Why Does Oil Float on Water

While dissolving a liquid in another, we should remember the rule, like dissolves like. But oil and water are different. The water molecules are polar. They are attracted to each other due to the intermolecular force of hydrogen bonding (the bond that is formed when the slightly negative oxygen side of a water molecule is attracted to the slightly positive hydrogen side of another water molecule).

On the other hand, the oil molecules are non polar and are not attracted to the water molecules as much since they are hydrophobic or water fearing. The concept of dipole moment helps us to understand it better.

Dipole Moment

A dipole can be thought of as a magnet. It has a positively and negatively charged end analogous to the north and south poles of a magnet. The product of the difference of the magnitudes of the 2 charges and the distance between them is known as the dipole moment or the strength of the dipole. The 2 liquids should have similar dipole moments to mix with each other.

Dipoles having similar strengths dissolve more rapidly with each other than those having dissimilar strengths. Since oil (like those which are hydrocarbon based) and water differ a lot in their dipole moments, they don’t dissolve in each other.

A video explanation is given to help you grasp the concept.

1. This property of oil is the basis of cleaning greasy clothes and dishes by detergents.This is because detergents are attracted to both oil and water. One side of the detergents is hydrophilic (or water loving) that binds with the water and the other end is hydrophobic and binds with the oil. This forms an emulsion that is nothing but a stable mixture of 2 or more liquids that would not have normally mixed with each other. Thus the detergent pulls off dirt and grime and washes them off with the water.
2. Animals living in the oceans or water bodies are benefitted by this property. The oil in their coats and feathers keeps them protected from the cold water in winter.
3. Salt water being denser than freshwater, oil floats on the surface of salt water too. You can check this behavior with corn oil, cooking oil, crude oil, etc.
4. If we immerse ice in oil in a container, it will float. This is because it is less dense than oil. But as it slowly melts into water, it will sink to the bottom.