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Name: Emily Hartman
Status: student
Age: 10
Location: N/A
Country: N/A
Date: Around 1999

Why don't oil and water mix?

Well, at your school, do the boys and girls mix? Probably not. And that is not because the girls don't like the boys (usually), or because the boys don't like the girls. It's just that young girls like spending time with other young girls more than with young boys. Boys and girls tend to like to do different things and talk about different things when they are young.

Imagine you are a girl going outside for recess. You look around. You see a boy or two, and a girl or two. Naturally you go over to the girl to talk. Later, another girl comes out. She, too, goes over to talk to the girls. After a while, after every child is outside, you have patches of boys and patches of girls all over the place, but not many areas where boys and girls are together.

The same thing happens with oil and water. Oil is made of molecules (which are like very small, sticky, rubber balls). So is water. But water molecules are not the same as oil molecules. Most importantly, while all molecules like to stick to each other, oil molecules like to stick to other oil molecules more than they like to stick to water molecules. Same with the water: water molecules like to stick to oil molecules, but they like to stick to other water molecules even more.

So, if you pour some water into some oil, the water molecules coming in see oil molecules and (at first only a few) water molecules. Naturally they prefer to join the other water molecules. After all the water is in, you have patches of water, and patches of oil, but nowhere the two kinds of molecules mixed up together.

It turns out that oil is lighter than water, so that the patches of oil tend to float up on top of the water, and join together into a big oil slick. If you get enough oil, you get a thick layer of it on top of the water. You can sort of mix up the oil and water by shaking the jar, but you will only break up the layer into smaller patches, and they will come back together again quickly because of the lightness business. Now, if you were on the Space Shuttle, the patches wouldn't form a thick layer, they'd just stay all jumbled up. But the oil and water wouldn't mix!

You CAN force oil and water to mix. What you need is another type of molecules which both the water and the oil like to stick to more than they like to stick to each other. Such a molecule is called an emulsifier, or, more simply, a soap. When you get a soap molecule in there, the water and the oil stick to it, and then the oil and water mix.

A lot of things can be emulsifiers. Soap is one of them. If you get grease on your hands, plain water won't wash it off -- the grease won't mix with the water and come off. But if you put soap on your hands, the grease will mix with the water, and come off.

Egg yolks are another thing that works. So if you mix salad oil and vinegar (which is mostly water), and then put some eggs in, the oil and vinegar will mix -- you get mayonnaise. You can make some with oil, vinegar, and eggs in your blender at home.


Water is a polar molecule which means that it has a negatively charged end and a positively charged end. So water molecules attract each other. They also attract other polar molecules. Oil is NOT a polar molecule-it doesn't have a separation of charge. So water and oil aren't attracted to each other. Just remember-like dissolves like.

Van Hoeck

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