CBSE Class 12-science Answered

Oil in water with vinegar for salad dressing is a common emulsion. However, this is not a stable colloidal system. Usually, salad dressings must be shaken vigorously to redisperse the oil before pouring. The kinetic energy of shaking breaks up the oil into small droplets. But the oil quickly separates again, aggregating into a large, hydrophobic phase. This is called coalescence for droplets since they not only attach but also merge into a single, larger drop. The interfacial tension between oil and water is very high; in other words, they do not mix easily. The acetic acid in vinegar can act as a surfactant, but a much better one is needed to reduce the oil—water interfacial tension enough for stable emulsion formation.