This happens because of an enzyme that’s released into the air when the onion is cut. That enzyme has the very catchy name of ‘Lachrymatory-factor synthase’!
When it’s released into the air, this enzyme takes bits of protein from the onion and makes a new, whole protein out of them. The new protein is called ‘Syn-propanethial-S-oxide’ (again, really catchy!).
This protein irritates and hurts the glands in our eyes that make tears. The glands then make tears to try and get rid of the irritant protein, so we cry to clean out our eyes.
If you’re quick and don’t have your head to close to the onion though, it doesn’t hurt 🙂
When you cut an onion, you break cells, releasing whats inside them.
Inside is an acid (sulfenic acid) with the enzymes eat and attack. This produces a gas called propanethiol S-oxide, a sulfur gas (like the gas you see out of steel works) – also sulfur can smell off onions, giving it that tangy smell.
This gas reacts with the water in your tears to form sulfuric acid. The sulfuric acid burns your eyes, can you cry cause we don’t want it touching our eyes and burning a hole through it :S
Would you want acid on your hand – I definately don’t want it in my eyes, so i let myself cry when I chop an onion
Cutting an onion breaks open their cells, releasing their contents. Amino acid sulfoxides form sulfenic acids. Enzymes that were kept seperate when the oinon was in tact are now are free to mix with the sulfenic acids to produce propanethiol S-oxide. This is a volatile sulfur compound that wafts upward toward your eyes. This gas reacts with the water in your tears to form sulfuric acid. The sulfuric acid burns, stimulating your eyes to release more tears to wash the irritant away.
But you can inactivate the enzyme by cooking the onion so it doesn’t burn your eyes. You could get some chemistry goggles and wear them when you cut onion, you might look funny but it would stop you crying; or, you can keep from crying by refrigerating your onion before cutting it (slows reactions and changes the chemistry inside the onion) or by cutting the onion under water.