Science Puzzle
How Does a Siphon Make Water Flow Uphill?
To empty a fish tank, fill a tube with water, put one end in the tank and the other in a bucket on the floor. Water flows up out of the tank, over the rim and down into the bucket, and it keeps on flowing until the tank is nearly empty. No pump, no squeezing.
Water doesn't normally flow uphill. How does a siphon work?
The Answer
The end in the bucket is lower than the water in the tank, so the water in the long, outside part of the tube weighs more than the water in the short part inside the tank. The heavier side falls and pulls the rest of the water after it, up and over the rim.
Think of the tube as a chain of water hanging over the rim. The side going down to the bucket is longer, so gravity pulls harder on that side, and the water slides that way. As it moves, it would leave an empty space at the top of the tube, but it can't: water molecules cling together, and the air pressing on the surface of the tank pushes more water up into the tube to take its place.
The flow keeps going as long as the outlet is lower than the water level in the tank. Lift the bucket above the tank and the flow stops, then runs backwards. There's a height limit too: at sea level, the top of the bend can't be more than about 10 metres above the water, or the water column breaks, for the same reason a suction pump can't lift water higher than that.
Scientists still argue about how best to describe siphons, because experiments have even made them work in a near vacuum, held together only by the pull between water molecules, just like the water rising up a tall tree.
The principle: The siphon. When a tube full of liquid runs from a higher surface to a lower outlet, the longer, heavier column on the outlet side keeps pulling liquid up and over any bend in between.