How is H2O formed?
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HOW IS WATER FORMED?
Water has two chemical components, oxygen and hydrogen, which gives us the formula H2O . (There are various joke names for water on the internet, but if you must name it from the elements, then the only chemically reasonable name would be hydroxic acid. But please don't do that, water is what it is).
The reaction stochiometry formula
2H2+O2→2H2O does not describe how the water is made, such formulas are "mass balance" equations only and say nothing about the process. So 2H2+O2 does not mean that two hydrogen or three oxygen molecules literally come together. In fact the low temperature combustion of hydrogen with oxygen is not a direct reaction at all, and involves the intermediate formation of hydrogen peroxide which then dissociates:
H2+O2⇝H2O2⇝O2+H2O
Now we can turn our attention to the real question, the origins of water.
The hydrogen is mainly left over from The Archaic Universe. That was too short lived and cooled too quickly to make heavier atoms other than hydrogen and some helium. The universe was initially structured into areas of matter density that formed galaxies, and star formation initiated in regions of higher density.
Oxygen is produced by stellar nucleosynthesis, mainly by an alpha process (fusion of helium), but it also builds up in the core of massive stars, and when large stars explode, matter is thrown out into space. We belong to the "local bubble" left by older explosions.
The main bubble ingredient is hydrogen gas and cosmic dust particles called stardust. This dust has been found to have hydrogen peroxide present H2O2 .
Atomic hydrogen interacts with frozen solid oxygen catalysed by the surface of dust particles.
Hydrogen peroxide is chemically reactive with organic molecules and water can be produced as a byproduct, certainly the periodic melting of comets could accelerate such processes. But also hydrogen peroxide is unstable and decomposes into water and oxygen molecules as shown above.
(We have to be careful making chemical assumptions here though. The conditions in space are very cold, radiation is a factor, improbable reactions may occur given very long times, and unstable is a relative term, and without something to interact with, a molecule could persist for very long times.)
Dust particles can eventually accumulate into large snowball structures, which form comets and even planet-sized objects. These do not necessarily undergo heating, as slow collisions can amalgamate stardust and Chondrules into larger bodies. It was thought that objects such as comets delivered a lot of earth's later water after earth cooled, but lately attention has shifted to the asteroid belt as a more likely candidate. Possibly also the large body that joined with the earth brought extra water to us.
The upper mantle of the earth is degassed, and water is cycled along with other elements in the upper crust. When the earth cooled it was largely covered in one big ocean, and since water absorbs heat, this water has long acted as a thermal buffer to keep our planet warm.
As the solar wind will blow away hydrogen atoms into space from the upper atmosphere, this has caused the planet to lose a quarter of its water, by removing an important ingredient. It is not replaced. This was catalysed by methane when it was once more abundant on earth. Luckily we avoided the fate of planets such as Mars that lost all the water.
Water can formed biologically during metabolic reactions through the action of enzymes on various compounds.
Water is formed synthetically by
- Elimination reactions from molecules that contain oxygen and hydrogen in the right proportion
- Combustion, burning hydrogen in oxygen. 2H2+O2⇝2H2O
- Catalytically combining oxygen and hydrogen to provide a power source in non polluting fuel cells that produce only water as the waste product.
- Bio-water can be made by burning hydrogen created by bacteria such as Halanaerobium hydrogeninformans.
- Addendum: Recent research on by the isotopic composition of water released from the surface of a comet has confirmed it is identical to earth water after all, and this points to a vast reservoir of frozen water in the outer regions of the Solar System, with the comet delivery option back on the table.
Explanation: to produce to molecules of water(H20), two molecules of diatomic hydrogen (H2) must be combined with one molecule of diatomic oxygen(02).
