Why Don't Pencils Explode (Air Pressure)?

There’s a surprising amount of air pressure, esp. the closer you are to earth. Reportedly enough to crush a steel drum. You just don’t feel it, because the pressure is the same inside and out your body.

Also, things like, say, a carton of milk aren’t affected for much the same reason. But put it in a total vacuum and boom! The contents aren’t drawn out. They’re pushed out.

That doesn’t seem to hold true for other items though. And I can’t figure out why.

Pencils don’t explode in vacuums. Neither does a piece of paper or a rock. Apples and plums don’t explode in vacuums, at least AFAIK.

Why the heck not?


Because they’re completely solid. They’re not kept together by air pressure except to a tiny extent. Putting them in a vacuum will raise the speed at which their components sublimate (evaporate), but not to rates which are anywhere near explosive.

Conversely, liquids and gases (including those in our bodies) are kept together by external pressure to a much greater extent. This is very useful, as it allows us to do things such as distill at low pressure and low temperature mixtures whose components wouldn’t survive high temperatures so well. A human body will, uh, leak a lot when put in a vacuum (so will an apple, or a plum) but it also will not explode.

If you put a carton of milk with the top open in a vacuum, it won’t explode - the air will just rush out the hole. The point is that with the top closed it’s airtight, the air can’t escape slowly, but since the structure isn’t strong enough to withstand the pressure it will rupture and make its own hole.

But I’m not convinced of your assumption that the other items would not rupture (I don’t know about “explode”). With a slow drop in pressure, since they are not air tight, they would release air slowly. But with a rapid drop in pressure I think something like a bell pepper would certainly rupture. I think an apple has a fair amount of air and a moderately impermeable skin, I wouldn’t be surprised if it the skin ruptured. Wood is also air permeable to varying degrees, but obviously wood is quite strong, so it would probably withstand the pressure differential as the air escaped.

But in any event, I think your premise that these things would definitely not rupture (ok, maybe not “explode”) is mistaken. It just depends how permeable they are to air, and how quickly it can escape, and how strong they are.

I suggest you write a grant and do some experiments.

It’s mostly about the compressibility of gases vs the nearly-non-compressibility of solids and liquids. A sealed milk carton that is full to the brim and contains no air will not behave the same* as one that does - the gases expand** in lower pressures, causing the rupture.

*The liquid in the carton might boil at low pressure, releasing gases, but that’s a different phenomenon.

**perhaps a better way to put that is that the contained gases are compressed under ‘normal’ atmospheric pressure, and they stop being compressed in a vacuum.

Solids and liquids are compressible, but only to a tiny degree in comparison to gases.

It works the other way around too. Let all the liquid out of a container without letting air in and unless the container is very strong, it will collapse - sometimes spectacularly.

All things change their volume with different pressures. For a gas, this change in volume is very significant: Put a gas in an environment with a different pressure, and it’ll change volume dramatically, until it matches the pressure. For a solid or liquid, though, the change in volume is tiny. Put a solid or liquid in an environment at a different pressure, and it’ll also change volume to match, but that change in volume will typically be imperceptible.

How do all you guys know what happens to a carton of milk in a vacuum??? Or what doesn’t happen to a pencil in a vacuum???

Is this an honest question?


I mean, I haven’t put a pencil in a vacuum chamber personally, but I’m pretty sure it doesn’t turn into a duck.

Some of us have actually worked with high vacuum. While my own work didn’t include those specific two objects, it did include solids, liquids and containers with a liquid inside. So I’ve got both a ton of physicist’s work and my own inductive abilities.

Gases are compressible. A defining characteristic of liquids is that they don’t compress very much at all. Gasses in a vacuum will dissipate very quickly. Ditto solids. if they are contained, then there is essentially pressure on the inside of the container not matched on the outside. Then the question is - what will the container solid do? Stretch? Burst? Rip in one spot?

Liquids have a boiling point that changes with pressure. So a human body, a milk carton, or most other objects containing water - the water will evaporate very quickly when pressure is released. Once a milk carton loses its integrity in a vacuum (pops/rips open), the liquid inside will evaporate very quickly. AFAIK the heat of vaporization still applies - it takes a significant amount of calories to transition from liquid to gas, hence cooling whatever it is evaporating from. For an apple, for example - I’m guessing - the surface layer will evaporate first, creating a less permeable layer for the interior water to evaporate - which may cause the apple to crack or blow open, from the pressure of the evaporating water that can’t get through the outer layer… your best analogy is like when you put an apple or egg in a microwave and the water turns to steam. Egg especially, because that eggshell is particularly a good barrier to expansion until the pressure gets too much).

Solids don’t do much differently in a vacuum. The exception would be those with embedded gas or liquid pockets.

I’ve got a nice vacuum pump, a Kinney KC-5. https://www.vacuumpumprebuilders.com/Kinney-Pumps/kinney-kc-5-rotary-piston-vacuum-pump.html

Anyone in Cleveland have a bell jar?


