# Why is liquid water considered a requirement for life?

Ive heard about different extremofiles living on earth. Is water a requirement for life because we have not seen life survive on earth without it, is it a chemical reason which makes life without water seem impossible? Could life exist with water, but in gas form? What makes the liquid part so important? Can life be based on other liquids on potential exoplanets?

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There are probably two separate questions to be answered here. 1) The requirement of liquid water for the genesis of life on a planet, and 2) how necessary a supply of liquid water is to the maintenance of life once it gets going. –  David H Feb 3 '14 at 18:55
This question appears to be off-topic because it is about biology and as such it should be asked on Biology.stackExchange. Chemistry.stackexchange also seems useful. –  Eduardo Serra Feb 3 '14 at 19:02
The definition of life is probably hard to pin down, since we only know terrestrian life. A hypothetical life form based on silicon, diamond chips or fullerenes as kind of advanced computers, if it ever could occur, wouldn't necessarily need water as basis. –  Gerald Feb 3 '14 at 20:37
@EduardoSerra I disagree with that assessment in this case. It is equally on topic here as a matter of astrobiology. –  called2voyage Feb 3 '14 at 21:04
This is a question frequently discussed in astronomy more specifically astrobiology. I believe this is on topic for this site –  Haaakon Feb 3 '14 at 22:38

"Life" seems to be an emergent phenomenon, a complex result of a very large number of simple parts or interactions. The whole is more than the sum of it's parts, unpredicted by the properties of the individual parts. If those individual parts are atoms and molecules, they have to move to be able to interact, to build larger, more complex structures. To move, they need to be suspended in something that permits movement and allows chemical reactions. On Earth, that would be liquid water.

Other liquids might work, but they'd need to allow for the movement and reactions life requires. It's hard to imagine something other than a liquid doing the job.

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Do you have some sort of argument for why a gas couldn't work because I don't see it. It would be slower because molecules would interact more rarely, but that just means it would evolve over longer timescales. –  ThePopMachine Jan 13 at 17:07

The reason for which water is needed for Life as we know it (and we have not been able to successfuly imagine a different model from that of DNA that has ability to self-replicate, react to the ambient, and evolve), is that water is the "universal solvent".

That means that most chemical reactions needed for life happen on a liquid environment, more specifically a water-based environment for most of them.

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I ask you to define life. –  Magno C Feb 6 '14 at 1:05
Schrödinger did it way better than I could: en.wikipedia.org/wiki/What_Is_Life%3F (I recommend reading this book. It's brilliant) –  Envite Feb 6 '14 at 8:04
.living matter, while not eluding the "laws of physics" as established up to date, is likely to involve "other laws of physics" hitherto unknown, which however, once they have been revealed, will form just as integral a part of science as the former.` We may have life without water, just only don't know at this moment how that can be. Not implies in affirm that does not exist. –  Magno C Feb 6 '14 at 10:47
@MagnoC Schrödinger defines DNA (the "aperiodic crystal) as a requisite for life, and DNA-driven reactions can happen only in a water-based medium. –  Envite Feb 6 '14 at 10:58
I'm okay with that. What I said is 'for now' is just what we know. All our premise is based on our observation of the surrounding world. As we are talking about the universe, may have things we do not know yet. But you're right, lets keep talking about what we know or soon we will philosophising about spiritual and digital life. –  Magno C Feb 6 '14 at 12:16

Water contains oxygen and hydrogen which are suitable for growth of life so yes it is necessary for life...at least now... We may find something else on different parts of universe which is necessary for growth of life.

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Water molecules are bipolar, a bit like a bar magnet. For this reason they form non-covalent bonds with other molecules. This makes both the breaking up and making of strong covalent chemical bonds easier. Water is not only a fluid in the meaning that things float in it. Water also increases the chemical turnover and this is important for biological processes where different molecules need to find each other.

The bipolarity of water helps make the right amino acids find their place along an RNA molecule, simply by facilitating more tests per millisecond until the right one comes along and sticks. Methane, for example, is not a bipolar molecule. It does not facilitate chemical recombinations in the way water does. Life would have a hard time to originate and survive in liquid methane. There are of course other bipolar volatile molecules than water.

I don't really know what I'm talking about here, but I heard someone who seems to do so... And animations like this help my impression that bipolarity is important in order to make the right stuff come together sooner rather than later.

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