Water is a fundamental substance on Earth, underpinning all known life forms and shaping geological processes. Its widespread presence and vital role stem from water’s unique properties, largely attributed to polarity. Understanding this intrinsic molecular feature provides insight into why water behaves as it does.
What Polarity Is
Molecular polarity refers to the distribution of electrical charge across a molecule, resulting in distinct positive and negative ends. This property arises from the unequal sharing of electrons between bonded atoms. When atoms with differing abilities to attract electrons, known as electronegativity, form a bond, the electron density shifts towards the more electronegative atom. This uneven distribution creates partial positive (δ+) and partial negative (δ-) charges, termed dipoles, within the bond. For a molecule to be considered polar, it must contain these polar bonds, and their asymmetrical arrangement must prevent the partial charges from canceling each other out.
Why Water Is Polar
Water (H₂O) is a polar molecule due to its atomic composition and molecular shape. The oxygen atom in a water molecule is significantly more electronegative than the hydrogen atoms. This means oxygen attracts the shared electrons in the covalent bonds more strongly, pulling them closer to itself. As a result, the oxygen atom develops a partial negative charge, while each hydrogen atom acquires a partial positive charge.
Water’s bent molecular geometry is also crucial for its polarity. Unlike linear molecules where bond dipoles might cancel out, the H-O-H bond angle ensures an asymmetrical distribution of these partial charges. This bent shape prevents the positive and negative poles from neutralizing each other, giving the water molecule distinct positive and negative sides.
The Impact of Hydrogen Bonds
Water’s polarity directly enables the formation of hydrogen bonds, which are attractions between water molecules. A hydrogen bond occurs when the partial positive hydrogen atom of one water molecule is attracted to the partial negative oxygen atom of an adjacent water molecule. Although weaker than the covalent bonds within a water molecule, these intermolecular forces are responsible for many of water’s unique physical properties.
These bonds allow water molecules to “stick” to each other, a property known as cohesion. This cohesive force contributes to water’s high surface tension, which is the resistance of a liquid’s surface to rupture. Water also exhibits adhesion, meaning its molecules are attracted to other polar substances. Cohesion and adhesion together facilitate phenomena like capillary action, where water moves through narrow spaces, such as in plants, against gravity.
Water as a Solvent
Water’s polarity makes it an exceptional solvent, often referred to as the “universal solvent” because of its ability to dissolve more substances than any other liquid. This dissolving power is attributed to the principle of “like dissolves like,” meaning polar solvents effectively dissolve other polar molecules and ionic compounds.
When an ionic compound, such as table salt (sodium chloride), is introduced into water, the positively charged sodium ions are attracted to the partially negative oxygen ends of water molecules. Simultaneously, the negatively charged chloride ions are drawn to the partially positive hydrogen ends. These attractions cause the water molecules to surround and separate the ions, pulling them away from the crystal lattice.
This process leads to the formation of “hydration shells,” where water molecules orient themselves around each dissolved ion. The formation of these shells stabilizes the ions and keeps them dispersed throughout the solution. This capability to dissolve and transport substances is fundamental to biological systems, enabling the movement of nutrients and waste.