W Water Saeid Eslamian 1 , Saeideh Parvizi 1 , Kaveh Ostad-Ali-Askari 2 and Hossein Talebmorad 1 1 Department of Water Engineering, Isfahan University of Technology, Isfahan, Iran 2 Department of Civil Engineering, Isfahan (Khorasgan) Branch, Islamic Azad University, Isfahan, Iran Synonyms Hydro Definition Water (H 2 O, meaning that each of its molecules contains one oxygen and two hydrogen atoms that are connected by cova- lent bonds) is a polar inorganic compound that is, at room temperature, a tasteless and odorless liquid, nearly colorless with a hint of blue. It covers more than 70% of the earth’ s surface. It is the only common substance to exist as a solid, liquid, and gas in normal terrestrial conditions (Reece 2013). Introduction The word “water” comes from Old English “wæter,” from Proto-Germanic watar (source also of Old Saxon watar, Old Frisian wetir, Dutch water, Old High German wazzar, German Wasser, Old Norse vatn, Gothic wato “water”), from PIE wod-or, suffixed form of root wed-... “water; wet.” Water is essential for life. With two thirds of the earth’ s surface cov- ered by water and the human body consisting of 75% of it, it is evidently clear that water is one of the prime elements respon- sible for life on earth. However, Contrary to the past, our recent developed technological society has become indifferent to this miracle of life. Our natural heritage (rivers, seas, and oceans) has been exploited, mistreated, and contaminated. Water is a liquid at the temperatures and pressures that are most conducive to life. But in general, there are three states of water in nature: solid (ice), liquid, and gas (steam or water vapor). Since the water molecule is not linear and the oxygen atom has a higher electronegativity than hydrogen atoms, it is a polar molecule, with an electrical dipole moment: the oxy- gen atom carries a slight negative charge, whereas the hydro- gen atoms are slightly positive (Fig. 1). This simplest hydrogen chalcogenide is by far the most studied chemical compound and is described as the “universal solvent” for its ability to dissolve many substances (Greenwood and Earnshaw 1997). This allows it to be the “solvent of life” (Reece 2013). Physical and Chemical Properties of Water Water has specific physical and chemical characteristics. The important physical properties of water are cohesion (water is attracted to water) and adhesion (water is attracted to other substances). Water quality are primarily chemical character- istics and include color, dissolved oxygen, electrical conduc- tivity, hardness, pH, saline water, suspended sediment, and turbidity (Swanson and Baldwin 1965). Color: Water is in fact not colorless; even pure water is not colorless but has a slight blue tint to it. Dissolved oxygen: Another common measurement often taken is dissolved oxygen (DO), which is a measure of how much oxygen is dissolved in the water. Electrical conductivity: Pure water is actually an excellent insulator and does not conduct electricity. Water that would be considered “pure” would be distilled water (water condensed from steam) and deionized water (used in laboratories), although even water of this purity can # Springer International Publishing AG 2018 P. T. Bobrowsky, B. Marker (eds.), Encyclopedia of Engineering Geology , https://doi.org/10.1007/978-3-319-12127-7_295-1