Question

while cutting onions our tears start flowing.Why?

Answer 1

It turns out when you cut an onion, it releases a gas called, ready for this, Propanethiol S-oxide. When mixed with certain enzymes in the onion, it creates a sulfur gas. These gases then get to your eyes and create a mild acid which irritates the eyes.
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Answer 2

Answer:

1) The cells of onion contains amino acids like mithionine and enzyme cystine They are kept separate in the onion cell.

2) When we cut the onion, enzymes start mixing and produce propanethiol. Sulphur oxide, which is a volatile compound that starts moving towards our eyes.

3) The gas that is emitted reacts with the water of our eyes and forms sulphuric acid.

4) The sulphuric acid thus produced causes burning sensation in our eyes and this in turn leads to the tear glands secreting tears.

5) Thus we end up with watery eyes every hme we cut onions at home.

hope it helps you...