Grades K–2 · Systems
Why does a sink pipe keep water in its bend?
A sink can look empty while water is still sitting in the pipe underneath it. That water has a useful job. It helps keep air from the drain system from taking an open path back into the room.
The pipe goes down, bends around and rises again before reaching its outlet. When water runs from the sink, it travels through the bend and onward. After the flow stops, water remains in the lowest part. It cannot all roll uphill and leave.
The leftover water fills the pipe across the low bend. This is a water seal. Air on the drain side meets that water instead of an empty tunnel leading to the sink. More sink water can still flow through, replacing some of the water already there.
Compare the blue water seal in the upper cutaway with the empty bend below. The orange arrows show where drain gases meet the seal or find an open route.
If the bend becomes dry, the open air route returns. The pipe's shape has not changed, but its watery barrier has gone. A simple bend and a small pool of water work together: the sink can let water leave while blocking an open route for drain gases.
About this explanation
Starting knowledge: Recognize a low bend and distinguish water from an empty air space.
Sources
- U.S. Environmental Protection Agency — Indoor Air Quality Design Tools for Schools: Drain Traps