Oil and milk behave differently when mixed with water due to their chemical compositions. Oil is immiscible in water, meaning it doesn't dissolve and forms separate layers. Milk, on the other hand, is miscible in water, meaning it dissolves and forms a homogeneous mixture.
Immiscibility of Oil in Water
When oil is added to water, it remains separate, creating distinct layers because of differences in polarity. Oil is nonpolar, while water is polar. These differing polarities prevent the molecules from interacting and mixing. This is why you see oil floating on top of the water. This is explicitly stated in one source: "When oil is mixed in water it doesn't dissolve in water. Thus, it is an immiscible liquid."
Miscibility of Milk in Water
Milk, unlike oil, is mostly water itself and contains other components like fats, proteins, and sugars that are capable of interacting with water. These components allow milk to mix relatively easily with water to form a uniform solution. Although one source mentions that "Ultimately there's not much difference between milk and water," this statement is true only regarding their use in certain baking recipes where water can often be substituted with milk without significant impact. However, this does not negate the crucial difference in their miscibility.
Practical Differences and Examples
- Appearance: Oil and water separate into distinct layers; milk and water form a uniform mixture.
- Texture: Oil in water will feel oily on the surface. Milk in water will have a smoother texture.
- Baking: While milk can sometimes replace water in recipes with minimal effect (as per one cited source), the presence of milk's components can alter the final product's texture and flavor.
- Emulsions: Milk is a natural oil-in-water emulsion, where tiny oil droplets are dispersed in water. Oil and water require emulsifiers to mix.
