Which term best describes a stable mixture of oil and water?

• A. Emulsion
• B. Solution
• C. Suspension
• D. Emulsifier

Answer: (A)

A stable mixture of oil and water is an emulsion. Oil and water don’t mix, so to keep them together you break the oil into tiny droplets that are dispersed throughout the water. When those droplets stay dispersed rather than coalescing and separating, the mixture is an emulsion. Often an emulsifier is needed to help keep the droplets from coming back together by reducing surface tension and forming a protective layer around the droplets—think of mayonnaise, where egg yolk acts as the emulsifier to stabilize the oil-in-water droplets.

A solution wouldn’t apply here because oil doesn’t dissolve in water at the molecular level. A suspension involves solid particles dispersed in a liquid that tend to settle out, not liquid droplets of one liquid dispersed in another. The emulsifier is the stabilizing agent, not the type of mixture itself.