Storm in a Teacup: The Physics of Everyday Life(52)

For a molecule to escape from the liquid, it needs enough energy to escape the attraction of the others. This is evaporation, and it happens at the moment a molecule has acquired just enough energy to escape from a liquid and float off by itself to join a gas. My wet clothes were full of liquid water, molecules sluggishly moving around each other but without the energy to escape.

For three days in that monsoon, I tried everything I could to dry my clothes off. Drying clothes generally means putting them in a situation that will give the liquid water molecules enough energy to escape, so that they just drift off elsewhere. During bursts of hot sunshine, the liquid water absorbed the sun’s energy and the water molecules would slowly escape. But when it was cloudy, I was fighting a losing battle. The problem was that there was just too much water in the air. The air blowing toward the beach from the ocean was full of it. As the sun shone on the hot ocean, it warmed the surface layer. The water molecules in the ocean are also playing bumper cars, and the hotter the water gets, the faster they move, on average. As the ocean surface heated up, more molecules happened to end up with enough speed to escape. These molecules drifted up into the atmosphere to become gas instead of liquid. So the warm, moist air that arrived at the beach was already full of escaped water molecules. They were now playing bumper cars with the other molecules in the air.

When I got rained on, the warmth from my body was heating up my clothes, giving some of the water molecules I was carrying around enough energy to escape into the air. That was making the clothes drier. But the kicker was that there were so many water molecules in the air that they were bumping into my clothes and sticking. When that happened, they would just sink into the liquid mob, making my clothes wetter. The reason my clothes never dried was that the numbers of water molecules evaporating from them into the air was exactly balanced by the number of water molecules that were condensing on to them from the air. This is what 100 percent humidity means: that every molecule that evaporates is replaced by another one condensing. If the humidity is lower than 100 percent, more molecules will leave the liquid than arrive. The bigger that difference is, the faster things dry.

At night, it got worse. As the air cooled, all the molecules slowed down. So even more of them slowed down enough to stick to my top and shorts, and the clothes got even wetter. The point at which more molecules are condensing than evaporating is called the dew point, and the liquid drops that form are dew. Occasional molecules will still have enough energy to leave the liquid and join a gas. But their numbers are insignificant compared to the molecules that are coming the other way. If I had been able to heat up the clothes, I would have increased the number of molecules evaporating, perhaps enough to tip the balance back so that the clothes got drier. As it was, I was stuck with the wet, and so was the rest of India.

The point is that there’s always an exchange going on. That statistical way of looking at a sea of molecules is important because the molecules aren’t all doing the same thing. At exactly the same time, in exactly the same place, some molecules will be evaporating and some will be condensing. What we see just depends on the balance between those two possibilities.

There are times when it’s really helpful that each molecule in a mob is behaving differently. For example, when sweat evaporates, it’s only the molecules with the most energy that escape. The consequence is that the average speed of the ones left behind decreases. That’s why sweating cools you down; the escaping molecules take lots of energy away with them.

Clothes generally dry pretty slowly. It’s a peaceful process. Once in a while, a particularly energetic water molecule finds itself at the water surface with enough energy to escape, and off it drifts. But it doesn’t have to be like that. And violent evaporation can be very useful, especially when you’re cooking. It turns out that frying food, generally classed as a “dry” cooking method, owes an awful lot to water.

My favorite fried food is halloumi cheese, something I’ve always thought of as the vegetarian’s answer to bacon. It all starts with oil heating in a heavy pan, while I chop up rubbery strips of cheese. The oil silently takes in enough heat to raise its temperature to about 350°F, and if I couldn’t feel the heat nearby, I’d never know anything was happening. But as soon as I drop in the first strips of cheese, the peace is smashed by loud crackling and sizzling. As the cheese touches the hot oil, its surface layer is heated up to almost the oil temperature in a fraction of a second. The water molecules at the surface of the cheese suddenly have loads of extra energy, far more than they need to escape the liquid and float off as a gas. And so they burst apart from each other, producing a series of mini gas explosions as the molecules in the liquid are liberated. These bubbles of gas are what I can see at the surface of the cheese, and this is where the noise is coming from. But the bubbles have an important role to play. As long as gaseous water is streaming out from the cheese, the oil can’t get in. It can barely touch the surface, only just enough to pass on heat energy. This is why frying food at too low a temperature makes it greasy and soggy; the bubbles don’t form quickly enough to keep the oil at bay. As the cheese cooks, some heat is transferred into the bulk of the cheese, heating it up. The outer edges give away lots of water, because it’s too hot for liquid water to remain there. This is why the outer surface becomes crispy—it’s dried out. The browning comes from chemical reactions that happen as the proteins and sugars in the cheese get heated up. But the sudden transition from liquid water to gas is at the heart of how frying works. And frying food has to involve sizzling—if you’re doing it right, there’s no way to avoid it.