It is because as we cut the onion, the onion itself contain oil. The gases release by the onion obtain sulfur which irrate our eyes and nose. When the gases Propanethiol S-oxide, is released and it rise upward encounter the water produced, the tear ducts is automatically blink, and produce tears which irriate the eye, and flush out the sulfuric acid in our eyelids, it produces sulfuric acid. In response to the sulfuric acid, our eye blinks and produce tears which irrate our eyes. Our tears contain the sulfuric acid. Thr chemical are Propnethiol S-oxide, sulfur , sulfuric acid and amino acid.
First we should put our onion in the cold water so the volative sulfured compound will react to the water. Then we froze the onion before cutting so the chemical reaction would be slow down.