Why is water essential to life?

It all has to do with its properties. For instance, it's polarity makes it a good solvent for ions and polar molecules. It also has the capacity to form hydrogen bonds (only possible if a molecule has H-N, H-O, or H-F bonds in it) which gives it cohesiveness and therefore surface tension. This is what makes capillary action possible--you can have a tiny strand of water inside a plant (or any small tube really) and if the molecules at the top are leaving (as with evaporation in plants), all the ones below it move up--AGAINST GRAVITY!--because of the hydrogen bonds that link each water molecule in the column to the ones around it.
Then there's water heat capacity--which is huge compared to most other substances. By this chemists mean that it takes a relatively large amount of energy to increase the temperature of water. This makes water an incredibly good temperature stabilizer--witness the differences in weather between places on the coast and places inland: the inland regions have much larger fluctuations in temperature than the coastal regions do because the ocean serves as a buffer against temperature change. So planets with water are more likely to have life than those without because (from what we can tell) organisms generally don't survive as well when there are large fluctuations in temperature. Water also serves as a temperature buffer inside organisms, so even when outside temperatures fluctuate quite a bit, having water inside the organism can help it survive
Water is also rather unusual in that it is denser as a liquid than it is as a solid, so ice floats. This matters because a layer of ice on top of a body of water insulates the water below it and keeps the lower stuff from freezing (including any organisms down there). On the other hand if ice were denser than water, water would freeze from the bottom up which would probably kill most anything living there.
There's even more to it than this, but basically water is a very special substance, and we have had no reason to think that any other compound could serve as the foundation for life. And yes, it's possible that we are limited in our ability to imagine life forms that wouldn't be based on water, but considering the time, effort, and cost of sending expeditions to other planets, testing for water (which is relatively easy) instead of looking directly for life (which is much much harder) is just a way of playing the odds: it is far more likely, given what we know about chemistry and molecular physics, that water would be necessary to life than it is likely for us to find a non-water-based lifeform.

Also, what's with your tag-line?

Roses are red, violets are blue.
All of my base, are belong to you.

If you put that into the English forum, someone would have a stroke (probably my sister, haha!).

Thanks for the response Eos.

As for the tag line, it's a computer nerd joke, and a very popular one at that. It's a modification from the phrase "All your base are belong to us", and if you google it you'd get a lot of explanations, but basically a few years back there was a video game on Sega, the intro clip of which was translated with astounding mediocrity. The transcript of the poor translation was so humorous it gained popularity online, and today there are "All Your Base" websites and even T-shirts. The tag line isn't my own creation, it's a quote from bash.org, which is a website that takes hilarious quotes from IRC chat rooms and posts them for all the world to enjoy.

I just found this website that explains it pretty nicely with animated gifs and all.

All your base are belong to us


That's a nice play on the original quote, Exception.

Re: Why is water essential to life?


Because the only type of life they have ever seen, evolved in water.

Eos has listed a number of unique characteristics of water, which may seem to be a prerequisite of life, but that does assume that life is limited to the type we are familiar with.

Other compounds have very unique characteristics at different pressures and temperatures. Robert L. Forward described life on a Neutron Star in his novel _Dragon's Egg_, and David Brin speculated on the possibilities of life in the fluid plasmas of our sun (and other stars) in his novel _Sundiver_, and Gregory Benford wrote of magnetic and mechanical life in various forms. All three men are physists who put a bit more than just random speculation into their models, even though they are fictional.

Until we have another type of "life" to compare to, it's reasonable to assume that our foundation is the most likely foundation, but we must always remind ourselves that we are working from a single data point, so we really don't know if we are "normal" or not in the grand scheme of things. This is one reason we search with such desperation to find even the merest scrap of life from somewhere else, for even two data points alone, are vastly more revealing than one.

Good question. Thanks.

Water is essential to life on earth, given, as rosborne has said, water's unique characteristics here on earth.

All the plant and animal forms of Earth require water. Off plant there may be substitutes, but biologists and chemists have few ideas on what chemical could perform even half of the functions of water in living organisms. Neil

Water isn't essential to life.
Only life as we are familiar with it.
In any case the important thing is to be conscious and aware and you don't have to have life for that either.

Ditto all other points. A very mundane addition to Eos's list of properties of water that are advantageous to our way of being alive: water is a reactant in many or most biochemical pathways, and is very useful for shuttling protons and electrons around. Life doesn't just use water as a solvent and an insulator: it's crucial to biochemical reactions, as well.