Communicating vessels

The surface of a liquid is always horizontal.

When several vessels connect with each other, they are communicating vessels. The surface of the liquid that fills each of the containers is the same level everywhere.  

An imbalance in levels is observed if:

• The containers are not interconnected or obstruction (valve) prevents the flow of liquid.
• The pressure is not the same in each container (this is the case if a container is stopped).

If one of the containers has a very small diameter, then capillary action may significantly alter the level of the liquid.