Benzene is a clear, colorless, volatile liquid used as a fundamental building block for numerous industrial chemicals, including plastics, resins, and synthetic fibers. This organic compound, characterized by its distinct six-carbon ring structure, is naturally present in crude oil and is a component of gasoline. When benzene comes into contact with water, it definitively floats on the surface.
The Reason Benzene Floats: Density
The fundamental reason benzene floats is relative density, which measures mass per unit volume. Any liquid with a lower density than water will form a separate layer on the surface. This phenomenon is governed purely by comparing the two substances’ masses in the same volume.
Water has a standard density of approximately \(1.0\ g/cm^3\) at room temperature. Benzene has a density of approximately \(0.88\ g/cm^3\). Since \(0.88\ g/cm^3\) is less than \(1.0\ g/cm^3\), benzene always rises to the surface when mixed with water.
This difference means a given volume of benzene weighs less than the same volume of water. The less dense benzene is physically displaced upward by the heavier water, causing it to pool on top. This creates a floating layer, classifying benzene as a Light Non-Aqueous Phase Liquid (LNAPL) when it is found in the environment.
Why Benzene and Water Do Not Mix
While density determines which liquid sits on top, the liquids do not blend due to immiscibility, explained by molecular polarity. Molecules are either polar, having a positive and negative end due to uneven electron distribution, or nonpolar, with a balanced charge. Water is a highly polar molecule because its oxygen atom strongly pulls electrons away from the hydrogen atoms.
Benzene is a nonpolar aromatic hydrocarbon, characterized by a highly symmetrical structure that results in a balanced charge. The chemical rule of “like dissolves like” explains mixtures: polar substances mix with polar substances, and nonpolar substances mix with nonpolar substances. Because water is highly polar and benzene is nonpolar, they repel each other, preventing them from dissolving into a single solution.
This incompatibility causes the liquids to remain separate, forming two distinct layers. Benzene has only slight solubility in water, around 0.18 grams per 100 milliliters. The molecular forces within the highly polar water are much stronger than the weak attractive forces between water and nonpolar benzene molecules.
Environmental Behavior of Benzene Spills
The combined properties of low density and immiscibility dictate the environmental behavior of benzene during a spill. Because it floats, spilled benzene spreads rapidly across the water surface, often forming a thin, expansive film. This rapid spreading covers a large surface area quickly, which is a significant factor in its environmental impact.
The benzene layer on the surface is highly volatile, meaning it evaporates easily and quickly into the atmosphere. This high volatility reduces the amount of liquid on the water but simultaneously creates a hazardous plume of flammable vapor above the spill site. The floating layer itself poses a substantial fire hazard because benzene is a highly flammable substance.
Cleanup efforts for floating pollutants like benzene are complicated by this behavior. Unlike denser substances that sink, benzene’s tendency to spread and rapidly evaporate requires specialized containment booms and skimming equipment. Furthermore, the small amount of benzene that dissolves into the water can contaminate the aquatic environment, even if the majority of the liquid mass is contained.