Water covers most of our planet and is fundamental to all known life forms. Its unique characteristics stem from how individual water molecules interact. Understanding these forces reveals why water behaves differently from many other liquids, enabling various biological and geological processes.
Inside a Water Molecule
A single water molecule (H₂O) consists of one oxygen atom bonded to two hydrogen atoms. These connections are called covalent bonds. In a covalent bond, atoms share electrons, though not always equally. Oxygen has a greater attraction for electrons than hydrogen, a property known as electronegativity.
Because oxygen is more electronegative, it pulls the shared electrons closer, resulting in the oxygen atom developing a slight negative charge. The hydrogen atoms each acquire a slight positive charge. The water molecule’s bent shape further contributes to this charge separation. This uneven charge distribution makes water a polar molecule, with one side slightly positive and the other slightly negative. These internal covalent bonds are strong.
The Hydrogen Bond
While covalent bonds hold atoms within a water molecule, a different attraction exists between neighboring water molecules. This attraction is known as a hydrogen bond. Hydrogen bonds form when the slightly positive hydrogen atom of one water molecule is drawn to the slightly negative oxygen atom of an adjacent water molecule. This is an electrostatic attraction between opposite partial charges.
Hydrogen bonds are not as strong as the covalent bonds within a water molecule. For instance, breaking an O-H covalent bond requires around 467 kJ/mol, while a hydrogen bond typically requires about 4 to 40 kJ/mol. Despite being weaker, these intermolecular attractions are important for water’s overall properties. In liquid water, these hydrogen bonds constantly form, break, and reform, allowing molecules to move past each other while remaining connected. Each water molecule can participate in up to four hydrogen bonds, creating an extensive network.
How These Bonds Influence Water’s Behavior
The collective strength and dynamic nature of hydrogen bonds give water many distinct characteristics. One property is water’s high specific heat capacity, meaning it absorbs a large amount of heat energy without a significant temperature rise. This occurs because much of the absorbed energy breaks hydrogen bonds before increasing molecular kinetic energy. This thermal buffering helps regulate Earth’s climate and maintain stable temperatures within living organisms.
Hydrogen bonds also lead to water’s cohesive and adhesive properties. Cohesion refers to water molecules sticking to other water molecules. This attraction is responsible for phenomena like surface tension, where water’s surface acts like an elastic film. Adhesion is water’s ability to stick to other polar or charged surfaces, also facilitated by hydrogen bonds. These properties are important for processes like capillary action, enabling water to move upwards through narrow tubes, as seen in plants.
Another property is that solid water, or ice, is less dense than liquid water. As water freezes, its molecules arrange into a more ordered, crystalline structure, held together by hydrogen bonds. This arrangement creates more space between molecules compared to liquid water, causing ice to float. This characteristic is important for aquatic life, as it prevents bodies of water from freezing solid from the bottom up, allowing organisms to survive beneath the ice.