Water, the liquid of life. One of the most versatile elements. It is a very common mixing element. It vaporizes into Steam when it's heated, and that steam condenses when it hits Cooler. It puts out fires and passes this property onto its compounds.
Water turns into a gas after touching Hot, and condenses when touching Cold. Sadly this does not produce much extra pressure. It is useful for moving Water from one place to another.
Water is useful for controlling explosions.
Water has a special "surface water" animation that makes some particles look more pale when they are on the "outside" of a drop of water. See the article on Null for more about surface water. This surface water effect can be seen with any mixture containing Water, for example Rigid Water (RQ) has an obvious surface, but also Tensile Water looks like large drops.