Chemistry categorizes substances based on their composition and physical behavior. They are classified as either pure substances, which have a uniform molecular composition, or mixtures, which involve two or more substances physically combined. When examining a familiar substance like oil, determining its classification—whether it is a pure compound, a solution, or a colloid—requires analyzing its chemical makeup and the size of its constituent particles.
Understanding Chemical Mixtures
Mixtures are classified into three main types based on the size of the dispersed particles suspended within a continuous medium.
The smallest particle size defines a solution, a homogeneous mixture where dispersed particles are individual atoms or molecules, typically less than one nanometer (nm) in diameter. These particles are invisible, and the mixture appears transparent, such as sugar dissolved in water.
A suspension represents the opposite extreme, featuring visible particles greater than 1,000 nm, like silt in muddy water. Due to their size, these particles eventually settle out of the medium under gravity, making the mixture opaque.
Occupying the intermediate space is the colloid, which consists of particles ranging from 1 nm to 1,000 nm in size. These particles are large enough to scatter light, producing the characteristic Tyndall effect, but small enough that they do not settle out over time, as seen in milk or gelatin.
The Chemical Structure of Oil
Oil, whether crude or refined, is fundamentally a nonpolar liquid mixture. The primary components of oil are hydrocarbons, compounds made almost exclusively of hydrogen and carbon atoms. These hydrocarbons exist in various forms, including long-chain paraffins, ring-shaped naphthenes, and aromatic compounds.
A single sample of crude oil contains thousands of different hydrocarbon molecules, making it a complex mixture rather than a pure substance. Since these individual molecules dissolve seamlessly into one another, most oils present as a homogeneous liquid. A refined motor oil or cooking oil is essentially a homogeneous solution where various nonpolar organic molecules are entirely dissolved within the liquid medium.
Classification of Oil
Typical oils, in their pure or refined state, are classified as solutions, not colloids. This determination rests entirely on the particle size of the components. The individual hydrocarbon molecules that make up oil, such as octanes or dodecanes, are molecular in scale, meaning their diameters are far less than one nanometer.
This molecular size places them firmly within the definition of a true solution. Because the particles are too small to interact significantly with light waves, a beam of light passed through oil will not be scattered, confirming the absence of the Tyndall effect. If oil were a colloid, its particles would be significantly larger, and the path of light would be clearly visible within the liquid.
Oil Emulsions and Colloidal Systems
The confusion regarding oil’s classification stems from its ability to form a colloid when mixed with a substance it does not dissolve in, such as water. When oil and water are forcefully mixed, the oil breaks down into microscopic droplets dispersed throughout the water, creating a specific type of colloid called an emulsion. These droplets fall directly into the 1 nm to 1,000 nm range, fulfilling the particle size requirement for a colloidal system.
Because oil is hydrophobic, the droplets would quickly coalesce and separate without assistance. To stabilize this mixture and prevent separation, a third substance called an emulsifier is added, which forms a protective layer around the oil droplets. Common examples of these oil-containing colloidal emulsions include mayonnaise, where oil droplets are dispersed in vinegar using egg yolk as the emulsifier, and homogenized milk, where fat globules are stabilized within water.