Written By: Georgia S
Have you ever tried mixing oil and water? No matter how much you shake, stir, or swirl, they just won’t stay together. But why? Let’s dive into the fascinating chemistry behind why some liquids mix and others don’t.
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Liquids are made of tiny particles called molecules, and these molecules behave differently depending on their structure. Some molecules are polar, meaning they have a positive and a negative side, almost like tiny magnets. Water is a polar molecule. Other molecules are nonpolar, meaning they don’t have charged sides. Oil is nonpolar. The rule of thumb in chemistry is "like dissolves like." Polar molecules mix with other polar molecules, and nonpolar molecules mix with other nonpolar molecules, but polar and nonpolar molecules don’t get along.
Water molecules love to stick together because their positive and negative sides attract each other. Oil molecules, on the other hand, stick to each other but not to water. When you pour oil into water, the water molecules push the oil away, forming separate layers. Oil floats on top because it is less dense, or lighter, than water.
You can see this for yourself with a simple experiment. Fill a clear glass halfway with water and add a few drops of food coloring. Then gently pour vegetable oil on top of the water. You’ll see the oil and water form two distinct layers. Now sprinkle some salt on top of the oil. The salt sinks through the oil, carrying some oil with it into the water. As the salt dissolves in the water, the oil floats back to the top. It’s a fun and easy way to watch chemistry in action.
This phenomenon is more than just a fun trick—it’s important in real life too. When you make salad dressing, you have to shake it because oil and vinegar won’t stay mixed. Soap works because it helps oils and water mix, allowing you to clean greasy dishes or hands. In environmental science, understanding why oil and water don’t mix helps experts manage oil spills in oceans and rivers.
The next time you see oil and water refusing to mix, remember that it’s not just a simple separation—it’s chemistry at work, showing us the hidden rules of how molecules behave. Chemistry is everywhere, even in your kitchen!
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Works Cited:
What Is Immiscibility? - Let’s Talk Science, letstalkscience.ca/educational-resources/hands-on-activities/what-immiscibility. Accessed 20 Jan. 2025.
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