Category: Writing

Hot Tub -> Shampoo

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Okay, so there’s the obvious route: Hot tubs are like bathtubs, where you’d usually use shampoo. But there’s not much science along that path. Let’s see if we can forge another one.

Hot tubs are so relaxing because heat loosens our muscles, which is a big-picture way of saying that blood vessels dilate when they’re warm—letting our bodies transport bad stuff out of muscles and good stuff into them. (That itself is a big-picture way of avoiding biology.) There’s probably some biochemistry that goes into dilating warm vessels (there definitely is), but there’s also some pretty straightforward physics. Something’s temperature is a measure of the internal kinetic energy of its atoms and molecules: The hotter the object, the more its molecules wiggle and jiggle. Cold things tend to be rigid because their molecules don’t have the energy they’d need to adjust their positions much. Warmer things loosen up, and they also tend to expand: their molecules can spread out now that they have the energy to push their surroundings out of the way. The same must be true for the walls of blood vessels.

This is fundamentally why we clean with hot water, too. It gives grimy molecules the energy they need to break free from whatever they’re affixed to. An awful lot of kinds of molecules dissolve in water, so anything loosened by the heat tends to be carried away by the water. Washing with soap and hot water is even better—at least as far as dirt is concerned. Soap molecules either bond with or otherwise break into bits of oil or dirt, and it’s easier of molecules to react and rearrange when they have a lot of energy.

Ridding any surface of oils and dirt is the central job of any soap. Soaps get specialized because we might not care very much if a soap is designed to mount an all-out attack on a plate (ceramic can handle it), but we’d probably care more about a soap that does such a good job cleaning our hair that it destroys the hair in the process. That’s why we use gentler stuff on our hair: Shampoo might not be quite as good at breaking bonds as dish soap, but the trade-off is worth it for something made to be gentle. We also want stuff in our hair that washes away easily, and shampoo is designed to be just that.

There. That was a more interesting route, huh?

Chimney -> Spoon

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Chimneys only work because hot air rises, which it does for reasons that I’m sure I’ll explain in more detail someday. But today is not that day. For now, we can just content ourselves with that statement and move on: Hot air rises. That’s not where my interest here lies, anyway. My interest lies at the bottom of the chimney. The fire in a fireplace can get pretty hot, anywhere from 500 to 1,100 or so degrees Fahrenheit. (That’s 260-600 Celsius, if you’re into that sort of thing.)

There’s a lot of complicated chemistry in a fire, but this is a physics blog. We can imagine the whole thing as reactions that combine oxygen from the air with carbon—making carbon dioxide—and hydrogen—making water—in the wood. Those reactions give off energy (loosely speaking), and that energy both sustains the fire and heats the surroundings. It can seem unbelievable at first that fires give off water, but you can prove it to yourself the same way Michael Faraday did: Put a piece of glass above a burning candle and, among the soot, you’ll see condensation. The soot is evidence of the chemistry I’m avoiding; the condensation is evidence of water.

Anyway, fireplace fires can be hot, but they’re nowhere near hot enough to melt most metals. Silverware used to be silver, sensibly enough, and silver melts at 1,700 degrees. Today’s utensils are often made of metals like aluminum instead of silver, but the melting point (and the broader point) is effectively the same. You’re not getting there in your home fireplace, no matter what kind of fancy system you’re running. Getting an oven that hot requires reinforcements. You often need to burn the remnants of ancient wood and other plant matter—what we’d more conventionally call coal. (Or propane. Or something else that burns that hot.) Once your oven is that hot, you’re able to melt all sorts of metals, pour them into molds, and form them into whatever shape you want.

The other option, of course, is to make your spoon out of something that melts at a low temperature, like gallium. Gallium spoons are a pretty popular gag among chemists, although it’s not an element you want sitting around at the bottom of your tea.