How does water stay in a straw when you place your finger over the opposite opening?

Question

My son just asked me at dinner and I do not have an exact answer.

In: 7

Answer 1

Air pressure is pushing it back up.

Air pressure is pushing on us from all sides, and it’s going to be pretty much the same at the top of the straw and the bottom since the straw us so short, so can ignore the difference. The air bubble at the top of the straw that’s trapped under your finger will be at that same pressure. If the water tries to run out, it will mean that same air will have to fill the new volume above the straw, reducing that pressure. The pressure at the surface of the water is going to be the same pressure at the bottom of air bubble, which means the same expansion would need to happen in the water. You may have heard that water is “incompressible” meaning that as water is put under pressure, it hardly changes in volume. The same applies in reverse, reducing pressure means very little expansion. If that expansion can’t happen, then that means that the system must already be in equilibrium, and therefore nothing should move. By removing the finger from the end of the straw, that air bubble no longer is under the reduced pressure, so the air pressure from above is pushing as hard as the air pressure from below, so gravity takes over and the water falls.

Here is a (https://youtu.be/jzZR0_NSUOg) of the same phenomenon happening in a jar.

The air bubble isn’t necessary for this to work, in fact it’s a stronger seal without that expansion of an air bubble because it’s only the water trying to expand, which it can’t do. I just used it to help explain, and it’s more similar to whats happening in your straw.

Surface tension is also the only force preventing air from bubbling up the water to equalize the pressure and allowing the water to pour out. In the jar video, you can see when the students poke the surface of the water, they momentarily break the surface tension, causing an air bubble to form.

I have a very fond memory of asking my dad the very same question about straws as a kid, and I think you have yourself a future scientist on your hands, and I hope you nurture this curiosity.