To piggyback on that, Let’s be generous and say that the pencil from the OP has a air pocket in it that’s maybe 0.1" in diameter. Assuming the pocket is spherical in shape, surface area is 0.031 in^2.

If the pocket was originally filled with air at 1 atmosphere (14.7psi) and then brought quickly to a vacuum, that works out to a total outward force that the air pocket is applying of about half a pound.

Now take a pencil, rest it on the table, and try pushing down on it with the force of half a pound, over the same surface area (about the size of the end of another pencil). What’s going to happen to it? Nothing. The pencil strength can probably support 10 times that amount without permanent distortion or failure.

You can easily extrapolate from the observed physical and chemical properties of similar items under similar circumstances that have been documented in a vast ocean of YouTube videos, textbooks, and research publications dating back to the invention of the vacuum pump. If you believe there’s some esoteric, hitherto unknown property of milk cartons and pencils that makes such extrapolation unwise, you can buy your own hardware (bell jar, vacuum plate, and vacuum pump) for a few hundred bucks and test it yourself.

Bell jar, $35

Vacuum plate, $230

Vacuum pump, $65

Milk carton and pencil sold separately, but should add no more than $5 at most to your budgetary requirements. If you discover something new about these items that imbues them with unique properties under a vacuum, you can publish a manuscript about your research in a science journal and garner some fame.

If OTOH you’re the kind of person who needs to personally witness any given natural phenomenon before regarding it as accepted fact (as opposed to trusting the work of some other researcher that has since been verified/reproduced by still other researchers), your lifetime accumulation of knowledge will be very, very limited; good science takes time and costs money, and you have a limited amount of both.

This may sound like a nitpick, but it’s a nitpick in good faith:

The contents of that milk carton are neither drawn out nor pushed out. They flow because there’s a pressure differential between the interior of the carton and the vacuum outside.

Thinking about “pushing” or “pulling” is attractive on an intuitive level, but it’s really a question of perspective. It’s also kind of a misleading false dichotomy. If you’re on the low-pressure side, it seems like the contents are pushed out by the high pressure in the container. If you’re inside the container, it seems like things are pulled out by the lower exterior pressure (cf. getting “sucked out” of a plane due to explosive decompression).

It’s not pushing or pulling: it’s a pressure differential, which could also be described as pushing and pulling. For myself, at least, it’s helpful to think about pressure differences rather than whether something is being pushed or pulled.

When trying to fix a non-running piston engine, it helps to consider the absolute minimum requirements to run any piston engine: fuel, air and spark. If your engine isn’t running, one of those three things is probably either absent or present in the wrong amount. Pressure differentials are similarly helpful in thinking clearly about things like the OP’s question.

The conventional explanation of how airplane wings generate lift focuses on Bernoulli’s principle, which gets garbled in such a way that many people have the impression that low pressure “sucks” the wing up. That’s not exactly wrong, but it fails to explain why planes can fly upside down and why paper airplanes (with no airfoil) can fly at all.

In reality, wings produce lift by keeping the air pressure above lower than the pressure below. With a true airfoil, Bernoulli plays a significant role in creating that pressure differential, but it can be done with angle of attack alone.

Similarly, in the early days of rocketry, some (ill-informed) people doubted that rockets would work in space because the exhaust doesn’t have an atmosphere to “push” against. Of course, rockets work just fine in space equal/opposite reactions tell you all you need to know, and considering the exhaust’s “push” on anything external is a red herring.

I missed the edit window, but in my post above, the third paragraph from the bottom should read this way:

In reality, wings produce lift by keeping the air pressure above lower than the pressure below and by deflecting a mass of air downward. With a true airfoil, Bernoulli plays a significant role in creating that pressure differential, but both a pressure differential and downward air deflection can be done with angle of attack alone.

Aerodynamic lift is a fairly complicated phenomenon. In my attempt to make things as simple as possible (but not simpler), I made them a little simpler than possible.

I don’t think this cuts it, nor anyone of the youtube videos showing air being pumped out. Obviously things that are air permeable are not going to rupture if air is slowly pumped out. The interesting experiment would be to subject things to a very rapid drop in pressure. With an apple, for example, there may be enough air in it that if the pressure dropped quickly enough the skin might rupture.

Where, exactly, is this “air” in the apple? Are you talking in the core around the seeds? or are you asserting that an apple has lots of air just floating around inside the apple flesh?

Maybe I’m mistaken, but the flesh seems less dense than (say) a grape or a plum that’s mostly water?

ETA: Here you go, cite!

I’d expect that an apple probably does have tiny air pockets in it, distributed uniformly throughout the flesh, like wood does. After all, what (other than air) are apples made out of? Mostly water, plus sugar and other carbohydrates, which are denser than water. And yet, apples float.

From that cite:

The results in Fig 6 show about 15% air content in Braeburn apples.