Hydrology is the study of the movement, distribution, and quality of water throughout the Earth. The study of the distribution of water is hydrography. The study of the distribution and movement of groundwater is hydrogeology, of glaciers is glaciology, of inland waters is limnology and distribution of oceans is oceanography. Ecological processes with hydrology are in the focus of ecohydrology. The Clausius-Clapeyron relation also applies to the boiling point, but with the liquid/gas transition the vapor phase has a much lower density than the liquid phase, so the boiling point increases with pressure. [64] Water can remain in a liquid state at high temperatures in the deep ocean or underground. For example, temperatures exceed 205°C (401°F) in Old Faithful, a geyser in Yellowstone National Park. [65] In hydrothermal vents, the temperature can exceed 400°C (752°F). [66] Water fit for human consumption is called drinking water or potable water. Water that is not potable may be made potable by filtration or distillation, or by a range of other methods. More than 660 million people do not have access to safe drinking water. [108] [109]

On a pressure/temperature phase diagram (see figure), there are curves separating solid from vapor, vapor from liquid, and liquid from solid. These meet at a single point called the triple point, where all three phases can coexist. The triple point is at a temperature of 273.16K (0.01°C; 32.02°F) and a pressure of 611.657 pascals (0.00604atm; 0.0887psi); [70] it is the lowest pressure at which liquid water can exist. Until 2019, the triple point was used to define the Kelvin temperature scale. [71] [72] Health and pollution An environmental science program – a student from Iowa State University sampling water In a vacuum, water will boil at room temperature. [69] Triple and critical points Phase diagram of water (simplified) Water is a diamagnetic material. [59] Though interaction is weak, with superconducting magnets it can attain a notable interaction. [59] Phase transitionsThe viscosity of water is about 10 −3 Pa· s or 0.01 poise at 20°C (68°F), and the speed of sound in liquid water ranges between 1,400 and 1,540 meters per second (4,600 and 5,100ft/s) depending on temperature. Sound travels long distances in water with little attenuation, especially at low frequencies (roughly 0.03 dB/km for 1 k Hz), a property that is exploited by cetaceans and humans for communication and environment sensing ( sonar). [100] Reactivity Water is fundamental to both photosynthesis and respiration. Photosynthetic cells use the sun's energy to split off water's hydrogen from oxygen. [106] In the presence of sunlight, hydrogen is combined with CO

In a water molecule, the hydrogen atoms form a 104.5° angle with the oxygen atom. The hydrogen atoms are close to two corners of a tetrahedron centered on the oxygen. At the other two corners are lone pairs of valence electrons that do not participate in the bonding. In a perfect tetrahedron, the atoms would form a 109.5° angle, but the repulsion between the lone pairs is greater than the repulsion between the hydrogen atoms. [90] [91] The O–H bond length is about 0.096nm. [92] In nature, the color may also be modified from blue to green due to the presence of suspended solids or algae. Because Earth's environment is relatively close to water's triple point, water exists on Earth as a solid, liquid, and gas. [22] It forms precipitation in the form of rain and aerosols in the form of fog. Clouds consist of suspended droplets of water and ice, its solid state. When finely divided, crystalline ice may precipitate in the form of snow. The gaseous state of water is steam or water vapor. to form water. Water is considered to be neutral, with a pH (the negative log of the hydrogen ion concentration) of 7. Acids have pH values less than 7 while bases have values greater than 7.At sea level, the boiling point of water is 100°C (212°F). As atmospheric pressure decreases with altitude, the boiling point decreases by 1°C every 274meters. High-altitude cooking takes longer than sea-level cooking. For example, at 1,524 metres (5,000ft), cooking time must be increased by a fourth to achieve the desired result. [67] (Conversely, a pressure cooker can be used to decrease cooking times by raising the boiling temperature. [68]) One factor in estimating when water appeared on Earth is that water is continually being lost to space. H 2O molecules in the atmosphere are broken up by photolysis, and the resulting free hydrogen atoms can sometimes escape Earth's gravitational pull (see: Atmospheric escape). When the Earth was younger and less massive, water would have been lost to space more easily. Lighter elements like hydrogen and helium are expected to leak from the atmosphere continually, but isotopic ratios of heavier noble gases in the modern atmosphere suggest that even the heavier elements in the early atmosphere were subject to significant losses. [29] In particular, xenon is useful for calculations of water loss over time. Not only is it a noble gas (and therefore is not removed from the atmosphere through chemical reactions with other elements), but comparisons between the abundances of its nine stable isotopes in the modern atmosphere reveal that the Earth lost at least one ocean of water early in its history, between the Hadean and Archean eons. [30] [ clarification needed] Seawater contains about 3.5% sodium chloride on average, plus smaller amounts of other substances. The physical properties of seawater differ from fresh water in some important respects. It freezes at a lower temperature (about −1.9°C (28.6°F)) and its density increases with decreasing temperature to the freezing point, instead of reaching maximum density at a temperature above freezing. The salinity of water in major seas varies from about 0.7% in the Baltic Sea to 4.0% in the Red Sea. (The Dead Sea, known for its ultra-high salinity levels of between 30 and 40%, is really a salt lake.)