If you ignite a piece of paper—mostly cellulose—it burns to produce carbon dioxide, water vapor, and ash; a mixture of carbon dioxide, water vapor, and ash never produces cellulose. An ice cube left on the counter turns into a puddle of water, but a puddle of water on the counter never turns into an ice cube. Cream mixes with your coffee; they never separate. Gas expands into a vacuum, but all the molecules of a gas never spontaneously move to one side of their container to leave a vacuum on the other side. Why not?
We are so accustomed to the fact that most processes are irreversible—that they have a one-way direction—that it seems almost silly to ask Why? Nonetheless, we’ll focus on that question now. It turns out to be a profound question with many implications ranging from how efficient an engine can be to the very existence of life.
If you think about it, the processes just mentioned wouldn’t violate any of the laws of physics we have studied so far if they ran backward. Energy is conserved as heat flows from a warm counter to a melting ice cube, and energy could be conserved just as well by heat flowing from a puddle of water to the counter, lowering the water’s temperature until it freezes. Yet that doesn’t happen. There must be another law of physics at work that prevents some energy-conserving processes from occurring.
This new law—the second law of thermodynamics—is subtle, so we’ll start with a qualitative overview before we dive into a quantitative analysis.