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

Option 3: Onions.
Why do onions make you cry?What is 'happening' in your eyes? What is/ are the chemical compound(s) that cause you to tear? How can you make onion-dicing less problematic?

Onion is just like other plants. It is made up of cells which are separated by membrane into two parts. One part contains enzymes while the other part contains the molecules that consist of sulfur compound. When the onion is cute, some onion cells break open and the contents (enzymes and molecules that consist of sulfur compound) of membrane mix together. The enzyme causes the sulfur compound, amino acid sulfoxides to undergo a few chemical reactions. Sulfenic acids are formed, which are immediately coverted into a volatile gas, propanethiol S-oxide. The gas gets in contact with your eyes and reacts with the water present in the eyes. Certain chemical changes are taken place inside the eyes and a mild sulfuric acid is formed. This causes irritation of the eyes. Your eyes twinge while cutting the onion because the nerve endings in the eyes are very sensitive. The tear ducts in the eyes produce more water in order to dilute the irritating acid. However, this is good as the eyes are protected.

The irritation can be prevented by cutting the onions by different ways. One of the easy ways is to keep your head as far away from the onion as possible. Gas will mostly disperse before it reaches your eyes. Another simple way is to wear goggles. Another way is to peel the onion and chill t in the refrigerator before cutting, then it woud minimize a release of gas. This is because a change in the temperature alters the compounds present in the onion. You can also cut the onions under water or run the tap over the onions as you cut it.

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