OIL AND WATER DON'T MIX - No matter how much you mix oil and water, they always separate. Chemicals that don't mix are said to be immiscible. The reason this happens is because of the chemical nature of oil and water molecules. Each H2O or water molecule is polar because it has a bent shape in which the negatively charged oxygen atom and the positively charged hydrogen atoms are on separate sides of the molecule. Water forms hydrogen bonds between oxygen and hydrogen atoms of different water molecules. When water encounters nonpolar oil molecules, it sticks to itself rather than mingles with the organic molecules.

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Oil and Water Don't Mix

Why Oil and Water Don't Mix

by Anne Marie Helmenstine, Ph.D. 

You may have experienced examples of how oil and water don't mix.

Oil and vinegar salad dressing separate. Motor oil floats on top of the water in a puddle or in an oil spill.

No matter how much you mix oil and water, they always separate.

Chemicals that don't mix are said to be immiscible. The reason this happens is because of the chemical nature of oil and water molecules.

Like Dissolves Like

The saying in chemistry is that "like dissolves like." What this means is the polar liquids (like water) dissolve in other polar liquids, while nonpolar liquids (usually organic molecules) mix well with each other.

Each H₂O or water molecule is polar because it has a bent shape in which the negatively charged oxygen atom and the positively charged hydrogen atoms are on separate sides of the molecule.

Water forms hydrogen bonds between oxygen and hydrogen atoms of different water molecules.

When water encounters nonpolar oil molecules, it sticks to itself rather than mingles with the organic molecules.

Making Oil and Water Mix

Chemistry has tricks for getting oil and water to interact.

For example, detergent works by acting as emulsifiers and surfactants.

The surfactants improve how well water can interact with a surface, while the emulsifiers help oil and water droplets mix together.

Density and Immiscibility

Oil floats on water because it is less dense or has lower specific gravity.

The immiscibility of oil and water, however, is not related to the difference in density.

Anne Marie Helmenstine, Ph.D.

Introduction

Ph.D. in biomedical sciences from the University of Tennessee at Knoxville - Oak Ridge National Laboratory.

Science educator with experience teaching chemistry, biology, astronomy, and physics at the high school, college, and graduate levels.

ThoughtCo and About Education chemistry expert since 2001.

Widely-published graphic artist, responsible for printable periodic tables and other illustrations used in science.

Experience

Anne Helmenstine, Ph.D. has covered chemistry for ThoughtCo and About Education since 2001, and other sciences since 2013. She taught chemistry, biology, astronomy, and physics at the high school, college, and graduate levels. She has worked as a research scientist and also abstracting and indexing diverse scientific literature for the Department of Energy.

In addition to her work as a science writer, Dr. Helmenstine currently serves as a scientific consultant, specializing in problems requiring an interdisciplinary approach. Previously, she worked as a research scientist and college professor. 

Education

Dr. Helmenstine holds a Ph.D. in biomedical sciences from the University of Tennessee at Knoxville and a B.A. in physics and mathematics with a minor in chemistry from Hastings College. In her doctoral work, Dr. Helmenstine developed ultra-sensitive chemical detection and medical diagnostic tests.

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https://www.thoughtco.com/why-oil-and-water-dont-mix-609193