Onions make us cry because propanethial-s-oxide interacts with the tears present in your eye to form sulfuric acid, which is battery acid. Onions absorb a lot of sulfur from the ground and when you cut an onion, you’re breaking the cell structure, this allows amino acid sulfoxides to form sulfenic acid, aka RSOH (Seeker, 2015) (Wikipedia, 2017). The difference between a plant cell and animal cell is that the plant cell has a cell wall, vacuole, and some chloroplasts. Now, a special enzyme called lachrymatory-factor synthase is released, which reacts with the sulfenic acid to create a chemical known as propanethial-s-oxide, aka C3H6OS (Seeker, 2015) (PubChem, n.d.). This chemical is volatile, which means it easily evaporates at normal temperature to create a gas (Smart Learning for All, 2017). The gas rises to meet with our eyes and reacts with our tears. There are three types of tears including emotional, which is for crying, basal, which is always present, and reflexive, which acts to remove an irritant (Seeker, 2015). Tears contain water, mucus, oil, nutrients, and antibacterial lysozymes. (Seeker, 2015) Your reflexive tears reacts with propanethial-s-oxide and in rearranging again, sulfuric acid is formed, aka H2SO4 or battery acid (Seeker, 2015). Sulfuric acid is made from sulfate and not sulfur since the chemical name does not have “hydro” in it. Also, the chemical formula has oxygen, making it polyatomic. It is also not made from sulfite because sulfite would be called sulfurous acid. The acid gives a burning sensation and more reflexive tears are filling the eye until there is no more room, which results in “crying” (Smart Learning for All, 2017). Battery acid has the pH of 1 making it very acidic, that can cause irritation and tissue damage (Healthfully, 2017). But according to Jayclinics, “While sulfuric acid can damage the eye permanently, this acid irritates the nerve on the eyes surface, making them sting. The tear gland produces an alkaline fluid in form of tears that will always neutralize, dilute the acid, and flush it out” (Jayclinics, 2014) So, onions will not damage your eye despite the fact that they can form sulfuric acid. Simple ways to avoid crying from cutting onions is by using goggles or by freezing the onion first. Freezing the onion will slow down the reactions because atoms move slower when cold, like how humans move slower when cold as well. Now you know why onions make you cry and how you can prevent it from happening.
Healthfully. (2017, July 27). Dangerous pH Levels. Retrieved January 27, 2018, from Healthfully: https://healthfully.com/dangerous-ph-levels-6301792.html
Jayclinics. (2014, July 26). Can Onions Damage Your Eyes? - Health - Nairaland. Retrieved January 27, 2018, from Nairaland: http://www.nairaland.com/1830310/onions-damage-eyes
PubChem. (n.d.). Thiopropanal S-oxide. Retrieved January 27, 2018, from PubChem: https://pubchem.ncbi.nlm.nih.gov/compound/Thiopropanal_S-oxide#section=Top
Seeker. (2015, May 25). Why Do Onions Make You Cry? Retrieved January 27, 2018, from YouTube: https://www.youtube.com/watch?v=ERtlCbByqVg
Smart Learning for All. (2017, October 18). Human Tears - Why do onions make you cry? Retrieved January 27, 2018, from YouTube: https://www.youtube.com/watch?v=9ZinH1jEhb4
Wikipedia. (2017, September 15). Sulfenic acid. Retrieved January 27, 2018, from Wikipedia: https://en.wikipedia.org/wiki/Sulfenic_acid