Oil and water. A separating funnel containing oil (yellow) and water. Oil and water do not mix as they are mutually insoluble. Water is a highly polar liquid, whilst the organic molecules in oil are non-polar or weakly polarised. To mix the two, a detergent is needed. This has molecules in which one end is polarised (like water), but the other end non-polarised (like oil). Also seen here is the meniscus, or curved surface, of the water/oil interface. The downward-curving meniscus of water is due to the adhesion force between the water molecules and glass. This is greater than the cohesive force between water molecules, so the edges of the water get 'pulled up' by the glass.