While I understand the principle behind a P-trap, I fail to understand why gasses downstream cannot pass through the liquid held in the trap? Gasses can bubble through and/or become soluable. Is the basis that these drawbacks to the P-trap are substantially minimized?
Exactly. The traps (also known as a seal) do not work if the associated vent becomes blocked by build up of leaves, bird’s nest, hair, etc. Also, in very high winds, it is possible to get enough pressure/vacuum to either push gas past the seal, or to suck the water out of it so it no longer functions.