Have you ever washed your hands covered in oil and realized that it cannot be cleaned with water? Or perhaps you have heard about oil spills and wondered why an entire ocean of water cannot dissolve that small amount of oil? There are two main reasons explaining why oil cannot dissolve in water, and both reasons are related to very small components called molecules.
Everything around us is made up of tiny particles called molecules. The way two substances interact with each other depends on the physical and chemical interactions between the molecules of those substances.
The reason oil and water do not mix is that the molecules of each substance bond in different ways.
Oil and water have different molecular densities
The first reason that oil and water cannot mix is because the molecules of each substance bond in different ways. Water molecules are bonded together more tightly. Imagine that a glass of water contains more molecules than there are stars in the sky. This means that if you compare the same amount of water and oil, the number of molecules in water will be greater than in oil. This explains why water always sinks and oil always floats on top.
Polarity
Another reason that water and oil cannot dissolve in each other is due to polarity. More specifically, the electronegativity of each element is different, and this difference in electronegativity will determine whether a molecule has polar bonds (ranging from 0.4 to 1.8). Water is a polar molecule. It consists of two hydrogen atoms and one oxygen atom. These three atoms do not bond in a straight line but rather form a V-shape. The electrons in the molecule are distributed more around the oxygen atom than around the hydrogen atoms, resulting in the oxygen having a negative charge and the two hydrogen atoms having a positive charge.
Polar molecules only dissolve in polar solvents, while non-polar molecules can only dissolve in non-polar solvents. Unfortunately, oil has a non-polar molecular structure. Therefore, when oil is added to water, it simply floats on the surface due to the first reason and does not dissolve in water.
Of course, with today’s advanced technologies, we have detergents and soaps that help us clean our hands or household items from oil stains.
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Birds that frequently dive into the water to catch fish also rely on the properties of oil to protect themselves. The feathers on their bodies are regularly coated with a special layer of oil secreted from their pores. If it weren’t for that protective layer of oil, their feathers would get wet, and they would drown immediately. You can observe that when it rains, ducks happily run around without getting wet, while chickens, lacking that oily layer, become soaked by the rain and turn into “drenched” chickens. |
