Final answer:
Salad oil and water separate immediately when mixed because they are immiscible, resulting from the difference in polarity and density. Oil is nonpolar and less dense than water, leading to separate layers without an emulsifying agent.
Step-by-step explanation:
When you mix salad oil and water, they immediately separate because these two substances are immiscible. The term immiscible means that the liquids do not mix to an appreciable extent and will form separate layers when combined. This occurs due to the differing polarities of the two liquids; water is a polar molecule with strong hydrogen bonds, while oil is a nonpolar compound with weaker London dispersion forces.
As a result, the attractive forces between the polar water molecules and the nonpolar oil molecules are not sufficiently strong to overcome the hydrogen bonding between the water molecules. Furthermore, oil is less dense than water and tends to form a layer on top when the two are combined. If an emulsifying agent, like bile or egg yolk in mayonnaise, is added, it can stabilize the mixture by interacting with both polar and nonpolar components, resulting in an emulsion that does not separate as readily.