If melting of ice into water increases entropy, then does its freezing due to naturally occurring cold temperatures decrease the entropy? And does the entropy of the ice decrease or that of the whole Universe too? If it does not in case of the Universe, then why?
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This is hard to translate to Eli5 because it involves some concepts of thermodynamics so this is going to be long. Tldr at the bottom.
First of all, entropy is a measure of how disordered or random a system is. The more possible ways a system can be arranged, the higher its entropy. For example, a gas has higher entropy than a liquid, because the gas molecules can move around more freely and occupy more space.
Melting of ice into water increases entropy, because the water molecules have more freedom to move than the ice molecules.
The water molecules can slide past each other and take different shapes, while the ice molecules are fixed in a rigid structure.
Therefore, melting of ice is a process that increases the disorder or randomness of the system.
However, entropy is not only a property of the system, but also of the surroundings.
The surroundings are everything outside the system that can exchange energy or matter with it.
For example, if you have a glass of ice water on a table, the system is the ice water and the surroundings are the table, the air, and everything else in the room.
When ice melts into water, it absorbs heat from the surroundings. This means that the surroundings lose some energy and become colder.
The loss of heat from the surroundings also means that they lose some entropy, because heat is a form of disorder or randomness.
Therefore, melting of ice is a process that decreases the entropy of the surroundings.
The second law of thermodynamics states that the total entropy of the system and the surroundings must always increase or stay the same for any spontaneous process.
A spontaneous process is one that happens naturally without any external force or intervention.
For example, ice melting into water at room temperature is a spontaneous process, because it happens by itself without any external heating or cooling.
So how can ice melting into water be spontaneous if it decreases the entropy of the surroundings? The answer is that it increases the entropy of the system (the ice water) more than it decreases the entropy of the surroundings.
The net result is that the total entropy of the system and the surroundings increases, which satisfies the second law of thermodynamics.
Now let’s consider the reverse process: freezing of water into ice.
This process decreases the entropy of the system (the ice water), because it makes the water molecules more ordered and less free to move. However, it also increases the entropy of the surroundings, because it releases heat to them. The heat makes the surroundings warmer and more disordered.
Freezing of water into ice can be spontaneous if it happens at low enough temperatures, such as in a freezer or outside in winter.
At these temperatures, the increase in entropy of the surroundings due to heat release is greater than the decrease in entropy of the system due to freezing.
The net result is that the total entropy of the system and the surroundings increases, which satisfies the second law of thermodynamics.
However, freezing of water into ice cannot be spontaneous at higher temperatures, such as at room temperature or above.
At these temperatures, the increase in entropy of the surroundings due to heat release is smaller than the decrease in entropy of the system due to freezing.
The net result is that the total entropy of the system and the surroundings decreases, which violates the second law of thermodynamics.
Tldr:
* Entropy is a measure of disorder or randomness.
* Melting of ice into water increases entropy of the system (the ice water) and decreases entropy of the surroundings (everything else).
* Freezing of water into ice decreases entropy of the system (the ice water) and increases entropy of the surroundings (everything else).
* The second law of thermodynamics states that the total entropy of the system and the surroundings must always increase or stay the same for any spontaneous process.
* Melting of ice into water is spontaneous at high temperatures, because it increases the total entropy of the system and the surroundings.
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