Final answer:
When oil and water are mixed, they form two distinct layers because oil has a lower density than water, and they are immiscible due to differences in their polarity and intermolecular forces. The nonpolar oil remains separate from the polar water, floating on top.
Step-by-step explanation:
When a mixture of oil and water sits undisturbed, two distinct layers will form. This phenomenon occurs because oil and water have different densities and immiscibility. Oil is less dense than water and therefore, floats on top. Furthermore, these two liquids are immiscible, meaning they do not mix to an appreciable extent due to their differing polarities and the inability of their molecules to form strong intermolecular attractions with each other. The water is polar with strong hydrogen bonds, while oil is nonpolar and is held together by dispersion forces.
The nonpolar nature of oil molecules does not allow them to dissolve in or mix with the polar water molecules. This lack of mixing leads to a separation into layers, with the less dense oil forming a layer above the denser water layer.