A siphon is used to empty containers of liquid. It consists of a pipe which leads from one container into another so that the end of the pipe leading out of the container is below the water level in the vessel to be emptied. The siphon is first filled with liquid, then the container will continue to empty as long as the end of the pipe is below the level of fluid in the tank.
Atmospheric pressure pushes down on the liquid surface A, pushing liquid into the pipe. The pressure is greater at the other end by an amount
and this excess pressure forces liquid out of the lower end of the pipe. Surface tension also plays a part, maybe a greater part than atmospheric pressure. We can imagine a greater force of gravity in the liquid in the right hand pipe, which moves down therefore and the cohesive force between the liquid molecules pulls the fluid in the left hand side of the pipe up.
If dissolved gases are present in the fluid, the cohesive force between the molecules is reduced and bubbles may form, maybe causing the liquid column to break up if the external pressure is low, so atmospheric pressure is necessary for impure liquids to be able to siphon. Pure liquids may siphon in a vacuum.