Why Onions Make Us Cry...and Other Food Mysteries Solved

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Why don't vinegar and oil mix?

Technically speaking, oil and vinegar can be whisked into submission to create a temporary cohesion. But like an on-again, off-again couple, they eventually finish by separating. So what gives? To put it simply, oil (a fat) and vinegar (which is mostly made up of water), are incompatible because fats, or lipids, are insoluble in water. Their molecules simply aren't attracted to each other. This is why you see oil floating on top of vinegar in your salad dressings when they've been stagnant, even within minutes of vigorously stirring, shaking or whisking them together. Moral of the story? Always give your dressing a good shake or stir just before serving.