When I see drops of oil floating on water, at times, oil comes across as a stubborn liquid, which refuses to mix with water. At other times, I wonder whether it is water, to be blamed for not allowing oil to mix with it. Perhaps, both oil and water do not want to mix with each other or science does not let them gel! Let's look at what exactly stops them from mixing.

Both oil and water are liquids, but their chemical compositions are very different from each other. Chemically, all kinds of oils are long chains of carbon atoms with hydrogen atoms attached to them. Oil is made of long non polar hydrocarbons, which means that they experience weak forces of attraction. On the other hand, water molecules are dipolar, which means that one of their ends is positively charged while the other is negatively charged. Also, oil molecules are much bigger than water molecules. The mixing of any two liquids is attributed to the similar nature of their molecules. The difference in the molecular structures of oil and water is one of the main reasons why they do not mix with each other. They are a well-known example of immiscible liquids.

Another important factor due to which water and oil do not mix, is surface tension. When water molecules come together, they form a network of hydrogen bonds which results in a very high surface tension. Both oil and water have high surface tensions, due to which the adhesion between them is weak and they do not mix. Why are patches of oil floating on water circular in shape? It's because of surface tension. The oil molecules cling to each other to form an elastic layer with the minimum possible surface area. Due to this interplay in the oil molecules, they form circular droplets over the surface of water.

For two liquids to mix, the chemical bonds holding the molecules together, need to be broken and new ones need to be formed. The bonds between water molecules are very strong, whereas, the bonds between oil molecules are weak. Hence, a lot of energy needs to be consumed in an effort to bring the water and oil molecules together. This explains why oil does not mix with water.

Molecules that carry charge are referred to as hydrophilic or water-loving, while molecules without charge are known as hydrophobic, which means they lack water affinity. As oil molecules (which are long chains of hydrocarbons) do not carry charge, they cannot bond with water. As a result, oil and water don't mix.

But why does oil float on water and not drown in it? It floats because its density is less than that of water. However, they can be forced to mix by adding an emulsifier to oil and water. An emulsifier is any substance that is used to cause the oil to mix with water. Try adding salt to oil and you will see the oil drowning in water. Adding a detergent also has the same effect on oil and it readily mixes with water. The detergent molecules bridge the 'rift' between the oil and water molecules and allow them to mingle with each other. Adding an emulsifier to oil and water gives a stable mixture.

One thing's for sure, although oil and water do not come to terms with each other under normal conditions, water is gracious enough to let the oil float atop.