Science Buddies: "Ask an Expert"

I am doinga projec on onions and have found out that you can prevent yourself from crying if you place a match in your mouth with the tip sticking out, as you chop the onion.
I know that sulphides are released from the onion - that is what causes you to cry. there is phosphates of some kind in matches that may act as an absorbant for the sulphides, but I have been unable to find out exactly what the reaction is that occurs.

I would appreciate any assistance received.

Thank you
Robin Bentele

Hi,

I am reposting this response from Matt Mulanax. It was accidentally added on to another post in a separate forum.

Tina
Science Buddies Staff

This will explain the tear response.

http://www.aarpmagazine.org/food/Articl ... tears.html
Stop crying when you cut onions

We've all heard dozens of Stupid Onion Tricks to eliminate the tears. Some say cut off the stem end before the root end. Others say let the water run in the sink, or fill the sink, and dice your onion underwater. (And then round up the wandering pieces with a butterfly net.) Oh, and if you wear contact lenses, either put them in or take them out. I don't know where those ideas came from, but as a chemist I can't think of any reason why they should work. And in fact, they don't.Let's analyze the problem scientifically. The lachrymator, or tear gas, in onion vapors is a chemical called thiopropanal S-oxide or propanethial S-oxide. It does not exist in onions as such, but is formed when the onion is cut—which breaks open its cells and releases two previously isolated chemicals that react with each other to form the irritating gas.

We can't prevent those two chemicals from reacting when the onion is cut, but we can certainly slow down the reaction. Lower temperatures slow down all chemical reactions. So try refrigerating your onion for a few hours before cutting it, and it won't be as potent a tearjerker.

The burnt match probably acts as an absorbant (charcoal) for some of the onion volatiles.

Sticking your tongue out exposes many taste buds that react rather than your eyes.

Matt Mulanax