Barometers, Atmospheric Pressure, and Super-Long Straws (PLA 9)

Ben Lear

Alright, welcome back to The Honors Element—Ben here, outnumbered by chemistry once again, and I’ve got Morgan Vincent joining me, as always. Today’s episode connects a physics mystery, barometers, to our chemistry toolkit. So, Morgan, did you ever build your own barometer back in middle school, or was that just students with questionable science fair ambitions?

Morgan Vincent

Oh, Ben, it definitely wasn't me. I think the closest I got was a rock-solid clay volcano that somewhat spewed fake lava. But the history behind the real thing? Super cool. Barometers go way back to the 1600s. And Torricelli’s the big name, right?

Ben Lear

Yeah! Evangelista Torricelli, Italian scientist, and an assistant to Galileo, actually. His experiment was a classic setup. He took a long glass tube, sealed one end, filled it with mercury, then flipped it over into a dish of mercury. Some of the mercury drops out sure, but a column stays. The column is about 76 centimeters tall. There’s a near vacuum at the top. That was all that was needed for the proof: the air around us has weight.

Morgan Vincent

Exactly, and the height of that mercury column, about 760 millimeters, balances the weight of the entire atmospheric column pushing down on the dish. It's a literal tug-of-war: mercury against air. If that balances out, it’s atmospheric pressure: one atmosphere, to be precise. And that’s why one atmosphere is defined as the pressure needed to support that 760 mm column of mercury at zero degrees Celsius.

Ben Lear

To this day, we still use those classic units, millimeters of mercury, often called mmHg, but also "torr" and, of course, the more modern pascal. Have you ever seen the old weather forecasts? That’s why they talk about inches of mercury too. And the barometer? It set the standard for measuring air pressure and, indirectly, for helping weather prediction and lots of science since.

Morgan Vincent

I always picture early scientists with mercury everywhere… no gloves or goggles. It just makes me shake. But it’s wild how such a simple balance gives us something so fundamental that we still use in chemistry labs, isn’t it?

Ben Lear

Absolutely, and to tie it back, barometers basically gave us the language for pressure that we still use in pretty much every gas law we’ll talk about all semester: Boyle's Law, Charles’s Law, everything. That’s all built on knowing that atmospheric pressure is this invisible force we can see, if you pick the right column of liquid. Mercury, preferably, not water from your garden hose.

Morgan Vincent

So, once you have your barometer and that classic 76-centimeter mercury column at sea level, what happens if you start climbing up a mountain? The height of the mercury column drops, not because mercury got lighter, but because there’s literally less air above you pushing the mercury up. A barometer is a direct readout of how much air is left overhead.

Ben Lear

Exactly. And that’s why hikers and our fellow frisbee golfers at elevation, will see lower readings the higher they go. What you’re measuring, technically, is the pressure from a “column” of air that stretches all the way up from your feet to the edge of the atmosphere, somewhere in the stratosphere, about 150 kilometers up. The higher you go, the shorter that column, so the pressure drops.

Morgan Vincent

Which brings us to something called the barometric formula, though don’t worry—we’re not going to throw integrals and exponents at you today. The important part is that as you go higher, atmospheric pressure decreases in a way that’s exponential. You can see that in weather balloons or when you’re flying on a plane: pressure drops fast at first, then slower the higher you go.

Ben Lear

Yeah, and if you’re curious about the math—like, really want to dive in—it’s there in our textbook and the online resources. It honestly wouldn't hurt to take a look not only at the equations, but also a picture of a barometer.

Morgan Vincent

Anyway for chemistry, what matters is this: the pressure at different heights can be linked back to the behavior of gases, especially if you treat air as an ideal gas and assume constant temperature. It’s the same ideal gas law we’ve been building on since Boyle and Charles: pressure times volume equals moles times our gas constant times the temperature. The barometer, in a way, just visualizes what those equations describe.

Ben Lear

And so much, in chemistry, circles back to this idea: pressure changes, altitude changes, but those changes are predictable if you keep track of your system, your assumptions, and, well, if you don’t forget gravity. For now just know, as you go up, the air thins out, the pressure drops, and that’s why your chips bag gets all puffy on a plane.

Morgan Vincent

I always love that chips bag example. Or those sealed water bottles that look like they’re ready to explode. I traveled a lot this summer and had a few that spit water out when I opened them. So, it all comes down to pressure: inside versus outside, and that’s what barometers capture so elegantly with just a column of mercury. Cool stuff.

Ben Lear

Let’s take this a bit more hands-on and a little less historical. No offense, Torricelli. Have you ever tried using a ridiculously long straw? Like, not just a Big Gulp straw, but the kind you might see in a science demo or a late-night internet video?

Morgan Vincent

Not past about a foot, honestly, but I’ve definitely seen those “world’s longest straw” challenges online. Someone on a balcony, iced tea on the ground, and pure stubbornness at play. But, Ben, the real question is: chemically and physically, why does the liquid even move up at all?

Ben Lear

That’s it. Everyone thinks they’re “sucking up” their iced tea, but you’re not. What actually happens is, when you create a vacuum or honestly, just lower the pressure inside the straw. So, the atmospheric pressure on the liquid in the glass pushes the liquid up the straw. You’re not yanking the water up with your mouth muscles. It’s the weight of the air around you shoving it up, because the pressure outside the straw is greater than inside.

Morgan Vincent

So, theoretically it sounds like you could make the tallest straw in the world, right? Except, there’s a catch. Even with superhero lungs and zero leaks, the atmosphere will only push water up to about 10.3 meters or about 34 feet. That matches the pressure of a one-atmosphere air column, just like with a barometer, except now, it’s a water column. Try for longer, and you just get air, not beverage, no matter how hard you “suck.”

Ben Lear

Exactly. There’s a neat symmetry here: Mercury barometers max out at 76 centimeters of mercury, that’s one atmosphere. A water barometer would reach about 10.3 meters. They have the same pressure, but different densities. So, next time someone tries to impress you with a 30-foot straw stunt, you can ask them about Boyle’s Law and atmospheric pressure and watch their eyes glaze over… or join in, I guess, but have some sympathy for gravity.

Morgan Vincent

And those limits aren’t arbitrary, right? It’s all about the physics. The higher the straw, the less pressure difference for the atmosphere to work with. It links everything we’ve talked about: barometers, pressure at altitude, Boyle’s Law, you name it. So if you want an outdoor chemistry experiment, bring a tape measure, a big bucket, and maybe a lifeguard. Otherwise, keep your straws responsible, folks.

Ben Lear

Well that’s a wrap for today! Next time, we will dive into the kinetic molecular theory, exploring how the invisible motion of gas particles explains pressure, temperature, and the very measurements we started with using barometers.

Morgan Vincent

Sounds good. Thanks for tuning in, and Ben—don’t bring any questionable straws to the next recording, alright? Catch you all next time on The Honors Element!

Ben Lear

You got it, Morgan. Bye everyone!