Water

A water molecule consists of two hydrogen atoms and one oxygen atom
The three common states of matter
Phase diagram of water (simplified)
Tetrahedral structure of water
Model of hydrogen bonds (1) between molecules of water
Water cycle
Overview of photosynthesis (green) and respiration (red)
Water fountain
An environmental science program – a student from Iowa State University sampling water
Total water withdrawals for agricultural, industrial and municipal purposes per capita, measured in cubic metres (m³) per year in 2010
A young girl drinking bottled water
Water availability: the fraction of the population using improved water sources by country
Roadside fresh water outlet from glacier, Nubra
Hazard symbol for non-potable water
Water is used for fighting wildfires.
San Andrés island, Colombia
Water can be used to cook foods such as noodles
Sterile water for injection
Band 5 ALMA receiver is an instrument specifically designed to detect water in the universe.
South polar ice cap of Mars during Martian south summer 2000
An estimate of the proportion of people in developing countries with access to potable water 1970–2000
People come to Inda Abba Hadera spring (Inda Sillasie, Ethiopia) to wash in holy water
Icosahedron as a part of Spinoza monument in Amsterdam.
Water requirement per tonne of food product
Irrigation of field crops
Specific heat capacity of water

Inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a solvent ).

- Water
A water molecule consists of two hydrogen atoms and one oxygen atom

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Ice

The three-dimensional crystal structure of H2O ice Ih (c) is composed of bases of H2O ice molecules (b) located on lattice points within the two-dimensional hexagonal space lattice (a).
Pressure dependence of ice melting
Log-lin pressure-temperature phase diagram of water. The Roman numerals correspond to some ice phases listed below.
An alternative formulation of the phase diagram for certain ices and other phases of water
Frozen waterfall in southeast New York
Feather ice on the plateau near Alta, Norway. The crystals form at temperatures below −30 °C (−22 °F).
Ice on deciduous tree after freezing rain
A small frozen rivulet
Ice formation on exterior of vehicle windshield
An accumulation of ice pellets
A large hailstone, about 6 cm in diameter
Snowflakes by Wilson Bentley, 1902.
Harvesting ice on Lake St. Clair in Michigan, c. 1905
Layout of a late 19th-Century ice factory
Loss of control on ice by an articulated bus
Channel through ice for ship traffic on Lake Huron with ice breakers in background
Rime ice on the leading edge of an aircraft wing, partially released by the black pneumatic boot.
Skating fun by 17th century Dutch painter Hendrick Avercamp
Ice pier during 1983 cargo operations. McMurdo Station, Antarctica

Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or 32 degrees Fahrenheit.

Fireless steam locomotive Despite the resemblance to a boiler, note the lack of a chimney and also how the cylinders are at the cab end, not the chimney end.

Steam

Fireless steam locomotive Despite the resemblance to a boiler, note the lack of a chimney and also how the cylinders are at the cab end, not the chimney end.

Steam is water in the gas phase.

An illustration showing groundwater in aquifers (in blue) (1, 5 and 6) below the water table (4), and three different wells (7, 8 and 9) dug to reach it.

Groundwater

An illustration showing groundwater in aquifers (in blue) (1, 5 and 6) below the water table (4), and three different wells (7, 8 and 9) dug to reach it.
Dzherelo, a common source of drinking water in a Ukrainian village
The entire surface water flow of the Alapaha River near Jennings, Florida, going into a sinkhole leading to the Floridan Aquifer groundwater
Groundwater may be extracted through a water well
Diagram of a water balance of the aquifer
Iron (III) oxide staining (after water capillary rise in a wall) caused by oxidation of dissolved iron (II) and its subsequent precipitation, from an unconfined aquifer in karst topography. Perth, Western Australia.
Groundwater withdrawal rates from the Ogallala Aquifer in the Central United States
Center-pivot irrigated fields in Kansas covering hundreds of square miles watered by the Ogallala Aquifer

Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations.

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Cloud

Aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space.

Aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space.

