Why are hydrocarbons insoluble in water? What determines solubility?
The correct answer and explanation is:
Hydrocarbons are insoluble in water because they are nonpolar molecules, while water is a polar solvent. The principle of solubility is often summarized as “like dissolves like,” meaning polar substances dissolve in polar solvents, and nonpolar substances dissolve in nonpolar solvents.
Explanation
The solubility of a substance is determined by the interaction between solute molecules (the substance being dissolved) and solvent molecules (the substance doing the dissolving). For a solute to dissolve, the intermolecular forces between the solute and solvent must be similar or strong enough to overcome the forces holding the solute molecules together and the forces keeping the solvent molecules bonded.
Water is a polar solvent due to its molecular structure. The oxygen atom in water is highly electronegative, creating a partial negative charge on the oxygen and a partial positive charge on the hydrogen atoms. This polarity allows water molecules to form strong hydrogen bonds with other polar molecules or ions.
Hydrocarbons, on the other hand, are composed solely of carbon and hydrogen atoms. The C-H bond has a small difference in electronegativity, making hydrocarbons essentially nonpolar. Since they lack polarity, hydrocarbons cannot form significant interactions with polar water molecules. Instead, water molecules preferentially form hydrogen bonds with each other, excluding nonpolar substances like hydrocarbons.
When hydrocarbons are introduced to water, the polar water molecules interact strongly with each other, forming a cohesive network that resists the inclusion of nonpolar molecules. This phenomenon leads to a separation of the two substances, with the hydrocarbons forming a distinct layer or phase.
Additionally, dissolving hydrocarbons in water would require breaking the strong hydrogen bonds between water molecules. The energy required for this process is not compensated by the weak dispersion forces (Van der Waals interactions) between water and hydrocarbon molecules. As a result, hydrocarbons remain insoluble in water, consistent with the “like dissolves like” principle. This fundamental property underpins many phenomena, from oil spills in the ocean to the separation of organic and aqueous layers in chemistry labs.