Liquid WaterEdit
Liquid Water
Liquid water is the liquid phase of the chemical substance water, with the chemical formula H2O. It is the most common solvent in nature and underpins a wide range of physical, chemical, and biological processes. On Earth, liquid water exists in oceans, rivers, lakes, soils, and the atmosphere, and it plays a central role in shaping climate, geochemistry, and living systems. As a solvent, transport medium, and regulator of temperature, liquid water enables the chemistry of life and the functioning of many planetary systems.
The distinctive properties of liquid water arise from its molecular structure and hydrogen-bonding network. The molecule consists of two hydrogen atoms covalently bonded to a central oxygen atom, forming a bent geometry that promotes extensive hydrogen bonding between neighboring molecules. This network gives water several remarkable traits, including a high specific heat, a high heat of vaporization, a high surface tension, and an unusually large liquid-gas phase range for a small molecule. These features make water an effective transporter of energy and nutrients, a stable medium for biochemical reactions, and a key driver of weather and climate patterns in a broad range of environments.
Physical properties
Molecular structure and hydrogen bonding
The water molecule is polar, with a partial negative charge near the oxygen and partial positive charges near the hydrogens. This polarity facilitates strong intermolecular attractions, especially hydrogen bonds, which transiently link water molecules into a dynamic three-dimensional network. The hydrogen-bonding arrangement gives water a high cohesive strength and contributes to its solvent properties, allowing many ionic and polar compounds to dissolve. The balance between hydrogen bonding and thermal motion also underpins water’s anomalous behaviors, such as its density changes with temperature.
Phase behavior and thermodynamics
Water exists in several phases depending on pressure and temperature. At standard atmospheric pressure, liquid water is stable between 0°C and 100°C, but it can persist in liquid form outside this range under different pressures. The temperature and pressure at which the solid, liquid, and gaseous phases are in equilibrium are described by the water phase diagram. Critical features include the melting point, boiling point, sublimation point, and the critical point where liquid and vapor become indistinguishable.
Water’s phase transitions are influenced by the strength of hydrogen bonding and the molecular structure of H2O. The liquid state allows for continuous changes in density, viscosity, and heat capacity with temperature, and sectioned regions of the diagram correspond to solid ice, liquid water, and gaseous water vapor. The liquid state supports a broad spectrum of environmental and technological processes, from planetary climate to industrial cooling.
Anomalies and notable properties
Water displays several well-documented anomalies that have profound consequences for natural and engineered systems: - Density maximum: Water reaches its maximum density near 4°C, which causes ice to float on liquid water and influences the stratification of lakes and oceans. - High heat capacity and heat of vaporization: Water stores and releases heat efficiently, moderating climates and stabilizing temperatures in organisms and ecosystems. - Surface tension and cohesion: A strong surface film supports droplets and capillary action, aiding transport of water through soils and plant tissues. - Dielectric constant: Water’s high dielectric constant makes it an excellent solvent for ions and polar substances, enabling a vast array of biochemical reactions. These properties, rooted in the hydrogen-bonding network, distinguish liquid water from most other small-molecule liquids and contribute to its essential ecological and geological roles.
Occurrence and distribution
Water covers a large portion of the planet’s surface, with vast oceans acting as major reservoirs that regulate climate and biogeochemical cycles. Most of Earth's water is saline and unsuitable for direct biological use, concentrated in the oceans. A smaller but critical fraction is freshwater, found in rivers, lakes, soils, glaciers, and groundwater. The distribution of freshwater has important implications for ecosystems, agriculture, industry, and human health.
The water cycle, or hydrological cycle, describes the continuous movement of water within the Earth–atmosphere system. Evaporation, condensation, precipitation, infiltration, runoff, and sublimation transport and transform water among oceans, atmosphere, land, and biosphere. This cycle is in turn coupled to weather and climate, supporting life and shaping landscapes. Key components of the cycle include Evaporation, Condensation, Precipitation, and Infiltration processes, all of which interact with land use, vegetation, and topography.
Important natural reservoirs include the oceans, freshwater lakes and rivers, wetlands, groundwater aquifers, and glaciers and ice caps. Human activities—such as agriculture, industry, energy production, and urban development—alter the distribution and availability of liquid water by changing infiltration, runoff, and storage patterns. For an overview of how water is stored and circulated, see the Water cycle.
Chemical and biological significance
Water is the solvent of life. Its polarity and ability to stabilize diverse solutes enable complex biochemistry to occur within cells and in extracellular environments. Many metabolic reactions depend on water as a reactant or product, while its solvent properties allow biomolecules to fold, interact, and function in an aqueous milieu. Water’s buffering capacity and neutral to slightly acidic or basic pH ranges provide a stable environment for enzymes and other catalysts that drive biological processes.
Beyond biochemistry, liquid water participates in geochemical processes that shape minerals, rocks, and soils. It mediates chemical weathering, transports nutrients and dissolved inorganic species, and participates in redox reactions that influence soil chemistry and groundwater composition. The interaction between water and minerals, ions, and organic matter is central to nutrient cycles, soil formation, and the long-term storage of carbon in natural systems.
Natural resources, management, and technology
Liquid water supports agriculture, industry, and daily life, making its management a central concern in many civilizations. Purified water, whether for drinking, cooking, or process use, requires treatment to remove contaminants and to adjust properties such as hardness or salinity. Technologies for water treatment, distribution, and sanitation are foundational to public health and economic activity. In modern systems, desalination, filtration, disinfection, and distribution networks are used to ensure access to clean water in urban and rural settings alike.
Water is also central to energy production, cooling, and manufacturing. The sustainable use of water resources involves balancing demand with ecological integrity, protecting watersheds, and maintaining groundwater reserves. Research and policy discussions around water resources address issues such as efficiency, pricing, infrastructure resilience, and the protection of aquatic ecosystems. See Water resources management and Desalination for related topics, and consider the role of water treatment and sanitation in public health.