How Does a Siphon Work?
A siphon is a self-powered pump. Fill a rubber tube with water and pinch the ends to keep the water in. Put one end in a glass of water on a table and the other end in a bucket at a lower level. Release both ends, and the water flows from the glass to the bucket.
Atmospheric pressure and gravity, caused by the unequal weight of water in the short and long arms of the siphon, cause the water to flow from the glass to the bucket. As the heavier column of water in the longer arm of the siphon falls, it causes a suction that draws the water from the glass.
Historians have traced the use of siphons back to ancient Egypt. Ancient Egyptians used siphons in agriculture to transfer water from canals to irrigation channels. Today’s cotton farmers still use siphons in a similar way to help irrigate their crops.
Ancient Egyptians (along with people today) also used siphons in the winemaking process. Siphons helped them transfer wine from a large container to smaller containers. Placing the siphon hose in the middle of the container allowed them to increase the wine’s purity by avoiding particles that floated at the top or sunk to the bottom of the container.
A basic siphon consists of a tube in a larger container that goes up over a hump (the edge of the container) to empty out into a container at a lower level.
When liquid is sucked through the tube over the hump and begins to empty into the other container, a decrease in atmospheric pressure is caused at the highest point in the tube (where it passes over the hump). This decrease results in the atmospheric pressure on the surface of the liquid pushing liquid up into the tube toward the area of lower pressure.
While the atmospheric pressure theory seems to make sense, some scientists noted that it requires the presence of air. When tested in a vacuum, a siphon still worked, so it seemed that some other force must also be at work.
More recently, scientists who have studied siphons have theorized that the key force is gravity. When liquid is sucked up the tube and over the hump, the force of gravity continues to pull the liquid through the tube. This theory relies upon liquid cohesion, which means a continuous chain of cohesive bonds must exist in the liquid.
Some scientists refer to this as the chain model, because you can think of the water like a chain being pulled through the tube instead of a liquid. When you begin to pull the chain through the tube and over the hump, gravity will take over and continue to pull the entire length of the chain through the tube.
Unfortunately, most liquids don’t necessarily have strong cohesive bonds to make them act this way. Other scientists have created flying droplet siphons and carbon dioxide gas siphons that feature gas bubbles that exist between liquid molecules.
It may be the case that atmospheric pressure, gravity, and liquid cohesion all work together to make siphons work the way they do. Scientists will continue to study siphons to figure out once and for all how they work.