Water is an abundant substance on Earth, covering about 71% of its surface and making up a significant portion of all living organisms. This molecule, composed of two hydrogen atoms and one oxygen atom, plays a fundamental role in sustaining life. Its unique chemical structure gives rise to several properties essential for biological processes and Earth’s environmental stability.
Temperature Regulation
Water exhibits a high specific heat capacity, meaning it can absorb or release a large amount of heat energy with only a small change in its own temperature. This property stems from the extensive network of hydrogen bonds between water molecules; absorbed energy breaks these bonds rather than immediately increasing molecular kinetic energy.
This high specific heat capacity helps moderate global climates, as large bodies of water like oceans absorb vast amounts of solar energy during the day and release it slowly at night, preventing extreme temperature fluctuations. Within living organisms, this property helps maintain a stable internal body temperature, protecting cells from drastic thermal changes. Water also has a high heat of vaporization, requiring substantial energy to change from a liquid to a gas. This allows organisms to cool effectively through evaporation, such as sweating in humans, as the evaporating water carries away a significant amount of heat.
Water’s Stickiness
Water molecules are attracted to each other, a property known as cohesion, resulting from hydrogen bonds. Water molecules can also stick to other polar or charged surfaces, a property called adhesion.
These cohesive and adhesive forces work together to produce capillary action, enabling water to move against gravity through narrow spaces. In plants, for instance, capillary action helps transport water and dissolved nutrients from the roots up to the leaves through specialized vascular tissues. Similarly, within animal circulatory systems, these properties contribute to the efficient flow of blood through tiny capillaries.
Versatile Solvent
Water is a polar molecule, with slightly positive hydrogen ends and a slightly negative oxygen end. This polarity allows water to interact with and dissolve a wide array of other polar molecules and ionic compounds. Because of its ability to dissolve more substances than any other liquid, water is often referred to as the “universal solvent.”
This solvent capability is fundamental for life, as it allows for the transport of nutrients, minerals, and chemical signals throughout biological systems. Within cells, metabolic reactions often occur in aqueous solutions, and water dissolves waste products for excretion. The presence of water ensures that necessary biochemical reactions can take place efficiently by keeping reactants dissolved and mobile.
Floating Ice
Unlike most substances, water becomes less dense when it freezes, causing ice to float on liquid water. As water cools to its freezing point, hydrogen bonds arrange molecules into a more open, crystalline lattice, occupying more space than liquid water’s randomly packed molecules.
The floating of ice has significant implications for aquatic life, particularly in colder climates. When lakes and ponds freeze, ice forms on the surface, creating an insulating layer that prevents the water below from freezing solid. This allows aquatic organisms, such as fish and plants, to survive through winter underneath the ice layer. If ice were denser and sank, bodies of water would freeze from the bottom up, potentially eliminating most aquatic life.
Strong Surface Film
Water exhibits high surface tension, a property resulting from the cohesive forces between its molecules. At the water’s surface, molecules are more strongly attracted to neighboring water molecules below and to the sides than to air molecules above, creating a net inward pull that forms a “skin” or film.
This strong surface tension is evident in the spherical shape of water droplets and allows small, lightweight objects or organisms to be supported on the water’s surface. For example, certain insects like water striders can walk across the surface of a pond without sinking, effectively utilizing this natural film. This property also plays a role in the formation of dew drops and helps in various biological processes that occur at air-water interfaces.