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Stratocumuliform cloudscape
Tropospheric cloud classification by altitude of occurrence: Multi-level and vertical genus-types not limited to a single altitude level include nimbostratus, cumulonimbus, and some of the larger cumulus species.
Cumulus humilis clouds in May
Windy evening twilight enhanced by the Sun's angle, can visually mimic a tornado resulting from orographic lift
Nimbostratus cloud producing precipitation
Cirrus fibratus clouds in March
Stratocumulus over Orange County.
Stratocumulus cloud
Cumulus humilis clouds
Cumulonimbus cloud over the Gulf of Mexico in Galveston, Texas
High cirrus upper-left merging into cirrostratus and some cirrocumulus upper right
A large field of cirrocumulus
Sunrise scene giving a shine to an altocumulus stratiformis perlucidus cloud (see also 'species and varieties')
Altostratus translucidus near top of photo merging into altostratus opacus near bottom
Cumulus humilis clouds over Jakarta, Indonesia
Stratocumulus stratiformis perlucidus over Galapagos, Tortuga Bay (see also 'species and varieties')
Stratus nebulosus translucidus
Deep multi-level nimbostratus cloud covering the sky with a scattered layer of low stratus fractus pannus (see also 'species' and 'supplementary features' sections)
Cumulus humilis and cumulus mediocris with stratocumulus stratiformis perlucidus in the foreground (see also 'species and varieties')
Towering vertical cumulus congestus embedded within a layer of cumulus mediocris: Higher layer of stratocumulus stratiformis perlucidus.
Progressive evolution of a single cell thunderstorm
Isolated cumulonimbus cloud over the Mojave Desert, releasing a heavy shower
Altocumulus lenticularis forming over mountains in Wyoming with lower layer of cumulus mediocris and higher layer of cirrus spissatus
Example of a castellanus cloud formation
Cumulus mediocris cloud, about to turn into a cumulus congestus
A layer of stratocumulus stratiformis perlucidus hiding the setting sun with a background layer of stratocumulus cumulogenitus resembling distant mountains.
Cirrus fibratus radiatus over ESO's La Silla Observatory
Altocumulus stratiformis duplicatus at sunrise in the California Mojave Desert, USA (higher layer orange to white; lower layer grey)
Cumulus partly spreading into stratocumulus cumulogenitus over the port of Piraeus in Greece
Cumulonimbus mother cloud dissipating into stratocumulus cumulonimbogenitus at dusk
Cirrus fibratus intortus formed into a Kármán vortex street at evening twilight
Global cloud cover, averaged over the month of October 2009. NASA composite satellite image.
Lenticular nacreous clouds over Antarctica
Noctilucent cloud over Estonia
Joshua Passing the River Jordan with the Ark of the Covenant (1800) by Benjamin West, showing Yahweh leading the Israelites through the desert in the form of a pillar of cloud, as described in
Stratocumulus stratiformis and small castellanus made orange by the sun rising
An occurrence of cloud iridescence with altocumulus volutus and cirrocumulus stratiformis
Sunset reflecting shades of pink onto grey stratocumulus stratiformis translucidus (becoming perlucidus in the background)
Stratocumulus stratiformis perlucidus before sunset. Bangalore, India.
Late-summer rainstorm in Denmark. Nearly black color of base indicates main cloud in foreground probably cumulonimbus.
Particles in the atmosphere and the sun's angle enhance colors of stratocumulus cumulogenitus at evening twilight

Water or various other chemicals may compose the droplets and crystals.

The Space Shuttle Main Engine burnt hydrogen with oxygen, producing a nearly invisible flame at full thrust.

Hydrogen

Chemical element with the symbol H and atomic number 1.

Chemical element with the symbol H and atomic number 1.

The Space Shuttle Main Engine burnt hydrogen with oxygen, producing a nearly invisible flame at full thrust.
Depiction of a hydrogen atom with size of central proton shown, and the atomic diameter shown as about twice the Bohr model radius (image not to scale)
Hydrogen gas is colorless and transparent, here contained in a glass ampoule.
Phase diagram of hydrogen. The temperature and pressure scales are logarithmic, so one unit corresponds to a 10x change. The left edge corresponds to 105 Pa, which is about atmospheric pressure.
A sample of sodium hydride
Hydrogen discharge (spectrum) tube
Deuterium discharge (spectrum) tube
Antoine-Laurent de Lavoisier
Hydrogen emission spectrum lines in the visible range. These are the four visible lines of the Balmer series
NGC 604, a giant region of ionized hydrogen in the Triangulum Galaxy
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Most of the hydrogen on Earth exists in molecular forms such as water and organic compounds.

