Why Does Vinegar & Oil Not Stay Mixed?

When baking or mixing your salad dressing, you may notice oil and vinegar have a tough time mixing. Because oil and vinegar are opposites, their molecules separate. It takes strong blending or whisking for the two to stay together.

Identification

"Oil is made of molecules which are like very small, sticky, rubber balls," according to Newton.gov. Oil has non-polar molecules. meaning the molecules have the same charge on each end. Vinegar is made mostly of water, which has polar molecules, meaning the molecules have one negatively charged and one positively charged end.

Separation

Because oil has non-polar molecules and vinegar has polar molecules, the two do not attract. This causes the separation of oil and vinegar. When you combine oil and vinegar, "oil will float on top of vinegar because it is less dense," according to The Houston Museum of Natural Science.

Emulsion

Despite being opposites, oil and vinegar will mix for a period of time if they are whisked or blended. By being mixed like this, "tiny droplets of each liquid become suspended within each other," according to the Houston Museum of Natural Science. This is known as emulsion. The Houston Museum of Natural Science reports that emulsion is "a suspension of two liquids within each other that normally would not mix."

References

• Newton: Ask a Scientist: Polar vs. Nonpolar Molecules
• The Houston Museum of Natural Science: Beyond Bones:The Science of Food: Emulsions on Your Salad
• High Beam Research: Article: Why: why do oil and vinegar dressings separate, stay blended, get thicker, get hard?