Solubility is the ability of one substance to mix uniformly into another. Ethane, a simple hydrocarbon, is virtually insoluble in water. Only a minute amount can dissolve, typically measured at around 60 milligrams per liter at standard room temperature. This extreme lack of miscibility is rooted in the opposing molecular characteristics of the two substances.
Understanding Ethane and Water Molecules
The molecular structure of water (H2O) gives it a highly polarized nature. Oxygen atoms strongly attract electrons, creating a slight negative charge near the oxygen and slight positive charges near the two hydrogen atoms. This uneven distribution of electrical charge makes water a polar molecule. Water is able to form strong, attractive forces with neighboring molecules called hydrogen bonds.
Ethane (C2H6) is classified as a non-polar molecule. It is a saturated hydrocarbon, part of the alkane family, featuring a symmetrical structure. The electron density across the molecule is distributed evenly, meaning there are no distinct positive or negative poles. The only intermolecular forces holding ethane molecules together are the very weak London dispersion forces.
The Governing Rule of Solubility
The principle dictating whether two substances will mix is summarized by the phrase, “Like Dissolves Like.” This rule means that polar solvents, such as water, dissolve polar solutes, while non-polar solvents dissolve non-polar substances. Since ethane is non-polar and water is polar, they are chemically mismatched.
For ethane to dissolve, its molecules would need to separate the water molecules from their strong hydrogen-bonded network. The energy released from forming weak London dispersion forces between ethane and water is not enough to overcome the substantial energy required to break water’s existing hydrogen bonds. Water molecules are preferentially attracted to each other, effectively pushing the ethane molecules out and maintaining their cohesive structure.
Where Low Solubility Matters
The low solubility of ethane has important practical implications, particularly in the energy industry. Ethane is a major component of natural gas, often comprising 5 to 10 percent of the gas stream. This lack of affinity for water is utilized in industrial processing to separate ethane from any water or moisture present in the natural gas.
The low mutual solubility between liquid ethane and water is a significant factor in the transportation and phase behavior of natural gas. This property also dictates the environmental fate of ethane if released into an aquatic environment. Because it cannot remain dissolved, ethane quickly escapes the liquid phase, bubbles out of the water, and rapidly evaporates into the atmosphere.