Water cycle

Time-mean precipitation and evaporation as a function of latitude as simulated by an aqua-planet version of an atmospheric GCM (GFDL's AM2.1) with a homogeneous “slab-ocean” lower boundary (saturated surface with small heat capacity), forced by annual mean insolation.
Global map of annual mean evaporation minus precipitation by latitude-longitude
Relationship between impervious surfaces and surface runoff
Diagram of the water cycle
Natural water cycle

The water cycle, also known as the hydrologic cycle or the hydrological cycle, is a biogeochemical cycle that describes the continuous movement of water on, above and below the surface of the Earth.

Transpiration of Water in Xylem

Transpiration

Passively transported into the roots and then into the xylem.

Passively transported into the roots and then into the xylem.

Transpiration of Water in Xylem
Stoma in a tomato leaf shown via colorized scanning electron microscope
The clouds in this image of the Amazon Rainforest are a result of evapotranspiration.
Some xerophytes will reduce the surface of their leaves during water deficiencies (left). If temperatures are cool enough and water levels are adequate the leaves expand again (right).
The effect of temperature on the transpiration rate of plants.
The effect of wind velocity on the transpiration rate of plants.
The effect of humidity on the transpiration rate of plants.

]]Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers.

Runoff flowing into a stormwater drain

Surface runoff

Runoff flowing into a stormwater drain
Surface runoff from a hillside after soil is saturated
Precipitation washing contaminates into local streams
Urban surface water runoff
Willow hedge strengthened with fascines for the limitation of runoff, north of France.
Soil erosion by water on intensively-tilled farmland.
Farmland runoff
Runoff holding ponds (Uplands neighborhood of North Bend, Washington)

Surface runoff (also known as overland flow) is the flow of water occurring on the ground surface when excess rainwater, stormwater, meltwater, or other sources, can no longer sufficiently rapidly infiltrate in the soil.

Joseph Priestley is usually given priority in the discovery.

Oxygen

Chemical element with the symbol O and atomic number 8.

Chemical element with the symbol O and atomic number 8.

Joseph Priestley is usually given priority in the discovery.
Antoine Lavoisier discredited the phlogiston theory.
Robert H. Goddard and a liquid oxygen-gasoline rocket
An experiment setup for preparation of oxygen in academic laboratories
Orbital diagram, after Barrett (2002), showing the participating atomic orbitals from each oxygen atom, the molecular orbitals that result from their overlap, and the aufbau filling of the orbitals with the 12 electrons, 6 from each O atom, beginning from the lowest-energy orbitals, and resulting in covalent double-bond character from filled orbitals (and cancellation of the contributions of the pairs of σ and σ* and π and π* orbital pairs).
Liquid oxygen, temporarily suspended in a magnet owing to its paramagnetism
Space-filling model representation of dioxygen (O2) molecule
Oxygen discharge (spectrum) tube
Late in a massive star's life, 16O concentrates in the O-shell, 17O in the H-shell and 18O in the He-shell.
Cold water holds more dissolved.
500 million years of climate change vs. 18O
Photosynthesis splits water to liberate and fixes into sugar in what is called a Calvin cycle.
build-up in Earth's atmosphere: 1) no produced; 2) produced, but absorbed in oceans & seabed rock; 3)  starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4–5)  sinks filled and the gas accumulates
Hofmann electrolysis apparatus used in electrolysis of water.
Oxygen and MAPP gas compressed-gas cylinders with regulators
An oxygen concentrator in an emphysema patient's house
Low pressure pure is used in space suits.
Most commercially produced is used to smelt and/or decarburize iron.
Water is the most familiar oxygen compound.
Oxides, such as iron oxide or rust, form when oxygen combines with other elements.
Main symptoms of oxygen toxicity
The interior of the Apollo 1 Command Module. Pure at higher than normal pressure and a spark led to a fire and the loss of the Apollo 1 crew.

Most of the mass of living organisms is oxygen as a component of water, the major constituent of lifeforms.

The Pacific Ocean, part of Earth's hydrosphere

Hydrosphere

The Pacific Ocean, part of Earth's hydrosphere
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The hydrosphere is the combined mass of water found on, under, and above the surface of a planet, minor planet, or natural satellite.