Order and disorder
June 09, 2020
Matter must approach a phase transition in one direction or the other. Either ice is melting into water, or water is freezing into ice. The original temperature, and the change in temperature, dictate which one is happening.
You can think of solid matter as being more ordered than liquid matter. It is less fluid and behaves less randomly. Liquid likewise behaves more predictably than a gas. A gas is the least predictable and most random of the three.
Think of it this way: If you drop a piece of ice, you can just pick it up again. If you wait for it to melt, you can mop it up, which requires more work. But once it has evaporated, you essentially can’t reconstitute it. It’s not that the water has actually disappeared, it’s just mixed in with the air in such a random fashion that it is impossible to separate.
This irreversibility is important. You can think of this as a move away from order and towards disorder. It an increase in entropy. For now, it’s just that it is harder to predict where any given molecule is in a gas than a liquid, and in a liquid than a solid. If you freeze water, that’s the opposite: a move from disorder to order. That’s a decrease in entropy.
I think that the directionality of phase transitions is important. Either something can freeze, becoming more ordered, or it can melt, becoming less ordered. In the most interesting cases, it will go through repeated cycles of freezing and thawing. It is in these cases that something new can emerge.