Wondering Why?

Why does Oil float on Water?

Why does Oil float on Water?

If two liquids with different densities are mixed together, then they get separated based on their densities.  The one with a lower density will float on the one with a higher density.  The density of liquids is measured in terms of kilograms per cubic meter.  The density of pure water is 1000 kilograms per cubic meter.  This means that the mass of water is 1000 kg on one cubic meter volume.  The density of oil is around 800 kilograms per cubic meter.  Various oils have varying densities.

The liquid with less density floats on another liquid which has a higher density.  Here, water has a higher density than oil.  The other reason for any liquid to float on another one is due to the greater buoyancy of it.  To understand the concept of buoyancy, let us talk about an example here.

A ship, in order to be on the water, has to displace an amount of water that is the same as the weight of the ship.  Only then can it successfully float, otherwise, it sinks.  Likewise, the ship on sea water will displace less water than when on river water in order to float.  This is because river water is less than sea water.  When the ship moves into river water, it sinks, by displacing more water in order to balance its weight.  This is the phenomenon of buoyancy force exhibited by the liquid in which the object is floating.

Similar to this phenomenon, oil is buoyant when placed on water, and hence, floats.  The other reason that is worth discussing here for oil and water to exist separately, is that like dissolves like.  Oil and water are not the same, and hence they don’t mix.  Water is a dipole with positively charged hydrogens and negatively charged oxygen ends.  The dipole strength or dipole movement depends on the magnitude of the charges at both the ends, and the distance between the charges. Liquids with near dipole strengths will mix easily.  Oil does not have well defined dipole movement, while water has much more dipole movement than oil.  So, they remain separate.  Oil floats on water as it does not mix with water, and weigh less than water for every cubic meter.


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