A cutlass is a sword, and swords have to be sharp. (It’s also a kind of car, which has to be sharp in a different way to cut through the air.) But we’ve been sharpening things far longer than we’ve had swords. Members of our genus have sharpened tools for millions of years. We’ve gotten good at it by this point. We’ve gotten so good at it that the Ancient Greek philosopher Democritus used a knife to reason his way to a fledgeling atomic theory.
Here’s what he said: When you cut into an apple, some of the apple ends up on either side of the knife. But the only way the knife can get between the two parts of the apple is if there was already space between them. So there must be bits of matter with empty space between them. QED—or whatever the Ancient Greek version of QED was. It’s not quite the argument we’d use today, but it’s neat, even when summarized as fleetingly as I just did.
Modern atomic theory came from the dual directions of physics—where atoms explained temperatures and sounds and a bunch else—and chemistry—where atoms explained reactions. Chemical reactions seemed to consume the input elements in certain ratios, and we now know it’s because a certain number of atoms of one element would always react with the same number of atoms of the other: Two oxygens and one carbon became carbon dioxide, say.
Fire, as we know from the other day, is a complex collection of chemical reactions involving oxygen. But fires need two other ingredients. They need heat and, of course, they need fuel.
I’m surprised “electrical outlet” was a “Hard” Pictionary result. That seems like a pretty easy thing to draw. Regardless, let’s get diving.
Scuba diving involves taking a tank of oxygen underwater, which is necessary because humans, tragically, are only fish in the most pedantic of senses. We need to breathe oxygenated air, and the oxygen we breathe comes from plants: We’d suffocate without them, while they’d be fine without us. They make oxygen using plentiful carbon dioxide and water. But where does that oxygen come from—the stuff in the carbon dioxide and water molecules? We’ve got a ways to go, so let’s make this answer quick: Mostly molten rocks, and also some dead stuff.
But that doesn’t satisfy me yet. Where do the actual oxygen atoms come from that have been kicking around our planet for billions of years? They don’t come from chemistry or geology. They come from physics. Stars bigger than the Sun make oxygen all the time out of carbon and hydrogen (and another hydrogen), although admittedly most of it then turns into nitrogen. Dying stars of all stripes make more lasting oxygen by smashing together carbon with helium. When really big stars die, they explode in a supernova, blasting oxygen and other elements out into the cosmos. (Sun-sized stars typically hold onto the heavier elements they make, although evidence is mounting that they often end up growing big after they die and exploding anyway.) We breathe—and are made from—the ashes of ancient stellar explosions.
Stars themselves are made of plasma, which is either a fourth state of matter or a super-hot gas, depending on whom you ask. If you ask someone who’s focused on molecular motion, plasmas and gases are the same thing: Substances whose molecules zing around so quickly that they don’t clump together. But if you ask someone focused on conductivity, they’ll say plasmas and gases are quite different. In a plasma, the molecules move so fast that electrons are knocked off of them, creating a sea of charge through which electricity can flow quite easily. Plasmas are conductors, while most gases are much too diffuse to conduct electricity.
Back down on Earth, of course, we don’t have to seek out a plasma to find something conductive. We have metals that we form into wires and plugs. Plug something in and electricity—electrons—flow down the conductive wire from the electrical outlet.
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.
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.