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Ever Wondered Why Oil and Water Don't Mix? We Have the Answer

Why Oil and Water Don't Mix
You must have seen oil floating on water. Why do you think they never mix?
ScienceStruck Staff
Last Updated: Dec 09, 2017
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. The sizes of oil and water molecules are different and so are their intermolecular forces of attraction.
Cumene molecule
Cumene molecule
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. Also, oil molecules are much bigger than water molecules.
Water molecules
Water molecules
Water molecules are dipolar, which means that one of their ends is positively charged while the other is negatively charged. They are more attracted to each other. And they are relatively smaller than oil 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.
One reason why water and oil do not mix, is surface tension. When water molecules come together, they form a network of hydrogen bonds which results in 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.
Oil drops on water
Oil drops on water
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 form an ionic or a hydrogen bond with water are known as hydrophilic or water-loving, while non-polar molecules that repel water are called 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.
Oil and water
Oil and water
Why does oil float on water and not drown in it? It's because its density is less than that of water. However, oil and water can be forced to mix with the help of an emulsifier. It is any substance that can cause oil and water to mix. Add salt to oil and you'll see the oil drowning. Add a detergent, and the oil will readily mix with water. The detergent molecules bridge the 'rift' between the oil and water and allow them to mingle. 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!