We will separate a mixture of mercury, oil and water by using a separating funnel. These three substances have different densities and boiling points. Mercury, being the heaviest among the three, comes out first via stop-cock. The density of water is lower than mercury but higher than oil. So, it forms the middle layer. Oil has lowest density among the three, and therefore it forms the upper layer. These liquids are then separated from the funnel layer by layer.