Today’s Wonder of the Day was inspired by Skylar from FL. Skylar Wonders, “Why don't oil and water mix?” Thanks for WONDERing with us, Skylar!

When it comes to getting along, sometimes people are just like two peas in a pod. That means they hang out and get along together just fine.

On the other hand, two people might get along like cats and dogs. If you've ever seen a dog chase a cat, then you know they usually don't get along very well. Cats and dogs are like oil and water: they simply don't mix!

But why don't oil and water mix? Did oil and water have a big fight thousands of years ago that caused them to hate each other for eternity? Are oil and water similar to like poles of a magnet that repel each other?

As far as we know, oil and water have never had a big fight. They just don't mix. And it's not because they repel each other similar to like poles on a magnet. Instead, there's much more interesting chemistry afoot, and it does involve polarity.

Water molecules are polar molecules, which means that one end of the molecule has a positive charge and the other end has a negative charge. When water molecules come together, the negative end of one molecule is attracted to the positive end of another molecule. This polarity is why only other polar molecules, such as salt or vinegar, can dissolve in water.

Oil molecules, on the other hand, are non-polar molecules. Since non-polar molecules only dissolve in non-polar solvents, this explains why oil molecules will not dissolve in — mix with — polar water molecules.

When oil and water are combined, the water molecules attract other water molecules and exclude the oil molecules. The oil molecules stick together and the two substances form their own layers. Since the water molecules pack together more densely, the less dense oil molecules will float to the surface while the water molecules sink below the oil.

You can get oil and water to mix together if you add an emulsifier, which is a molecule with hydrophobic (repels water) and hydrophilic (attracts water) ends. Soap is an example of an emulsifier that can attract oil particles and suspend them in a way that allows them to mix with water particles. That's what it does when it cleans your hands!

So the next time you're in a restaurant and you see a container of oil and vinegar dressing sitting on the table, you'll know why the oil congregates on the top while the vinegar and water stays on the bottom!

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