Oil and water do not mix because their molecules interact with very different strengths. Water molecules are polar, meaning they have slightly positive and slightly negative ends that attract each other. Oil molecules are nonpolar, so they do not form strong attractions with water.
This matters in nutrition because fats, sauces, dressings, digestion, and cell membranes all depend on how water and oil behave together.
When oil is poured into water, the water molecules stay close to other water molecules instead of surrounding the oil. Oil also floats because it is usually less dense than water. In foods, emulsifiers such as egg yolk or lecithin can help oil and water stay mixed by having one part that interacts with water and another part that interacts with oil.
This is the science behind mayonnaise, salad dressing, milk, and the way the body transports fats.
Key Facts
- Water is polar, so its molecules attract each other through hydrogen bonding.
- Oil is nonpolar, so it does not mix well with polar water.
- Like dissolves like: polar substances mix with polar substances, and nonpolar substances mix with nonpolar substances.
- Density = mass / volume, and many oils float because their density is lower than water.
- An emulsion is a mixture of tiny droplets of one liquid spread through another liquid.
- Emulsifiers have hydrophilic and hydrophobic parts, allowing them to connect oil and water.
Vocabulary
- Polar molecule
- A molecule with uneven charge distribution, giving it slightly positive and slightly negative regions.
- Nonpolar molecule
- A molecule with charge spread out fairly evenly, so it does not have strong positive or negative ends.
- Hydrogen bond
- A weak attraction between a hydrogen atom in one molecule and a negative region of another molecule.
- Emulsifier
- A substance that helps oil and water stay mixed by interacting with both types of molecules.
- Density
- The amount of mass in a given volume of a substance, usually calculated as density = mass / volume.
Common Mistakes to Avoid
- Thinking oil floats because it is lighter in total. Floating depends on density, not just the total mass of the sample.
- Saying oil and water repel each other like magnets. The main reason they separate is that water molecules strongly attract each other and exclude nonpolar oil.
- Assuming shaking permanently mixes oil and water. Shaking makes temporary droplets, but the liquids separate again unless an emulsifier is present.
- Confusing dissolving with mixing. Oil may break into droplets in water, but it does not truly dissolve because its molecules are not evenly surrounded by water molecules.
Practice Questions
- 1 A student pours 40 mL of vegetable oil with a mass of 36 g into water. Calculate the density of the oil and state whether it is likely to float on water with density 1.0 g/mL.
- 2 A salad dressing contains 30 mL of oil and 70 mL of vinegar and water. What percent of the dressing volume is oil?
- 3 Explain why adding egg yolk helps mayonnaise stay creamy instead of separating into oil and water layers.