Question

Why we have a pain in eyes , when we chopping onion ?

Answer 1

Onions 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.
Normally, your body would signal you to close your eyes. This, of course, is not a good idea if you are cutting an onion. The next best thing your eyes can do is to make tears to protect the eyes. Rubbing your eyes is a bad idea, since your hands are likely full of the tear making onion juice. So how do you keep from tearing up? There are many theories out there. I’ve heard some have had luck burning a candle near the chopping site. (Don’t try that without an adult.) Wearing safety goggles, however, should reduce the tears every time.

Answer 2

I think you have mistaken pain for burning...
It burns because,
-When you cut an onion, it releases a gas called, 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.