•Onions and garlic both contain derivatives of sulphur-containing amino acids.
•When an onion is sliced, one of these compounds, S-1-propenyl-cysteine sulphoxide, is decomposed by an enzyme to form a volatile compound called propanethial-S-oxide.
•This is the irritant, or lacrimator (the name for any substance that causes tears to flow).
•When the compound comes in contact with water – in this case in your eyes – it hydrolyses to propanal, sulphuric acid, and hydrogen sulphide.
•A hydrolysis is a chemical reaction in which a water molecule is used to break chemical bonds.
•Tearfully, the eyes try to dilute the acid.
•However, it is these same sulphur compounds that produce the nice aroma when onions are being cooked.