How does siphoning work?
The short answer
A siphon is a tool to transfer liquid from a higher elevation to a lower one, primarily by using gravity. Siphoning works through the combination of gravity, atmospheric pressure, and the forces of cohesion and adhesion.
The long answer
You probably know the scene in the movies when someone drops a tube into a full gasoline tank, sucks on the other end (and spits out the gasoline that went into their mouth), and watches as the tube starts to fill a bucket on the ground. That is siphoning, but how does a siphon work?
The key reason why siphoning works is gravity. As liquid travels up and over the crest of the tube, the weight of the liquid going into the lower container starts to pull the rest of it over the crest, causing the upper container of liquid to drain. No pump needed! This gravitational pull creates a continuous flow until the upper container is empty or the liquid level of the lower container is even with the upper container.
Gravity explains how liquid goes down the tube, but let's take it a step further into the two leading physics explanations that help explain how liquid goes up the tube.
The dual theories of siphoning
Theory #1: Atmospheric pressure
When you suck on the end of a straw, you lower the pressure within the straw. Since the pressure outside of the straw is greater, liquid is forced up the straw and into your mouth. The same thing is happening with a siphon. When you suck on the end of the tube, the higher atmospheric pressure on the outside causes the liquid to push the liquid into the siphon and promote flow.
Here's a very simple diagram showing how atmospheric pressure explains how straws work. The higher/lower pressure differences are also at work in a siphon.
What's interesting is that a siphon can also function in a vacuum (where there's basically no atmospheric pressure)! So Theory #1 doesn't fully explain how siphons are able to continuously pull liquid up, indicating there's another mechanism at work. Enter, Theory #2: Cohesion and adhesion.
Theory #2: Cohesion and adhesion
Zooming way down to the molecular level, liquids have the characteristics of cohesion and adhesion. Cohesion is the tendency of liquid molecules to stick together (think beads of water). Adhesion is the tendency of liquid molecules to stick to a surface (think water on concrete). Both of these forces aid in liquids being siphoned. Cohesion results in a chain-like force where liquid is pulled together up and over the crest of the siphon tube. Adhesion results in the attraction of the liquid to the tube's material, which is necessary to ensure the liquid column in the tube remains unbroken.
In summary, siphoning works through the combination of gravity, atmospheric pressure, and the forces of cohesion and adhesion. Now go forth and learn how siphons make toilets possible.
Sources
Boatwright, A., Hughes, S., & Barry, J. (2015, December 2). The height limit of a siphon. Nature News. https://www.nature.com/articles/srep16790
Evans, J. (2017, November 24). Siphons and the Siphon Effect: That Sucking Sound. Pumps and Systems Magazine. https://www.pumpsandsystems.com/siphons-and-siphon-effect-sucking-sound
ScienceOnline. (2010). The Siphon. YouTube. Retrieved August 19, 2023, from https://www.youtube.com/watch?v=CZmP0vsRBZ8&ab_channel=ScienceOnline
Trethewey, R. (2021, January 11). How Does a Siphon Work?. This Old House. https://www.thisoldhouse.com/plumbing/22217954/how-does-a-siphon-work