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Spontaneous Process: Definition & Examples

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  • 0:05 What is a Spontaneous Process?
  • 1:04 The Reverse Process
  • 2:32 Purpose of Spontaneous…
  • 3:32 Freezing and Melting
  • 4:28 Lesson Summary
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Lesson Transcript
Instructor: Nichole Miller

Nichole is a research scientist with a PhD in Materials Science & Engineering.

This lesson defines and gives examples of spontaneous processes, explains entropy, and discusses what the second law of thermodynamics says about spontaneous processes.

What Is a Spontaneous Process?

Imagine you are holding a glass of water and you let go. What will happen? The glass will fall to the ground, break, and spill the water. This process happens because this is a spontaneous process. Will the reverse ever happen? Will the glass shards reassemble into a glass, the spilled water go back into the glass, and the glass leap up from the ground into your hand? You could painstakingly pick up the pieces, glue them back together, and collect the water back into the glass, but this reverse process will not happen on its own. It needs intervention, and therefore, it is not a spontaneous process.

A spontaneous process is a process that occurs on its own without outside intervention. Outside intervention is something that changes the process after it has started. In this example, the process starts as soon as you let go and leave the glass unsupported in midair. After this point any input from the outside, such as you catching the glass as it falls, is outside intervention.

The Reverse Process

A process that is spontaneous in one direction is not spontaneous in the reverse direction. Here are a few examples:

  • A sugar cube will spontaneously dissolve in hot coffee, but dissolved sugar will not spontaneously come out of the coffee and form a sugar cube.
  • Popcorn kernels will spontaneously pop when heated, but the popped popcorn will not spontaneously go back to being a kernel.
  • If you add a drop of blue food coloring to a glass of water, the dye will spontaneously mix with the water to form blue water. In contrast, a cup of blue water will not spontaneously separate into clear water and a drop of blue food coloring.

It's also important to remember that the direction of the spontaneous process can depend on temperature: below freezing water spontaneously freezes, whereas above freezing ice spontaneously melts. In both of these cases, the reverse process is not spontaneous.

Just because a process is spontaneous, doesn't mean it has to happen quickly. Spontaneous processes sometimes happen very slowly. For instance, have you ever had to polish your grandmother's tarnished silver? I have, and I am very thankful that even though the tarnishing of silver is a spontaneous process, it's also a very slow process. I therefore only had to help my grandmother polish silver about once a year. Some spontaneous processes happen very quickly, like the example of the glass of water falling to the ground; others, like the rusting of iron, can happen slowly over the course of years.

Purpose of Spontaneous Processes

We now know that spontaneous processes occur, but why? The second law of thermodynamics gives us the answer. The second law of thermodynamics states that the entropy of the universe increases for any spontaneous process. Let's break that down a bit.

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