Water molecules are electrically neutral overall, but possess a unique internal distribution of charge. Specific regions within the molecule carry slight, temporary charges. This characteristic internal charge distribution is fundamental to many of water’s distinctive properties.
Understanding Water’s Molecular Polarity
A water molecule consists of two hydrogen atoms bonded to one oxygen atom (H₂O). These atoms are connected by covalent bonds, where electrons are shared between them. Oxygen atoms have a stronger “pull” on shared electrons compared to hydrogen atoms, a property known as electronegativity. This causes electrons in the covalent bonds to spend more time orbiting the oxygen atom.
This unequal distribution of electrons results in a slight negative charge on the oxygen atom (δ-). Conversely, the hydrogen atoms, having less access to shared electrons, develop a slight positive charge (δ+). Because of its bent shape, the water molecule has a positive side (where the hydrogen atoms are) and a negative side (where the oxygen atom is), making it a polar molecule. The molecule as a whole remains electrically neutral, as the total number of protons and electrons is equal.
How Water’s Polarity Influences Its Behavior
The polarity of water molecules allows them to interact strongly with each other and with other charged or polar substances. The slightly positive hydrogen end of one water molecule is attracted to the slightly negative oxygen end of another, forming hydrogen bonds. These hydrogen bonds are responsible for many of water’s unique properties.
Water’s polarity enables it to dissolve a wide range of substances, earning it the title of “universal solvent.” Its partial charges allow it to surround and pull apart the ions or polar molecules of other compounds, enabling them to dissolve. For instance, when salt (sodium chloride) dissolves, the positive hydrogen ends of water molecules attract negative chloride ions, while the negative oxygen end attracts positive sodium ions, pulling the salt crystal apart.
Water’s polarity also gives rise to cohesion and adhesion. Cohesion refers to water molecules sticking to each other through hydrogen bonds, which contributes to surface tension—the ability of water’s surface to resist external force. Adhesion describes water molecules sticking to other surfaces, particularly those with charges. The combined effects of cohesion and adhesion enable capillary action, where water can move upwards against gravity through narrow tubes, which is essential for transporting water in plants.