Water is the most common liquid on earth. Although often perceived to be pretty ordinary, water is the most remarkable substance. We wash in it, fish in it, swim in it, drink it and cook with it, although probably not all at the same time. We are about two-third water and require water to live. Life as we know it could not have evolved without water and dies without it.
Three-quarters of Earth’s surface is covered by water. Although most of this water is in liquid form, water is also present on Earth as a solid (ice) and a gas (water vapor). Water is the only common substance to exist in the natural environment in all three physical states of matter. Furthermore, the solid state of water floats on the liquid, a rare property emerging from the chemistry of the water molecule.
Water seems, at first sight, to be a very simple molecule, consisting of two hydrogen atoms attached to an oxygen atom and indeed, few molecules are smaller. Its size, however, belies the complexity of its properties, and these properties seem to fit ideally into the requirements for carbon-based life as can no other molecule. Now let’s focus on what properties of the simple water molecule make it so indispensable to life on earth.