Polarity, in the context of a solution, is a fundamental property that governs how well different substances can dissolve in each other, directly influencing the physical characteristics of the resulting mixture.
Based on the general definition, polarity refers to the physical properties of compounds such as boiling point, melting points, and their solubilities. This characteristic mainly arises from the act between molecules and atoms with various electronegativities. In simpler terms, polarity describes the distribution of electrical charge within a molecule. If the charge is unevenly distributed, creating distinct positive and negative poles, the molecule is considered polar. If the charge is evenly distributed, it is nonpolar.
Polarity and Solubility in Solutions
The concept of polarity is crucial for understanding solutions because it dictates interactions between solute and solvent molecules. The general rule governing solubility is "like dissolves like":
- Polar solvents (like water) tend to dissolve polar solutes (like salt) and ionic compounds.
- Nonpolar solvents (like oil or hexane) tend to dissolve nonpolar solutes (like fats or waxes).
This occurs because the attractive forces between polar molecules (dipole-dipole forces) or between nonpolar molecules (London dispersion forces) are strong enough to overcome the forces holding the solute molecules together and the forces holding the solvent molecules together. Mixing polar and nonpolar substances typically results in separation because the attractive forces between unlike molecules are weaker than the forces between like molecules.
Examples
Understanding the polarity of common substances helps predict solubility:
| Substance | Polarity |
|---|---|
| Water (H₂O) | Polar |
| Ethanol (C₂H₅OH) | Polar |
| Table Salt (NaCl) | Ionic (behaves like polar) |
| Sugar (C₁₂H₂₂O₁₁) | Polar |
| Oil | Nonpolar |
| Hexane (C₆H₁₄) | Nonpolar |
| Iodine (I₂) | Nonpolar |
- Water dissolves salt because both are polar/ionic.
- Water does not dissolve oil because water is polar and oil is nonpolar.
- Oil dissolves iodine because both are nonpolar.
How Polarity Influences Solution Properties
Beyond just solubility, the polarity of the solute and solvent affects other physical properties of the solution, consistent with the general definition of polarity:
- Boiling and Melting Points: Solutions often have different boiling and melting points than the pure solvent (e.g., adding salt to water raises its boiling point and lowers its freezing point – colligative properties). These changes are related to the interactions between solute and solvent molecules, which are governed by their polarity.
- Miscibility: This refers to the ability of two liquids to mix in all proportions. Polar liquids are generally miscible with other polar liquids, and nonpolar liquids are miscible with other nonpolar liquids.
- Surface Tension and Viscosity: The intermolecular forces arising from polarity also influence properties like the surface tension and viscosity of a solution.
In summary, the "polarity of a solution" refers less to a single measurable value for the solution itself and more to how the polarities of the components (solute and solvent), originating from electronegativity differences, determine whether a solution forms and what its resulting physical characteristics will be, particularly its solubility.
