Have you ever been in tears while you're cutting up onions or even standing near someone cutting them? Ever wondered why onions make your eyes water? Let's look at the chemistry behind it.

Onions are make up of millions of tiny cells. The cells are divided into 2 sections which are separated by a membrane (a thin wall). One side of the membrane contains an enzyme and the other side of the membrane contains molecules that contain sulfur. When you cut an onion, these cells are broken and their contents are released. The enzyme and the sulfur containing molecules mix together and form volatile (goes from liquid to gas very easily) compounds called amino acid sulfoxides (sul-fox-ide). This gas gets into the air around you and reacts with the water in your eye to form sulfuric acid! Sulfuric acid is one of the components of acid rain.

The sulfuric acid irritates your eye and your eyes produce tears in order to wash it away.

Have you noticed that the watery effect stops when you cook the onion? This is because cooking the onion stops the enzyme from working so it can't produce the amino acid sulfoxides anymore.

Cutting up onions under water will also stop the tear producing reaction, as the sulfur containing compounds dissolve in water easily and will be rinsed away before they can reach your eyes. Freezing your onion for 10 minutes before you cut it will also help as cold temperatures slow down the reaction between the enzyme and the sulfur compounds.

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