When oil and water are combined, they do not blend into a single substance. Instead, they quickly separate, forming distinct layers. This common observation occurs whether you are mixing salad dressing or witnessing the aftermath of an oil spill. Understanding this phenomenon involves exploring the fundamental properties of these two liquids.
Water’s Unique Nature
Water molecules possess polarity, meaning they have an uneven distribution of electrical charge. This occurs because the oxygen atom in a water molecule attracts electrons more strongly than the hydrogen atoms. As a result, the oxygen side gains a slight negative charge, while the hydrogen sides acquire a slight positive charge. This charge difference allows water molecules to act like tiny magnets, with the positive end of one molecule attracting the negative end of another.
These attractions between individual water molecules are called hydrogen bonds. Hydrogen bonds are strong attractions that cause water molecules to stick together, a property known as cohesion. This strong internal attraction influences water’s ability to dissolve certain substances and its interactions with other liquids.
The Characteristics of Oil
In contrast to water, oil molecules are non-polar, meaning they have an even distribution of electrical charge. Oils are primarily composed of hydrocarbons, which are long chains of carbon and hydrogen atoms. These carbon-carbon and carbon-hydrogen bonds have minimal differences in electron attraction, preventing distinct positive and negative ends within the molecule. The overall balanced charge makes oil molecules neutral.
Oil molecules do not form strong attractions like the hydrogen bonds found in water. They interact with each other through weaker intermolecular forces. This property makes oil hydrophobic, meaning “water-fearing” and indicating its aversion to mixing with water.
Why They Don’t Mix
The principle of “like dissolves like” explains why oil and water do not mix. Polar substances mix with other polar substances, while non-polar substances mix with other non-polar substances. Water, being polar, is strongly attracted to other polar molecules and ions. Conversely, non-polar oil molecules are attracted to other non-polar molecules.
When oil and water are brought together, water molecules prefer to bond strongly with each other through their hydrogen bonds. Breaking these strong water-water attractions to interact with non-polar oil molecules would require a significant amount of energy, making it energetically unfavorable. Water molecules essentially exclude the oil molecules, pushing them away and forcing them to cluster together.
This exclusion occurs because the attractions between water molecules are much stronger than any potential attractions between water and oil molecules. Water molecules maintain their strong internal bonds, pushing the oil molecules together into separate droplets or layers.
How They Form Layers
Once oil and water have separated, they arrange themselves into distinct layers due to differences in their densities. Density is a measure of how much mass is contained within a given volume. Water has a density of approximately 1 gram per cubic centimeter (g/cm³). Most oils have a lower density, typically ranging from 0.75 to 0.95 g/cm³.
Because oil is less dense than water, it floats on top. The oil molecules cannot pack together as tightly, resulting in less mass per unit of volume. This density difference explains the visible layering, with lighter oil forming a surface layer above denser water.