Why Does Oil Float On Water?

Oil and water not mixing is one of the most common science experiments you’ll see out in the real world.

So why does oil float in water?

Oil floats on water because of different densities and polarity. Density means the substance will either displace or float on another substance due to gravity. Polarity refers to certain molecules attracting or repelling each other.

We’ll look at exactly what density and polarity are and how they factor into oil floating on water, as well as how these principles can be taken advantage of to make useful products such as detergents.

Why Does Oil Float On Water?

There are two primary reasons why oil floats on water. 

The first is that oil is less dense than water. This feeds into the calculation of the buoyancy force.

The buoyancy force is what is referred to when we say that objects float in water. What is actually happening requires some scientific explanation that will be explored below.

The other factor that causes oil to float is the notion of polarity. Fundamentally oil and water are different types of substances, which won’t mix well.

What Property Makes Oil Float On Water?

It is important to consider what exactly constitutes an oil when considering why oil floats on water.

In terms of chemistry, any thick, viscous hydrocarbon with a certain amount of carbon atoms joined to hydrogen atoms is an oil. There are also some further specifications, like it must be liquid at ambient temperatures and there has to be a certain ratio of atoms.

This constitution leads oil to be both hydrophobic and lipophilic. This means it does not mix in water but will do so in other lubricants or oils.

Polar And Nonpolar Molecules

Due to being composed of hydrocarbons, oil is referred to as nonpolar.

The explanation behind this is complicated, so just keep in mind that there is a common refrain in chemistry that like dissolves like, and oil being a nonpolar substance has consequences for mixing.

A nonpolar molecule is different from a polar molecule like water, that is to say there are no positive or negative poles.

This means that the electrical charges of nonpolar molecules are evenly distributed throughout the substance, whereas polar molecules are charged.

This difference in polarity means that substances like oil and water will not mix. It is possible for the polarity to be overcome but this requires a lot of energy, and molecules will just tend to prefer to stick to their own type.

Water is a polar molecule due to the interaction of the electrons in the oxygen and hydrogen atoms. This occurs because the oxygen atom tends to pull the hydrogen atoms, useful for bonding. 

Referred to as dipole-dipole forces, these act to cause the positive ends of one molecule to be drawn towards the negative end of the nearby molecule. In reality, this means the hydrogen atoms are strongly attracted to the oxygen atom, causing them to bond.

This process however does not occur between oil and water naturally. Dipoles with similar strengths mix poorly with dipoles with very different strengths, as is the case with oil and water.

Buoyancy Force

Buoyancy force is the explanation for why some objects float and why some sink. 

  • As oil is less dense than water, 1 cubic cm of oil weighs less than 1 cubic cm of water. Suppose you want to dissolve this amount of oil into the water. 

In order for the oil to dissolve, it must first displace 1 cubic cm of water. To do so, the force on the oil, i.e. the buoyancy force, must be equal to the weight of that 1 cubic cm of water. 

However at the same time, there is the downward force of gravity on the oil, you can think of this as the weight of the oil. In effect, the weight of water displaced is greater than the downward force of gravity on the oil because oil is not as dense as water.

This leads to an inequality of forces that causes the oil to float on the water, or another way of thinking is that the oil is not heavy enough to sink through the water.

Does Vegetable Oil Float On Water?

Oil is a very broad term, and the various oils have different densities. The density of vegetable oils is still lighter than water.

Vegetable oil has the hydrophobic properties of oil and has an average density of approximately 0.9 to that of water. This means that the buoyancy and density properties as outlined above will cause vegetable oil to float on water.

Why Does Oil And Water Not Mix?

Oil and water will not mix mainly due to polarity. While the water molecules will be attracted to each other through hydrogen bonding, oil molecules are not.

Hydrogen bonds are formed when the slightly negative oxygen side of a water molecule is attracted to the slightly positive hydrogen side of a separate water molecule.

Given that there is no attraction, water and oil are quite happy to sit in a container without interacting with each other and not mixing. 

This property of oil is why we can clean clothes with detergents and get oil off our hands with soap.

Detergents in effect have two sides, making them attractive to both oil and water. As the detergent attracts the oil, and then mixes with the attracted water, it forms what is called an emulsion.

An emulsion is in effect a mixture that allows the dirt or oil to be bonded to the detergent and then washed away with the water. It overcomes the hydrophobic qualities of the oil.

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.

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