The principle of "like dissolves like" is fundamental in chemistry and helps explain why non-polar substances tend to dissolve in non-polar solvents. This principle is based on the interactions between molecules, particularly the types of intermolecular forces at play.
Key Concepts:
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Non-Polar Molecules: These molecules do not have a significant charge separation; their electrons are distributed more symmetrically. As a result, they do not form strong dipole-dipole interactions or hydrogen bonds. Common non-polar molecules include hydrocarbons (like benzene) and many gases (like oxygen).
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Non-Polar Solvents: Similar to non-polar molecules, non-polar solvents, such as hexane or benzene, have no significant electrical charge and exhibit weak intermolecular forces like London dispersion forces.
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Intermolecular Forces: The dominant interactions in non-polar systems are London dispersion forces, which arise from momentary fluctuations in electron density. These forces are relatively weak compared to stronger interactions found in polar solvents.
Why Non-Polar Dissolves in Non-Polar Solvents:
When non-polar substances are mixed with non-polar solvents:
- Similar Forces: Both the solute (the substance being dissolved) and the solvent (the liquid doing the dissolving) experience similar types of interactions. The London dispersion forces between the solute molecules and the solvent molecules are of comparable strength.
- Energy Consideration: The energy required to separate solute molecules and break the interactions to facilitate mixing with solvent is compensated by the energy released when new interactions (the solute-solvent interactions) are formed. Because both solute and solvent are non-polar, this process is energetically favorable.
Classical Examples:
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Hexane and Oil: Hexane, a non-polar solvent, can effectively dissolve oils (which are also non-polar substances). For instance, cooking oils like olive oil can be dissolved in hexane because both exhibit negligible polarity.
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Benzene and Aromatic Compounds: Benzene is a common non-polar solvent that can dissolve non-polar aromatic compounds like toluene. Their similar structures and non-polar nature allow for effective solvation.
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Non-Polar Gases and Organic Solvents: Many gases like oxygen (O₂) and nitrogen (N₂) are also non-polar and can dissolve in non-polar solvents like hydrocarbons.
Summary:
In summary, non-polar substances dissolve in non-polar solvents due to similar intermolecular forces which facilitate the mixing and interaction between solute and solvent. The principle of "like dissolves like" is a key concept that explains these solubility behaviors.