Why Do Oil and Water Separate?

The molecules have different properties

Water molecules have an uneven charge distribution and form hydrogen bonds with each other. Typical oil molecules are largely nonpolar. These differences are useful clues to their limited mutual solubility, often summarized by the rule of thumb that like dissolves like. ^[1]

The full explanation includes entropy

Saying water simply repels oil is misleading: attractions exist between molecules of both substances. The thermodynamics of mixing includes both energy and entropy, which concerns molecular arrangements. Research discussions of the hydrophobic effect emphasize that entropy is important to oil–water separation at room temperature. ^[2]

Floating is a separate question

Common cooking oils are less dense than water, so their layer rests above it. But being less dense does not automatically make a liquid insoluble. Density explains the vertical order after separation; molecular interactions and the free energy of mixing explain limited solubility. ^[1]

Shaking does not make a permanent solution

Shaking breaks a layer into small droplets dispersed through the other liquid. Afterward, droplets can join and the layers reappear. This temporary dispersion differs from a molecular solution. Emulsifiers can help keep droplets dispersed by changing what happens at the oil–water interface. ^[1]

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