Question

Why does urine smell strongly if allowed to stand strongly

Answer 1

Substances diffuse according to their concentration gradient; within a system, different substances in the medium will each diffuse at different rates according to their individual gradients.

After a substance has diffused completely through a space, removing its concentration gradient, molecules will still move around in the space, but there will be no net movement of the number of molecules from one area to another, a state known as 

Answer 2

substance according to the quantity diffuse immediately and another main reason is when urine mixes with water the uric acid in the urine reacts with water and thus producing ammonia gas which is a sharp pungent chocking smell