Water holds energy for reactions

Water may be a reactant or product

Water must be present for many biochemical processes to take place

Water can dissolve many substances

It is less dense than water

It is more dense than water

High boiling point

It sinks

Weak bonds between the oxygen atom of one water molecule and the hydrogen atom of another.

Hydrogen bonding between water molecules.

The oxygen atom in the molecule

The hydrogen

High boiling point.

two

three

four

12098346

3 percent

10 percent

100 percent

0.1203%

3

2

4

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Fals

Water is NOT an inorganic compound with the chemical formula H₂O. It is NOT transparent, tasteless, odorless,^[c] and nearly colorless chemical substance, and it is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a solvent^[19]). It is vital for all known forms of life, despite not providing food energy, or organic micronutrients. Its chemical formula, H₂O, indicates that each of its molecules contains one oxygen and two hydrogen atoms, connected by covalent bonds. The hydrogen atoms are attached to the oxygen atom at an angle of 104.45°.^[20] "Water" is also the name of the liquid state of H₂O at standard temperature and pressure.

Because Earth's environment is relatively close to water's triple point, water exists on Earth as a solid, liquid, and gas.^[21] 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.

One factor in estimating when water appeared on Earth is that water is continually being lost to space. H₂O 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.^[28] 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.^{[29][clarification needed]}

Any water on Earth during the latter part of its accretion would have been disrupted by the Moon-forming impact (~4.5 billion years ago), which likely vaporized much of Earth's crust and upper mantle and created a rock-vapor atmosphere around the young planet.^[30][31] The rock vapor would have condensed within two thousand years, leaving behind hot volatiles which probably resulted in a majority carbon dioxide atmosphere with hydrogen and water vapor. Afterward, liquid water oceans may have existed despite the surface temperature of 230 °C (446 °F) due to the increased atmospheric pressure of the CO₂ atmosphere. As the cooling continued, most CO₂ was removed from the atmosphere by subduction and dissolution in ocean water, but levels oscillated wildly as new surface and mantle cycles appeared.^[32]

This pillow basalt on the seafloor near Hawaii was formed when magma extruded underwater. Other, much older pillow basalt formations provide evidence for large bodies of water long ago in Earth's history.

Geological evidence also helps constrain the time frame for liquid water existing on Earth. A sample of pillow basalt (a type of rock formed during an underwater eruption) was recovered from the Isua Greenstone Belt and provides evidence that water existed on Earth 3.8 billion years ago.^[33] In the Nuvvuagittuq Greenstone Belt, Quebec, Canada, rocks dated at 3.8 billion years old by one study^[34] and 4.28 billion years old by another^[35] show evidence of the presence of water at these ages.^[33] If oceans existed earlier than this, any geological evidence has yet to be discovered (which may be because such potential evidence has been destroyed by geological processes like crustal recycling). More recently, in August 2020, researchers reported that sufficient water to fill the oceans may have always been on the Earth since the beginning of the planet's formation.^[36][37][38]

Unlike rocks, minerals calledzirconsare highly resistant to weathering and geological processes and so are used to understand conditions on the very early Earth. Mineralogical evidence from zircons has shown that liquid water and an atmosphere must have existed 4.404 ± 0.008 billion years ago, very soon after the formation of Earth.^{[39][40][41][42]}This presents somewhat of a paradox, as thecool early Earthhypothesis suggests temperatures were cold enough to freeze water between about 4.4 billion and 4.0 billion years ago. Other studies of zircons found in Australian Hadean rock point to the existence ofplate tectonicsas early as 4 billion years ago. If true, that implies that rather than a hot,moltensurface and an atmosphere full of carbon dioxide, early Earth's surface was much as it is today. The action of plate tectonics traps vast amounts of CO₂, thereby reducinggreenhouse effects, leading to a much cooler surface temperature and the formation of solid rock and liquid water.^[43]