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Water in three states: liquid (including the clouds, which are aerosols), solid (ice) and gas (water vapour)

Water (H₂O) is a transparent, tasteless, odourless, and almost always colourless chemical substance and covers about 71% of Earth's surface.

No known life can live without it. Water is essential for life. This has to be qualified a bit. There are some forms of life which can survive without it, but cannot reproduce without it. Since reproduction is a central part of life, it is clear that water is essential for an organism to survive and reproduce.