I`ve 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?
"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.
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.
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.