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Because water ice is less dense as a liquid than a solid (as an aside - this is due to the basic structure of the oxygen atom) it floats when frozen. Without this property ice would sink, basically filling up every ocean and lake from the bottom up. In any biosphere, a basic solvent is required, which is water on Earth. What other solvent in another "bio-mix" (for example, a methane-ethane atmosphere) would also possess that seemingly critical property of floating when frozen?

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    $\begingroup$ This was flagged as unclear. I think people are confused because the body is written as an adjunct to the Title, rather than being readable on its own. Look over some of the other questions here to see how they are normally composed. $\endgroup$ – JDługosz Mar 19 '17 at 20:28
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From Wikipedia, via Physics.SE, acetic acid, silicon, gallium,germanium, bismuth, plutonium. Also, Silicon dioxide this property.

Of these, acetic acid (16C) and gallium (30C) both have reasonable melting points. Bismuth melts at 271 C, and germanium at 938 C. Acetic acid has a boiling point of 118 C. The metals, on the other have boiling points of 2400 C, 2833 C, and 1564 C, so even if you could plausibly have an ocean of gallium, you would never have gallium rain (which doesn't sound that great anyways).

Silica and Silicon dioxide have similarly huge melting and boiling points. Plutonium has the problem that an ocean thereof will a. radioactively decay to lead in a few million years, and b. destroy itself in a nuclear blast well before then.

So acetic acid is really our only reasonable contestant for a 'water' cycle like on Earth. With gallium unable to evaporate, there doesn't seem to be any way to get gallium onto land, making life outside the oceans mostly impossible. So any gallium creatures will be 'oceanic'.

The last bit is you want to life-solvent to be able to interact with a lot of other compounds to be able to support metabolic processes. Water, for example, can dissolve almost anything, and reacts with lots of compounds to make interesting and potentially life-sustaining substances. Gallium does not really work in this way. It is related to Aluminum chemically, and can make a lot of crystalline compounds, but those are all solids. It doesn't really make any special liquids or gases, the way the hydrogen and oxygen and water do. On the other hand, as a metal, gallium has a lot of interesting properties of electron transfer that covalent compounds like water do not have. I do not know enough chemistry to speculate on how electron transfer between the metallic bonds could be used to make metabolic processes, but there may be a way.

For a more traditional solvent, acetic acid is the way to go. I can't find anything about the chemical properties of pure acetic acid, since it is usually used in solution with water (as vinegar). However, the acetyl and hydroxyl groups that form it are both chemically active in a great many biological processes. Acetic acid seems like your best bet.

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    $\begingroup$ Plutonium?! Well, now THAT would be an incredible type of life. $\endgroup$ – Andon Mar 19 '17 at 17:35
  • $\begingroup$ TY - It might come to nothing about my question per se, but I do know that the Defense Department investigated using gallium arsenide instead of silicon in computer chips to better harden them against electromagnetic radiation. $\endgroup$ – Ozymandias Mar 20 '17 at 4:54

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