Why does the water boil if it sucked beyond 34 feet in height?

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I saw this video on YT shorts where the guy demonstrates how it is impossible to suck water beyond 34 feet from the level it is stored at.

[This is the YT short](https://youtube.com/shorts/rWij9gJWcTg?si=5nTRvPUVFUKSaJmH)

On Google it says that the atmospheric pressure equals the pressure exerted by water column.

I don’t understand how that matters! Because in the end the guy attached a vacuum pump to pull the water up.

Also, if the water boils then it should also make steam.
Does PV=nRT apply there? I know it’s not ideal gas but for all practical purposes we use this equation.

In: Physics

7 Answers

Anonymous 0 Comments

When water boils, the bubbles aren’t air but steam. The energy required for the water molecules to escape the liquid as a gas at lower altitudes is higher because there is more atmospheric pressure on them, keeping them condensed. As you go up in elevation, the boiling point goes down because now there is less energy required to escape the water. Eventually you’ll come to a point where there’s not even enough pressure to prevent the water from boiling which is what happens in the straw, it has no ability to stay together without simply dissipating because there’s no pressure on it. If you went to space without an astronaut suit, every liquid in your body would boil before you froze or anything like that.

Anonymous 0 Comments

The first thing to realise is air has weight.
The atmospheric pressure of kilometers of air above you makes water stay in liquid form. Without this pressure water would turn into gas immediately without any heat at all, even at the lowest temperature possible.
Edit: apparently it does need some heat, well above absolute zero , but quite below the normal freezing point under atmosphere.
The pump creates a vacuum and thus removes the pressure that holds water in liquid form.

Anonymous 0 Comments

To understand that you first need to understand what “sucking” actually is.

It all comes down to pressure. Pressure is atoms and molecules pushing against something, so there can only be positive pressure. By having pressure from the atmosphere and less pressure somewhere else the atmosphere will push the matter towards that point. So if we reduce pressure we are sucking matter towards that point. So when one end of the straw has normal pressure and the other end has no/low pressure the water gets pushed up towards the no/low pressure by the atmosphere.

Now the maximum pressure differential we can achieve in a straw is atmosphere vs nothing, so 1atm or 1,013 hPa. That is enough energy to push up the water 34 feet. After that you only have vacuum above the water, but it can’t go higher because the water is too heavy for the atmosphere to push it higher up. We could go higher by applying more force on the source at the bottom.

Now why does the water boil? Because the boiling point of matter is determined by outside pressure. The less pressure the lower the boiling point becomes. With your mouth or a regular pump you can’t create an actual vacuum with no matter in it(nobody can, it’s just different degrees of how good of a it vacuum is), but with the pump the pressure gets low enough to put the boiling point of water below ambient temperature making it boil at the surface where it touches the vacuum.

Anonymous 0 Comments

When you suck on a straw, you lower the pressure above the liquid, and there’s atmospheric pressure on the other end. So if imagine that in your mouth there is vacuum, and atmospheric pressure at the bottom of the straw, there’s a 1 atm pressure difference pushing the water up. But the water is being pulled down by gravity. So the force on the water is 1atm minus the weight of the water. Once the weight of the water is equivalent to atmospheric pressure, which occurs when the water is 34 ft tall, there is no net force pushing the water up anymore.

It doesn’t actually have to deal with water boiling. Even if water didn’t boil under those conditions (0atm and room temperature) you still couldn’t pull water more than 34 feet up with vacuum.

A (non vacuum) pump applies a positive pressure on the source of the water, while a vacuum pump lowers the pressure at the destination. Vacuum pumps are inherently limited since pressure can never be lower than zero. Traditional pumps are not limited by atmospheric pressure, since they you can always keep raising pressure.

Anonymous 0 Comments

Liquids are liquid because the pressure of the atmosphere smooshes them together. Nearly all liquids start boiling in vacuum until they’re all gas or their container fills up with enough vapour that the pressure is enough to keep the rest liquid.

So, if you start sucking too hard on the straw, you’ll remove enough gas that the liquid will no longer be opposed in its efforts to vaporize.

Anonymous 0 Comments

Ignoring the boiling part for a moment, you can only suck water up a tube because air pressure is pushing it into the tube from the outside. The maximum distance is when the weight of the water in the tube equals the weight of a column of air outside the tube, with the same cross section, all the way up to space.

As for the boiling part, take a look at a phase diagram for water. As you decrease pressure, the boiling point drops. Below a pressure of around 611 pascals water only exists as either a solid or a vapor. So if you put a vacuum pump on it, and keep it running, the water will boil, losing heat until it freezes.

Anonymous 0 Comments

Firstly, hydrostatic pressure. The higher the water column, the greater the pressure exerted by the weight of the water at the base of the column. But as a corollary, the greater the vacuum created at the top of the column.


It’s a bit like gravity pulling on a piston to create a vacuum in a chamber.

>On Google it says that the atmospheric pressure equals the pressure exerted by water column.

That’s not true. If the system is airtight, there’s no atmospheric pressure involved. When the system is an open reservoir, atmospheric pressure is added to absolute hydrostatic pressure to obtain absolute pressure. This is not the case with a depression tube used for pumping.

When the water column exceeds a certain size, the pressure becomes so low at the upper surface that the water evaporates at room temperature. This is what happens when cavitation occurs on a propeller.


This is why pumps are always installed at the lowest point of hydraulic systems. The aim is to avoid putting circuits under negative relative pressure, as this is inefficient and increases the risk of cavitation. We don’t like syphons and anything that can put circuits under vacuum in general, as most vessels are not designed to it and would end up like a crushed Coke can.

TL/DR:The guy in the YouTube video is making up nonsense.