Before we can answer the question," what does entropy mean? " we need to explain what we mean by " order tends to disorder. "
Some things happen in a specific sequence and could never happen on their own in a reverse sequence.
For example, a glass leaves your hand and breaks after it hits the floor.
The reverse cannot happen on its own! The glass could never unbreak itself and go back to your hand.
This example also shows something else. Order tends to disorder.
Another example of the fact that order tends to disorder is a balloon filled with helium in a closed room.
If you puncture the balloon, the helium atoms will spread in the room. Now, the helium atoms are all over the place in the room. Moreover, the helium atoms will never gather together in the balloon shape again.
When the glass is broken or when the helium atoms are spread throughout the room, they have taken a form that is not useful.
When the gas is burned in a car engine, some of the energy does work. However, some of the energy will go through the exhaust and it takes a non-useful (or unorganized) form that can no longer be used to power the car.
Here is what we learn from these examples and really this is another way of stating the second law of thermodynamics.
Entropy is a measure of the amount of disorder. The amount of entropy depends on the amount of disorder. As disorder increases, the amount of entropy will increase as well.
The amount of entropy also depends on time. If your desk is messy and you don't do anything about it, it may get worse in time.
Furthermore, the messy desk is a closed system because the mess will not fix itself unless you decide to bring some energy or do something about.
A closed system in thermodynamics is one that is isolated from its environment. No energy can enter or leave the system.
All the examples, that we mentioned in this lesson such as the glass or the balloon are also irreversible process.
Combining a closed system with a process that is irreversible, we get the following property