A solvent is a substance, typically a liquid, that dissolves another material (the solute) to form a homogeneous solution. For polar solvents, the ability to dissolve a solute is governed by the molecular structure of the solvent, specifically an uneven distribution of electrical charge (polarity). This polarity determines which substances can be incorporated into the liquid.
The Underlying Principle of Polarity and Solubility
The governing concept for predicting dissolution is that substances with similar chemical properties tend to mix, often summarized as “like dissolves like.” A solvent is considered polar when its molecules exhibit a net dipole moment, meaning the unequal sharing of electrons creates a slight negative charge on one side and a slight positive charge on the opposite side. Water, for example, is highly polar because the oxygen atom pulls electrons away from the two hydrogen atoms, creating a bent shape where the charges do not cancel out.
When a polar solvent encounters a solute, the partial positive end of the solvent is attracted to any negative charge on the solute, and the partial negative end is drawn to any positive charge. For dissolution to occur, these attractive forces must be strong enough to overcome the forces holding the solute molecules together. These strong intermolecular forces include dipole-dipole interactions and hydrogen bonding, which occurs when hydrogen is bonded to highly electronegative atoms like oxygen or nitrogen.
Dissolution involves the solvent molecules surrounding the solute particles, isolating them from one another. As the solvent molecules cluster around the solute, they form a solvation shell (or hydration shell in water), stabilizing the dispersed state. The high dielectric constant of a polar solvent, which reduces the force between charged particles, further enhances its capacity to dissolve charged substances.
Categories of Solutes Polar Solvents Target
Polar solvents are highly effective at dissolving two main categories of substances: ionic compounds and polar covalent compounds. Ionic compounds, such as table salt (sodium chloride), are composed of positively and negatively charged ions held together by powerful electrostatic forces. The strong dipoles of the polar solvent orient themselves around these ions. The negative end of the solvent is drawn to the positive ion, and the positive end to the negative ion, overcoming the strong ionic bond holding the crystal lattice together. The separated ions are then encased in a shell of solvent molecules, which keeps them stable and dissolved within the solution. This strong interaction is why substances like sodium chloride are readily soluble in water.
The second category includes polar covalent compounds, which are molecules that share electrons but still possess a net dipole moment. These substances do not break into ions but dissolve because they can engage in the same strong intermolecular attractions as the solvent. Common polar covalent solutes, like sugar (sucrose) and ethanol (drinking alcohol), are highly soluble in water because they contain functional groups such as the hydroxyl group (-OH). These hydroxyl groups allow the solute molecules to form hydrogen bonds directly with the polar solvent molecules. The presence of multiple hydrogen-bonding sites ensures that the attraction to the solvent is sufficient to achieve dissolution.
Substances Polar Solvents Cannot Dissolve
Polar solvents reach their limitation when attempting to dissolve nonpolar solutes, which are characterized by a balanced or symmetrical distribution of electrical charge. Nonpolar molecules, such as oils, fats, and waxes, are often composed of long carbon and hydrogen chains where electrons are shared equally. These molecules lack the permanent partial charges necessary to establish strong dipole-dipole interactions or hydrogen bonds with the polar solvent. Instead, nonpolar molecules interact primarily through weak, temporary forces called London dispersion forces.
When a nonpolar substance is introduced to a polar solvent, the strong attractive forces between the polar solvent molecules themselves are far greater than the weak forces they could form with the nonpolar solute. The polar solvent molecules preferentially stick together, effectively excluding the nonpolar molecules out of the solution. This exclusion is why nonpolar substances like cooking oil, gasoline, or paraffin wax do not mix with water and instead form a separate layer.