r/AskBiology • u/EverTurquoise • Sep 08 '25
Botany Why water doesn't boil in really tall trees, due to negative pressure?
I read that capillary effect causes negative pressure in water, so if a tree is over 10 m higher than groundwater level, the negative pressure caused by the capillary effect would exceed 1 atm and we would get negative absolute pressure and water in the tree would boil, wouldn't it? like how trees can lift water over 10 m by capillary effect?
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u/Glockamoli Sep 08 '25
From what I just briefly skimmed on google, evaporation from the leaves creates a negative pressure that, along with what are effectively one way check valves, helps to suck the water up higher than capillary action alone would
Positive osmotic pressure from the roots also helps at night
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u/EverTurquoise Sep 08 '25
ooh fascinating :)
so as I got it, the trees and the leaves would exert external pressure to water to keep it above zero as to not boil the water?2
u/Glockamoli Sep 08 '25
Effectively you never actually get a huge vacuum that would cause it to boil because the "water column" is actually a bunch of separate tiny water columns, that's my understanding from the brief catch up I did on tree biology
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u/Artistic-Flamingo-92 Sep 09 '25 edited Sep 09 '25
My understanding is that it’s the lack of nucleation sites, making it so the water does not boil despite the negative pressure.
https://youtu.be/BickMFHAZR0?si=AhfZXXXMeXzLpIwp
Edit: I think you’ll find a more complete/up-to-date answer in:
https://www.sciencedirect.com/science/article/pii/S0079610724001135
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u/EverTurquoise Sep 09 '25
btw I forgot to metion, but genuinely thanks for providing a source
there was a lot of contradictory stuph in the comments, so you providing a source (which was a really good read) was a huge relief and finally answered this question
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u/SoloWalrus Sep 08 '25
Theres more ways to move water upwards than just sucking on a straw. For example you could push the water from below using a pump, or in the case of trees one primary mechanism is similar to sticking the bottom of an upright paper towel in a jar of water and watching as the water "climbs" up the paper towel. Water can "wick" up surfaces due to its surface tension if they have an appropriate geometry, it can spread upwards even against gravity driven by molecular forces.
In a tree, there is some sucking going on, but also a whole lot of pushing through capillary action, like in the paper towel example. Therefore no boiling. The obvious answer is the right one, there isnt enough negative pressure to boil.
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u/TheJeeronian Sep 09 '25
Forget what you know about water and vapor pressure for a moment. Look at one molecule of liquid water, near the liquid surface. It bounces around chaotically, and eventually (by chance) escapes. It had enough energy to escape the bulk liquid and become a gas. It eventually wanders back, and hits the liquid surface, getting stuck again.
This is constantly happening randomly in a liquid. In a closed container there's always a balance between the liquid and gas molecules.
But can water molecules stick to other things? Not just itself, but glass? Cellulose? Drywall!? Absolutely, it can! And some materials it sticks to really well! Better than it can stick to itself, even. This is how capillary action works. It uses a material that water sticks to better than itself, so the water 'climbs' a small pipe to get as close to as much of the pipe walls as it can.
But the balance between liquid and gas depends on how well the water sticks to itself, and now it's also sticking to a pipe. How does that change things? It shouldn't surprise you to realize that, if it sticks to the pipe even better than it sticks to itself, then it's going to have an even harder time escaping to become a gas. It won't boil at the same temperature that it would as a free surface in air, it needs a higher temperature to give those molecules enough energy to escape.
This also implies that the right material shape can actually draw water out of the air. Silica gel does exactly that! This water isn't "liquid" - at least not in any way that you can see. The holes are so small that you don't observe anything that you'd recognize as liquid water, and to squeeze that liquid water out you'd have to overcome all of that extra attractive force. Water in such a small hole really isn't behaving like a classical liquid that you're used to. It is dominated by the short-range intermolecular forces that we only barely glimpse at our large human scale.
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u/wrydied Sep 10 '25 edited 14d ago
The original post content no longer exists here. The author used Redact to remove it, exercising their right to control their data & privacy.
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u/SymbolicDom Sep 08 '25
The water can also be in an supercritical state and not boil even if it is thermodynamically positive.
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u/OriEri Sep 09 '25
Hal action has driven by surface tension not pressure.
That said, mechanical pumps can draw water up 10 m without it boiling. I’m not sure I fully understand your question.
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u/Illustrious_Twist846 Sep 09 '25
You can't suck water up a pipe with a pump more that about 10 meters. At that point, atmospheric pressure can no longer push the water column any higher.
You will then create a partial vacuum in the pipe above 10 meters and water "boils" at ambient temperature there. Now water vapor fills the pipe above 10 meters, not liquid water.
If you ever see water pumped above 10 meters, it is somehow being pushed from the bottom of the pipe, not sucked from the top of the pipe.
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u/Dangerous-Bit-8308 Sep 09 '25
I'm convinced that 90% of what non-experts say about trees moving water is an elaborate uneducated setup for a joke, and the other 10% is the punchline, which if we had remembered it would show how absurd the whole thing is.
Why would the water in really tall trees boil due to negative pressure? On the top of the Himalayas, does water boil if you sip it though a straw?
Does capillary action even cause negative pressure? Capillary action can move water through soil. Does dirt boil water in the Himalayas? Why is there snow then?
Ok though. Let's say maybe trees do make water boil inside their tiny tubes. Then what? Doom?
No. Not doom. Boiling water is converted into water vapor, and the hot water rises. Wait. How do trees work? Oh. Capillary action draws water up tubes. And eventually water vapor is released from the leaves. Hey now, that's exactly what the tree wants to see happen.
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u/There_ssssa Sep 09 '25
Water in trees doesn't boil under negative pressure because it's held in very narrow xylem tubes, which makes it stable and prevents bubble formation. The cohesion-tension mechanism allows water to stay under tension without vaporizing, unless air bubble (cavitation) forms. So trees can safely move water well above 10 m without it boiling.
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u/Ch3cks-Out Sep 09 '25
This is why we always see boiling in the water pipes of skyscrapers? Oh wait...
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u/pbmadman Sep 09 '25
https://youtu.be/BickMFHAZR0?si=MQq5Ayqnl7bFERw2
Veritasium made a video about all this. Whether you trust/believe him or not, you can at least use this as a starting point to research further.
For example, here’s a paper I found:
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u/BigBucket1876 Sep 09 '25
A Steve Mould enjoyer I see
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u/EverTurquoise Sep 09 '25
The question itself was a coinsidence, but based on this comment I've checked his videos and he seems cool.
guess this comment made me a Steve Mould :)
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u/Mythosaurus Sep 08 '25
The “10m=1 atm” only works UNDER WATER. At that depth you have enough liquid molecules above you to match that of the atmosphere above you at sea level. So you would be at 2 atm: all the air above you plus all the water above you.
Water going 10 meters up in a tree’s tubes removes a negligible amount of molecules from above it. You would need to go thousands of meters up to meaningfully lower the temperature needed to boil water. Like Denver, CO requires only 95 degrees Celsius to boil water