Butane (\(\text{C}_4\text{H}_{10}\)) is a colorless, easily liquefied gas widely known as a fuel source in lighters and portable stoves. Butane is virtually insoluble in water, measuring only about 61 mg per liter at \(20^\circ \text{C}\). This extremely low solubility results directly from the fundamental chemical properties of both molecules.
The Fundamental Rule of Solubility
The mixing of any two substances is governed by a basic principle in chemistry often summarized as “like dissolves like.” This rule means that substances with similar molecular properties, particularly their polarity, tend to dissolve in one another. Solubility occurs when the attractive forces between the solvent and solute molecules are strong enough to overcome the forces holding the molecules together.
Molecular polarity describes the uneven distribution of electrical charge within a molecule, resulting in a positive end and a negative end. Water is an example of a highly polar solvent, while substances that lack this charge separation are classified as nonpolar. A solute will readily mix with a solvent only if their respective forces of attraction are comparable in strength. This comparison determines whether a mixture will form a homogeneous solution.
The Molecular Structure of Butane
Butane is a member of the alkane family, composed solely of carbon and hydrogen atoms in a chain-like structure. It contains four carbon atoms and ten hydrogen atoms. The bonds connecting the carbon and hydrogen atoms (C-H bonds) have a very small difference in electronegativity.
This slight difference means the electrons are shared almost equally, resulting in nonpolar bonds. Because the butane molecule is also structurally symmetrical, the minimal bond polarities cancel each other out. This classifies butane as an overall nonpolar molecule, meaning it has no distinct positive or negative ends.
Why Water and Butane Do Not Mix
The significant difference in polarity between water and butane is the primary reason they do not mix. Water is highly polar due to its bent shape, which creates strong intermolecular forces called hydrogen bonds. These hydrogen bonds are powerful cohesive forces that hold water molecules tightly together.
In contrast, nonpolar butane molecules interact through much weaker London dispersion forces. For butane to dissolve, water molecules must break their strong hydrogen bonds to make space for the butane. The weak forces that would form between water and butane are insufficient to compensate for the energy required to break water’s strong hydrogen bonds.
The water molecules prefer to stay clustered together rather than separate to accommodate the nonpolar butane molecules. This mismatch in intermolecular forces causes a phase separation when the two substances are placed together. Butane, being less dense, will form a separate layer or escape as a gas rather than integrate into the water structure.
Practical Implications of Butane’s Low Solubility
The low solubility of butane in water has several important real-world consequences, especially regarding safety and the environment. Because butane is a gas at standard temperature and pressure, its low solubility allows it to be collected efficiently over water in laboratory settings using the water displacement method. The gas simply bubbles up and does not dissolve.
If liquid butane were to spill in the environment, it would quickly separate from any body of water. Since butane gas is denser than air, a leak can cause the flammable gas to accumulate in low-lying areas, increasing the risk of fire or explosion. The substance does not dissipate by dissolving but instead rapidly volatilizes into the atmosphere.