Important annoucement - Chemistry

Dear 3E2,

See you guys for extra lesson on 5 May (Wed) 2.30pm.

Jia you in your exam preparation!

Eat well and sleep well too!

Mrs. Yeoh

Thursday, April 22, 2010


Why do onions make us cry?
Like most other plants, onion is made of cells. These cells are separated by a membrane into two sections. One side of the membrane consists of enzymes, while other side contains the molecules that consist of sulphur compound. This is because when you cut the onion, a number of onion cells break open and the contents of both sides of membrane mix together and it would release a gas that will cause us to tear.This is because the onion contains oil, sulfur which will irritate both our eyes and nose.This causes a chemical interaction, a substance called prophanethiol s-oxide with the enzymes in the onion will emit a passive sulfur compound.

What happens in our eyes?
When this gas reach the water in our tear ducts in our eyelids, it would release a sulfuric acid.The nerve endings in the eyes are very sensitive and hence, your eyes twinge while cutting the onion.Our eyes would automatically blink and produce tears to flush out the sulfuric acid. This would also cause a burning sensation in our eyes. The brain would also send signals to the eyes to produce more tears.
What is/are the chemical compounds that cause us to tear?
The chemical compounds are synthase enzyme that converts the sulfoxides (amino acids) of the onion into sulfenic acid. The unstable sulfenic acid rearranges itself into a volatile gas, syn-ropanethial-S-oxide.

How can we make onion dicing less problematic?
As the onion has a pungent smell which will also irritate our eyes, we can try boiling it instead of directly slicing or dicing it first. We can also cut the onion under water as the water would wash away most of the sulfuric compound. Peeling the onion and storing it in the refrigerator before chopping may also help as the temperature change would be altered. Some people also tried biting a slice of lemon or bread may help to keep us from crying when cutting an onion. Wearing safety goggles may sound ridiculous but people wear it to prevent themselves from tearing too!

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