Water the Most Substance on Earth

Water is the most common substance on Earth, covering more than 70% of the planet’s surface. All living things consist mostly of water; for example, the human body is about two-thirds water. Earth, air, fire, and water were considered by the ancient philosopher to be the four chemical elements, a concept that persisted through the middle Ages. It was not until 1781 that Henry Cavendish showed that water was obtained by the combustion of hydrogen. Shortly afterward the discovery was made that water could be electrolytically decomposed into two volume of hydrogen and one volume of oxygen, but the chemical formula of water was a matter of controversy until 1860, when Stanislao Cannzzaro established it as H₂O.

Water is a clear, colorless liquid that appears blue when its depth is at least 6 m (20 ft). The color results not only from physical causes but also from suspended impurities. In terms of its chemical composition, water has an unusually high boiling point (100^o C/212^o F) and freezing point (0^o C/32^o F). These discrepancies result from the strong attraction that each water molecule has for others, so that the energy required to melt the solid and boil the liquid is greater than might be predicted. Water also shows unusual volume changes with temperature. As warm water cools, it contracts until it reaches its maximum density at 4^oC (39^oF); further cooling effects an expansion occurs when the liquid phase, and another expansion occurs when the liquid freezes to ice. Both of these peculiarities occur because of the formation of highly ordered arrangement of hydrogen bonding of the water molecules.

Pure water is a poor conductor of electricity, but the impurities found water transform it into a relatively good conductor. The ability of water to act as a universal solvent as well as its high dielectric constant (about 80) are a result of the polar nature of the water molecule.