CBSE Class 10 Answered

When an onion is cut, the cells of onion are broken. Some of these cells contain enzymes which, when the cell was whole, had been carefully kept separate from the amino acid sulfoxides that also reside in the cell.  When the cell is cut the enzymes and amino acid sulfoxides mix together and form sulfenic acids.  The sulfenic acids, in the presence of oxygen, rearrange themselves to become a volatile gas called propanethiol S-oxide.  When this gas reaches the water that keeps our eyes moist forms mild sulfuric acid. The sulfuric acid burns, stimulating our eyes to release more tears to wash the irritant away. It is the volatile gas, propanethiol S-oxide that makes our eyes burn when we cut onions