begin making progress toward fixing this mess, prominent young-earth creationists can explicitly reject the general claims that — when they are examined by scientists who understand thermodynamics — are obviously wrong, and explain why these claims should be rejected. They can say "oops" and apologize for the confusion caused by the errors of their predecessors.

APPENDIX

Entropy and Disorder — Creationist Misconceptions
In addition to oversimplistic generalizations — by declaring that the Second Law "describes a situation of universally deteriorating order" — Henry Morris also illustrates entropy with everyday analogies. For example, in 1973 he quoted Isaac Asimov, a non-creationist writer of popularized science: "The universe is constantly getting more disorderly! ... We have to work hard to straighten a room, but left to itself it becomes a mess again very quickly and very easily. ... How difficult to maintain houses, and machinery, and our own bodies in perfect working order; how easy to let them deteriorate. In fact, all we have to do is nothing, and everything deteriorates, collapses, breaks down, wears out, all by itself and that is what the Second Law is all about." Although it is true that "we have to work hard" to maintain order, this has nothing to do with the Second Law because it's about energy states, not messy rooms.
For several decades after 1961, Morris was the most influential advocate for young-earth creationism, and his books and papers (such as those quoted in this page, 1973 & 1976 & 1985) contain oversimplistic "appeals to intuition" that are still commonly used. Inspired by Morris, other young-earth creationists now use everyday examples to mis-illustrate the Second Law and mis-educate their readers. In June 2007, for example, a page from Christian Answers Network — which in a Google search ["second law of thermodynamics" evolution] is the #3 website — says the Second Law "describes basic principles familiar in everyday life. It is partially a universal law of decay, the ultimate cause of why everything ultimately falls apart and disintegrates over time. Material things are not eternal. Everything appears to change eventually, and chaos increases. Nothing stays as fresh as the day one buys it; clothing becomes faded, threadbare, and ultimately returns to dust. Everything ages and wears out. Even death is a manifestation of this law. ... Each year, vast sums are spent to counteract the relentless effects of this law (maintenance, painting, medical bills, etc.). Ultimately, everything in nature is obedient to its unchanging laws." This is very sloppy science, because the Second Law is about energy distribution not faded clothing.

Defining Entropy — Molecular Microstates and Probability
In an excellent book, Introduction to Chemical Thermodynamics, William Davies shows how entropy is mathematically related to "the number of microstates corresponding to each distribution, and hence is [logarithmically] proportional to the probability of each distribution." Each microstate is a different way to disperse the same amount of energy in the microscopic realm of atoms and molecules.
Davies explains how the number of microstates — with energy dispersed in all possible ways throughout the molecules' energy levels — depends on the properties of molecules, such as the magnitude and spacing of their energy-levels. He also explains how microstates are related to entropy and to the equilibrium state that the chemical system will reach after its molecules have finished reacting. And he describes a useful application of the Second Law: As a chemical system moves toward its equilibrium state, the number of possible microstates the system could be in (and still have the same overall macro-state) will increase with time, because entropy (which depends on the number of microstates) increases with time, and total entropy (of the universe) is maximum at equilibrium.

Entropy Numbers for a Simple Reaction
The section about why things happen concludes by explaining that, during a reaction to form water, the system's entropy "decreases by a small amount" but the surroundings' entropy "increases by a larger amount" so for the whole universe (= system + surroundings) entropy increases.
But what is a "small amount" and "larger amount"? For this reaction, here are the relative size of three entropy changes: -327 (decrease for the system due to constraint-changes), +1917 (increase for the surroundings due to temperature-change, which occurs because electrical "chemical bonding" has become stronger during the reaction), and these combine to give +1590 (increase for the universe).
Section 3B of my longer page shows the data for another formulation of the Second Law that is used in most first-year chemistry textbooks, with the Second Law stating that a reaction occurs naturally when the system's free energy decreases, which occurs when the universe's entropy increases. Or, in mathematical form, "DH (heat change of system) - T DS (entropy change of system) = DG (free energy change of system)" and G (free energy of system) decreases during a reaction.
At normal temperatures these sizes for entropy change (with 327 being much smaller than 1917) are typical, because most chemical reactions are "driven forward" by an increase in bond strength, not by a decrease in constraints. But constraint-changes become more important as temperature increases, as shown by T in the "T DS" term, which is why water forms ice (with strong bonds) at low temperatures, or gas (with minimal constraints) at high temperatures, or liquid (a compromise between strong bonds and minimal constraints) at in-between temperatures. { Section 3B also takes a brief intuitive look at how liquid water turns into gaseous water, and vice versa, due to interactions between the two thermodynamic factors, in a system's tendency toward getting strong bonds and minimal constraints. }

This page is an overview, summarizing the main ideas from my longer page about The Science of Entropy and Evolution that covers most topics in more depth, and adds some new topics. This appendix has summarized two topics and shows areas where the longer page offers "added value":
• Why am I writing these pages about thermodynamics? (your browser will take you to this section if you replace the URL-ending, #i , with #motives)
• Since theology is a major source of young-earth ideas, there is a brief summary of my views about Thermodynamics and Theology. (to find this, change the URL-ending to #theology)
• A brief introduction to energy-levels and entropy. (change ending to #levels)
• Why the concept of "disorder" is not important (although it sometimes is used) in Conventional Thermodynamics, but is very important in Creationist Thermodynamics. (#disorder, and a summary is below)
• Some details about Information and Entropy (and the boy who cried wolf). (#info)
• More about Fixing a Four-Alarm Mess. (#mess)
• An entire section (3B) with details about reactions in systems that are isolated, semi-open, and open. (#3b, and a summary is below)
• A simple example to illustrate activation energy, which is an obstacle that prevents reaction. (#obstacle)
• When is a mechanism needed? some examples of Creationist Confusions. (#confusions)
• Does life violate the Second Law? The answer (NO) is illustrated by the growth of a baby animal. (#growth)
• The difference between life and death is equilibrium, not the Second Law, which is operating throughout life from conception through childhood, maturity, old age, death, and decay. (#lifedeath)
• An Appendix covers these topics: Thermodynamics and The Origin of Life, Information and Entropy (What is the relationship?), A Range of Quality in Creationist Thermodynamics, The Second Law is Statistical, Free Energy Changes (Standard and Actual), Irreversible Reactions & Reversible Reactions, Sometimes entropy is important at low temperatures, Three Sets of Terms (for Three Types of Systems).
And, as described above, the longer page "covers most topics [that are in this introductory page] in more depth."

Entropy-and-Evolution and

The Second Law of Thermodynamics