How come, if you put water and oil together, the water will rise and the oil will sink? asks Walton Rea, a student in Brookville, NY.
Actually — although it seems unlikely — the opposite occurs. It’s oil that does the rising. Just check out any oil-and- vinegar salad dressing in your refrigerator. (Note: vinegar is mostly water.) Unless the bottle was recently shaken, the oil should be floating neatly on top.
Shake the bottle vigorously, and the oil will (sluggishly, reluctantly) mix in. But set the bottle down, and the oil will eventually makes its triumphant way back to the top. It’s like an inflated raft held underwater in a swimming pool. Left to its own devices, the oil stubbornly bobs up.
Is it just that oil feels the need to be above it all? Or that water is easily intimidated? And why can’t the two just get along, mixing nicely in the bottle (and staying mixed)?
Surprise: Like that inflated raft, which is full of air, oil is actually less dense than water. That means that in an equal-sized parcel — say, a tablespoon — there is a bit more matter packed into the water than in the oil. So oil is actually buoyed up by water — it floats.
But that doesn’t explain why oil is so hard to mix into water. Think about sugar. Sugar crystals are denser than water, which is why they sink to the bottom of your iced tea. But stir long enough and the crystals will mix in and dissolve, creating a sweet solution.
Oil and water keep to themselves because of how their molecules are constructed. Molecules are made of atoms. Depending on how those atoms are arranged, some molecules have opposite electrical “poles,” rather like the magnetic north and south poles of a refrigerator magnet. One end of the molecule has a positive charge, the other a negative charge. But in nonpolar molecules, electrical charges aren’t separated, so the molecules don’t have positive and negative ends.
Water molecules are polar. A water molecule is shaped like a V, with an oxygen atom at the bottom and a hydrogen atom on each of the two ends. The bottom of the water molecule has a negative electrical charge, while the top carries a positive charge.
Since opposite charges attract, the negative area of a polar molecule is attracted to the positive area on another polar molecule. So it’s natural for polar molecules to be drawn together and mix it up in a solution.
Oil — you guessed it — is nonpolar. Since water’s polar molecules tend to stick together, nonpolar molecules, like those in oil, can’t easily mix in. On the other hand, alcohol and water, both polar, mix seamlessly. (Scientists say that alcohol and water are “miscible,” oil and water “immiscible.”)
Sugar molecules are weakly polar. So while it takes a bit of mixing, sugar will dissolve in water. And, of course, it’s also easy to mix two oils, like olive oil and vegetable oil, or mineral oil and motor oil.
But oil and vinegar? You’re fighting both the buoyant force and immiscibility. Better shake hard and pour fast.