Most things in our world seem to happen gradually.
Consider temperature. If you want the room to be a little bit warmer, you can turn a thermostat up that controls the heating. In the days before electrical heating, this might mean throwing on one more lump of coal to make it just a hair warmer, or throwing another log on a fire that’s starting to lose heat.
Move your icy hands a bit closer to that fire, and they’ll slowly start to heat up. This mirrors the gradual change in seasons most of us experience, where summer slowly gives way to fall, before winter grips us with its chilly clutches. Even on a day with wild temperature changes, it’s really not an all-at-once type change.
All of this is easy for our minds to grasp, but there’s something funny about water, isn’t there?
Sure, it cools down gradually as the temperature drops. That’s very intuitive—if it’s snowing outside and that lake is not frozen, you almost certainly don’t want to jump into that lake for a nice relaxing swim. You know the water has gradually cooled down to near-freezing temps by now.
Water is thicker than air, so it takes a bit longer for temperature to spread. When it gets much colder outside, the water is going to take a while to cool down as much as the surrounding air. What happens when the water temperature crosses that threshold from near-freezing to freezing?
Suddenly, things aren’t gradual any more. Suddenly, the very nature of the water itself transforms.
What’s going on here? Why should water be all right at a tenth of a degree above freezing, but solid as a rock at a tenth of a degree below?
Instead of what we’re used to, it freezes in a near-instant, like a snap of the fingers. Everywhere else we look, it seems that things happen gradually. In our world, water is very much the exception to the rule.
This is called a phase transition. Down at the smallest scales of nature, things are famously weird, and phase transitions are the rule, not the exception. It’s worth diving in here to look at the most famous of all these transitions—when water freezes.
Water seems kind of lame, right? If you’re a kid in the 80s at the Pizza Hut Buffet, the last thing you want to drink is a glass of water.
And yet, here it is, right in front of us—this ultra-common fluid that lives right inside of us behaves in the most nonintuitive way! What’s going on?
Shenanigans, that’s what.
Water is a dance of molecules just bouncing around off of one another like a crowded mosh pit. It’s probably a mosh pit at a pop-punk show, though, so very few molecules get hurt or thrown out of the club.
As the temperature goes down, it’s like a slow song comes on. There’s still some of that pushing and shoving, but it’s nowhere near as frenetic. Things are starting to slow down. Maybe it’s like a pop-punk reunion tour in 2024 now.
Once you cross the zero Celsius threshold, there is a microscopic (and yet also titanic) struggle going on between forces. Gradually (at the tiny level), the molecules in the mosh pit slow down. It might help to remember that heat is really just movement, so when the temperature goes down, it means that the movement down at that level goes down too.
Meanwhile, the attractive force between water molecules keeps right on doing its thing. They’re basically like little magnets, drawn together by the electromagnetic force, and they’re just way too tired to mosh now. Some kind of sappy song just started playing, and the 50 year old punks at the reunion show are just kind of hugging one another now.
Although a body of water takes a while to freeze, each individual molecule snaps into this orderly arrangement more or less instantaneously. There’s no moment where two punks with bad backs slowly help each other into chairs, but more like you blink and then they’re sitting down and hugging one another.
As more and more punks start hugging one another, a critical mass happens where the whole room just kind of stops moving all at once. This is the best example I know of for when something happens gradually, then suddenly.
It’s closing time at the punk club, which is like 9 PM for 50 year old punks, or so I hear. Everyone is just frozen in place, and it’s time to go home.
Water molecules have a particular structure that allows them to grab onto four other water molecules at a time. I’m going to do the slightest bit of hand-waving here and not worry much about precisely how, but that’s really just in the interest of moving this piece along. Suffice it to say, these little clumps draw individual groups of water molecules together into clumps.
Only, they’re not really drawing all that close inward toward one another. It’s more like the old punks are holding hands with their arms completely outstretched now. There’s space in between them, in the middle of the groups of four water molecules… er, punks.
This means the old punks need more room than the young moshers. They take up more space, even though they’re not moving around!
If you’ve enjoyed this weird blend of physics and punk nostalgia, you might enjoy reading some of the other things I’ve written. Here are seventeen pieces I’ve tagged with the #punk moniker, and here are ninety-six pieces on physics.
Another phase change I remember from chemistry is liquid -> gas. Is that also instantaneous? Baby punks?
“A watched pot never boils”