Water molecules are made up of two hydrogen atoms and one oxygen atom each. Since only opposites attract, the water molecules stick to each other. Polar molecules only dissolve in polar solvents.
Similarly, non-polar molecules only dissolve in non-polar solvents. Oil is made up of non-polar molecules having negative charges, or electrons, surrounding the molecule. And so oil molecule stick to each other. Even if you stir water and oil together, they will eventually separate in to two different layers.
But they can be forced to mix together by adding an emulsifier. This emulsion will create a stable mixture of water with droplets of oil spread through it or oil with droplets of water spread through it, does not settle out.
Why oil floats in water? This is because oil is less dense than water and therefore it floats to the surface. These beads, however, did not mix with the water. The detergent molecules can form bonds with both water and oil molecules. This activity brought to you in partnership with Science Buddies. Already a subscriber? Sign in. Thanks for reading Scientific American.
Create your free account or Sign in to continue. See Subscription Options. Go Paperless with Digital. Materials 2 clear plastic water bottles with lids 2 cups of water One-half cup of oil olive, cooking or vegetable oils will all work Liquid dishwashing soap Clock or timer Permanent marker Measuring cup Measuring spoon Food coloring optional Preparation Remove any labels from your water bottles.
Pour one cup of water into each bottle. Allow the bottle to sit on a countertop or flat surface while you observe the water and oil. Does the oil sink to the bottom of the bottle, sit on top of the water or mix with it?
Common food emulsions are stable vinaigrettes that contain mustard and mayonnaise, which uses the molecule lecithin from egg yolks as the emulsifier. Sarah Zielinski is an award-winning science writer and editor. She is a contributing writer in science for Smithsonian. Feedloader Clickability After tackling the phrase " comparing apples and oranges " a couple weeks ago, a co-worker suggested I take a look at "mixing like oil and water. Here goes: The phrase, as we know, is applied to any two things that don't get along together.
Anne Marie Helmenstine, Ph. Chemistry Expert. Helmenstine holds a Ph. She has taught science courses at the high school, college, and graduate levels. Facebook Facebook Twitter Twitter. Featured Video. Cite this Article Format.
Helmenstine, Anne Marie, Ph. Why Oil and Water Don't Mix. Avogadro's Number Example Chemistry Problem. Chemistry Jokes and Puns With Explanations.
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