Why do onions make your eyes water?

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@alfiestoppani

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Why do onions make your eyes water?
« on: 01/10/2010 19:30:04 »
@alfiestoppani asked the Naked Scientists:
   
Why do onions make your eyes water?

What do you think?
« Last Edit: 01/10/2010 19:30:04 by _system »

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Offline chemgeek

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Why do onions make your eyes water?
« Reply #1 on: 02/10/2010 11:28:18 »
When you cut an onion you break its cells and so the contents are released. Amino acid sulphoxides form sulphenic acids. Enzymes that were previously kept seperate can now mix with the sulphenic acids and this produces propanethiol S-oxide. This is a volatile sulphur compound which wafts towards your eyes. The water in your eyes reacts with this gas to form sulphuric acid which stimulates your eyes to produce tears to wash the irritant away.

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Offline Bored chemist

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Why do onions make your eyes water?
« Reply #2 on: 02/10/2010 16:25:38 »
propanethial S-oxide can't react with water to form sulphuric acid because it's the wrong oxidations state.
However the stuff causes irritation to the eyes and it is the reason why onions make you cry.
Of course, from the onion's point of view, the reason they make you cry is to stop you eating them.
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Offline chemgeek

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Why do onions make your eyes water?
« Reply #3 on: 02/10/2010 16:29:35 »
Ahhh ok- thanks for pointing that out  [:)]

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Offline mike4mayo

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Why do onions make your eyes water?
« Reply #4 on: 04/10/2010 11:48:51 »
When you cut into an onion, you slice through cells which contain sulfur compounds. Sulfur is a noxious element.The compounds released are highly volatile, meaning they quickly enter the air in their vicinity.  Your eyes produce tears because your body wants to wash or dilute the chemicals before they effect your eyes. [>:(]
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Offline Bored chemist

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Why do onions make your eyes water?
« Reply #5 on: 04/10/2010 20:30:13 »
"Sulfur is a noxious element."
Nonsense; I rather like it. Also we would all be dead without it.

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