Entropy is like a teenager's bedroom 2


Entropy refers to the amount of unusable energy in a system

When water flows over a dam, it looses some of its useful energy.
Solid wood burns to become ash.
Ice melting.
Converting the useful energy to do work, some of the energy is lost in heat.
The decrease in energy over time is referred to as Entropy.


First of all, Entropy is a concept that everything in the universe tends towards disorder or chaos. For example, all living things age and die, they don’t get younger. Mechanical things tend to wear out and break down. Bricks are not naturally stacked up nicely or become part of a building unless someone has put a lot of effort into making it. So, chaos or disorder is more probable in the universe than order.

Entropy Change from Viziscience on Vimeo.


Thermodynamics is just a branch of physics that studies heat and other forms of energy (mechanical, electrical or chemical) and their relationships between each other.

  • 1) The Law of Conservation
    The First Law of Thermodynamics states that matter/energy cannot be created or destroyed.
  • 2) The Law of Increased Entropy
    The Second Law of Thermodynamics states that matter/energy gradually deteriorates over time.
  • While quantity remains the same (first law), the quality of matter/energy deteriorates. As energy is used for productivity – such as grow new cells or repair tissues, useful energy is converted to unusable energy.
  • The Second Law of Thermodynamics states that once energy is changed from usable form to unusable form, it cannot be recovered.
  • And Entropy is a measure of that unusable energy.
  • When unusable energy increases, it means Entropy also increases.
  • See http://www.allaboutscience.org/second-law-of-thermodynamics.htm

Energy transfer is studied in three types of system. It is important to understand what they are.

  • Open
  • Closed
  • Isolated

Click here for more information on Thermodynamic SystemsType of Systems - Themodynamic - Open, closed and isolated systems

Entropy and Disorder

There are quite a few variations for the definition of Entropy. One idea that often causes misconceptions is “Entropy is a measure of disorder”. One tends to think about patterns when the word “disorder” or “randomness” is used rather than thermodynamics. As described earlier, how do you measure disorder? Is it by how far molecules are apart from each other? Is it by arrangement or pattern of molecules, particles or objects? If you have a pack of cards randomly placed on a table next to a pack of cards neatly stacked up on the table, you may say one is more random than the other. As you can see below, group B & C cards are more randomly placed but the properties of the cards have not changed. All three groups still have the same arrangement of molecules more or less and they have not undergone any heat or energy redistribution. So change in Entropy has not taken place in all three groups of cards regardless of how they are arranged.

entropy in deck of cards

So, what is a good definition for Entropy? A good approach is to think in terms of heat or energy redistribution, not randomness. If you decide to burn one set of cards, now you’re talking! The set of cards that you decide to burn would undergo an entropy change.

First, to burn the cards,  you apply activation energy by means of lighting a match to it. The cards would then catch fire and burn spontaneously. The cards made of paper have potential chemical energy. Burning objects causes the potential energy in the materials to change. Some of the energy is transformed into thermal enery and dispersed into the atmosphere. Entropy is said to have increased. And overall, the burning of the set of cards adds to the increase in Entropy in the universe slightly. So, the more people burn cards, the more entropy is increased in the universe!

Entropy is a measure of the quantity of heat energy dispersal during a physical or chemical process happening at a constant or specific temperature.


When energy is dispersed, useful energy is turned into unusable energy. So if you burn a pack of cards, the bonds in the molecules of the cards are broken and some new molecules like carbon dioxide and water vapor are made. The heat produced from the process escapes into the atmosphere. Eventually, no more burning takes place. The cards become ashes and the temperature of the ashes become the same temperature as the atmosphere. Eventually, the differential temperature between the cards and surrounding air evens out and thermal energy movement ceases. Heat can only move from a place of higher temperature to lower temperature. If everything is at the same temperature, then heat energy cannot transfer. At this point you can say work is done – completed! And equilibrium is reached and you won’t need to call the fireman out to kill the fire.

A good explanation for Entropy:  http://www.theguardian.com/science/2013/dec/01/what-is-the-second-law-of-thermodynamics

entropy change in burning cards

You can see now that  usable energy decreases, unusable energy increases. The amount of unusable energy generated as a result of the process is related to Entropy. So Entropy increases when the amount of unusable energy increases.

For the burned cards, no matter how long you wait, the chances of them becoming “unburn” will never ever happen. Energy cannot flow back into the ashes to make the cards whole again by a natural process. The probability to reverse the process by itself is zero.

The Second Law of Thermodynamics states that Entropy in the universe always increases. This is true if the universe is an isolated system that you cannot put any more energy into it. It starts off with a certain amount of energy, and due to decay, all the usable energy will eventually be converted into unusable form. So even if you don’t burn the cards, over time, after many years, perhaps thousands or millions of years, the cards will eventually disintegrate. So Entropy increases over time. In that sense, naturally everything tends towards chaos, but not chaos in the sense of arrangement or patterns of things but chaos in the breaking down and degeneration of matter. And energy converted from usable to unusable form. And you can understand why scientists say time cannot go backwards, it only proceeds forward!

Definition from Chemistry textbook – Matter and Change by Glencoe

The textbook defines Entropy as the measure of the amount of “disorder” in a system.

Remember the discussion above says that disorder doesn’t mean the pattern or arrangement of particles. But sometimes the arrangement of particles can tell you about Entropy.

High entropy = lots of disorder
Low entropy = less disorder or more order

High and low entropy cartoon

How do you measure entropy?

One useful way of measuring entropy is by the following equation:

DS = q/T

  • DS represents the change in entropy
  • q represents heat or energy transfer
  • T is the temperature.

Using this equation it is possible to measure entropy changes using a calorimeter. The units of entropy are J/K.


The law of disorder states that the more energy you feed into any part of the universe (for instance your cup of water by heating it), the more rapidly disorder occurs in that part of the universe (the molecules in the cup of water get excited from the heating,  and thus move faster and thus lead to more chaos).

Everything tends to move towards disorder or randomness.

The law of disorder universe


How do you predict if a chemical or physical change is going towards orderliness or randomness? There are three principals you can apply.

1) Changing states (solid to liquid, liquid to gas)

Entropy becomes greater changing from solid to liquid, and liquid to gas.

In solids, the molecules are arranged more orderly. The molecules in liquids are more random than in in solids. In gas, the molecules are moving around very randomly.

(s) —> (l)     or  (l) —> (g)

2) Changing from a solute to an aqueous solution.

Entropy increases when you dissolve a solid in a liquid, or combining a liquid into a solution with other solvent.

It increases in entropy because now the solute particle can move more freely and randomly in a solution.

(s) —> (aq)     or  (l) —> (aq)

3) Increasing the number of particles (provided no change in physical state)

Example below shows the product number of particles is greater than the reaction particles.

The more particles (especially in a gaseous state) the more random movement of particles.


ΔSsystem > 0 Increase in entropy

ΔSsystem < 0 decrease in entropy




  • When a substance changes from solid to liquid or liquid to gas, entropy increases as the molecules become more random.
    ΔSsystem > 0
  • When a gas becomes a solvent, the entropy decreases. Gas is more free to move in the air than in liquid.
    ΔSsystem < 0
  • If the product number of particles is greater  than reactant number of particles, then entropy increases.
    ΔSsystem > 0


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