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From my understanding, many of the features of water that make it very important for Earth's ecosystem are a result of oxygen's high electronegativity, which results in hydrogen bonding.

As nitrogen and fluorine also have some similar properties (High electronegativity, hydrogen bonding, stable hydrides, etc.), could they serve the same role water does in an alien ecosystem? (Obviously under different temperature and pressure conditions)

(I was directed to here from biology)

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    $\begingroup$ Related: What would a world whose atmosphere is made up of primarily ammonia be like? $\endgroup$ – Alexander Jun 25 '18 at 16:15
  • $\begingroup$ ammonia is already a major biological product. Most aquatic life excretes it. $\endgroup$ – John Jun 25 '18 at 16:32
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    $\begingroup$ Ammonia exists in very small amounts compared to the sheer amount of water present on the planet $\endgroup$ – SwagMcMuffins Jun 25 '18 at 16:49
  • $\begingroup$ Hello, user43479, and welcome to Worldbuilding. Please take our tour and visit the help center to learn more about the site. Have a nice day! $\endgroup$ – Gryphon Jun 25 '18 at 18:45
  • $\begingroup$ As regards ammonia, you might want to read Rocheworld by Robert L. Forward - this is a fiction account, and the life is purely speculative, but Forward's science is solid. $\endgroup$ – Jeff Zeitlin Jun 25 '18 at 19:27
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The book Xenology has a section about alternatives to water, and this boiling point calculator helps show the variance due to pressure.

Now, in my estimation, life will use whatever is the easiest in its current conditions. The reason why terrestrial life uses water is because most of the planet is not boiling or freezing (currently). In order for life to prefer ammonia or hydrogen fluoride instead, it would have to be perpetually in conditions where water is never available in liquid form.

Ammonia and hydrogen fluoride have boiling points around the freezing point of water, all at one atmosphere. Increasing the pressure will increase the liquidity range, roughly speaking. So an ice planet is probably the most likely to harbor ammonia or HF-based life.

Above the boiling point of water you are better off using something like sulfuric acid. Curiously enough, acids lose their acidic properties in environments devoid of liquid water.

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    $\begingroup$ “Curiously enough, acids lose their acidic properties in environments devoid of liquid water.” Depends on what you mean by that. Acids and bases work by basically reacting with water to split it into H+ or OH-, bonding with the one and leaving the other free to chemically interact with whatever else you drop in there. So, yeah, they don’t do that exact thing if there’s no water around, but the stronger ones do tend to be fairly reactive in and of themselves, so it’s not like acids or bases (generally) do nothing in the absence of water, either - they can still react chemically. $\endgroup$ – HopelessN00b Jun 26 '18 at 1:21
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    $\begingroup$ Depends on the definition of acid or base. Arrhenius defined acid to be a proton-donor when added to water. An Arrhenius acid gives an H+ ion when in water. Bronsted defined an acid to be a proton-donor in any solute. Lewis defined an acid as something that accepts an electron pair regardless of solution or the like. These definitions are complementary not contradictory. They apply to slightly conditions. $\endgroup$ – ttw Jun 26 '18 at 1:50
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Water is important for life not only for the factors you listed, but also because:

  • its high boiling point
  • its rather high specific heat
  • its volume increase upon freezing

Just to give you a measure of how these properties impact life, note that upon freezing ice floats on water, with the maximum density being achieved at around 4 degrees.

If that wouldn't be the case, ice would sink into the water, with the obvious result that any water body in winter would totally freeze.

Also the high specific heat allows water to be that important for mitigating climate next to large bodies of water.

As far as I know, ammonia and HF do not have the same behavior, that's why it is more difficult to base life on those.

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    $\begingroup$ +1 IMHO Water is the best choice for life because it's the Universal Solvent and on top of it works like it's perfect acid and alkaline at the same time (H+ & OH-). Serious it's unique properties makes it hard to replace and ammonio is a far "second place". I believe ammonia life forms are possible but I guess a lot more rare because water per si willl allow a lot more of solutions and chemical reactions $\endgroup$ – jean Jun 25 '18 at 17:58
  • $\begingroup$ High specific heat is also valuable for buffering organisms (many of which are largely water) against damage from heat or cold. $\endgroup$ – R.. GitHub STOP HELPING ICE Jun 26 '18 at 13:40
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Ammonia has some impressive properties as a solvent, including solvating metals (sodium, potassium, aluminum). It's true that ammonia ice doesn't float, but...

Many discussions of this question seem fixated on water's particular set of properties (expansion on freezing, acid/base balance, etc.). These strike me as Earth-chauvinism. I can just as easily imagine an ammonia-based race dismissing water as a basis for life. "Too hot (denatures proteins before it even boils)! Too acid! Too chemically limited (only two instead of three hydrogens)! And cells would rupture every time they froze!"

HOWEVER, I notice that the last few decades have brought us news of numerous water worldlets (Jovian moons, possibly even Ceres), but not much about worlds that could bear ammonia oceans. It may be that water life wins from simple abundance.

One good science-fiction read on the topic: Still River by Hal Clement.

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  • $\begingroup$ Welcome to worldbuilding. I am afraid this is just an elaborate comment, and not an answer to the OP's question. $\endgroup$ – L.Dutch Jun 26 '18 at 16:11

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