Communicating Vessels

Who hasn’t yet experienced the fact that it is particularly easy to spill from a very full teapot or watering can?
The reason is that the water in the spout is at the same level as the water in the rest of the can or pot. If you tilt the can, the water level remains in the same horizontal plane. So, to spill it out you only have to tilt the can a little.

This is called the principle of communicating vessels. In vessels which are connected to each other, or communicating vessels, the surface of the liquid remains always at the same level horizontally in each of the vessels, no matter how the vessels are formed or how they are tilted.

Why Is It That Way?

To understand this effect, you have to take a closer look at the hydrostatic pressure in the connecting pipe. Every spot in the vessel is subjected to a pressure, which is caused by the weight of the water lying on top of it. The pressure only depends on the depth, not the amount of water.
In order to maintain an equilibrium in the entire vessel, the same pressure has to be exerted on every point in the water column in each of the three vessels. This is the case when the point in each of the three vessels is at the same vertical depth in comparison to the water level. No matter how the vessels are formed, the water level has to be at the same height everywhere.

Communicating Vessels in Everyday Life

Apart from pots, communicating vessels occur in several other occasions in everyday life as well:
Level indicators of containers for liquids (e.g. kettles), in which the liquids are at the same level as inside the container, or hose scales, which can be used to check if two points are at the same height, function according to this principle, for example. In locks, the height of the water is adjusted to the water level outside by connecting channels in this way.

The Artesian Well

If ground water that is connected to groundwater at higher terrain is enclosed under a layer of impermeable rocks, pressure builds up. If a hole is drilled into this layer, the water spurts out of the borehole on its own. This is called an artesian well.

You can build such an artesian well yourself as well - best to do it outside:

You need a funnel and a water hose as thin as possible. If you fill the funnel with water now, and hold the end of the hose at a level below the water level in the funnel as shown in the picture, the water will spurt out of the hose on its own.

The explanation is simple: It is a matter of connected vessels, and the water level tries to adjust itself to the same height on both sides!