Water is a tasteless, odorless substance that is essential to all known forms of life and is known as the universal solvent. It appears colorless to the naked eye in small quantities. The UN Environment Program estimates there are 1.4 billion cubic kilometers (330 million mi3)[1] available on Earth, and it exists in many forms. It appears mostly in the oceans (saltwater) and polar ice caps, but it is also present as clouds, rain water, rivers, freshwater aquifers, lakes, and sea ice. Water in these bodies continuously moves through a cycle of evaporation, precipitation, and runoff to the sea. Clean water is essential to human life, and in many parts of the world, it is in short supply. Significant quantities exist on the moons Europa and Enceladus. Thales of Miletus, an early Greek philosopher, known for his analysis of the scope and nature of the term "landscaping", believed that "all is water."
Chemical and physical properties
Water has the chemical formula H2O meaning that one molecule of water is composed of two hydrogen atoms and one oxygen atom. It can be described ionically as HOH, with a hydrogen ion (H+) that is bonded to a hydroxide ion (OH-). It is in dynamic equilibrium between the liquid and vapor states at standard temperature and pressure. Water alone is a colorless, tasteless, and odorless liquid, but upon standing it takes on the traces of carbon dioxide in the air and tends toward a sour solution of carbonic acid that is unpleasant-tasting and more inhospitable to life.
Water is often referred to in the sciences as the universal solvent and the only pure substance found naturally in all three states of matter; however, "found" should not mean that water is the only such natural substance that can be in three states at regular Earthly conditions, as its two elements are much more abundant than those of at least ten other molecules that share water's range but that are often found dissolved in water or shale. Examples are acetic acid, formic acid, hydrazine, dioxane, and benzene.
Freezing point
A simple but environmentally important and unique property of water is that its common solid form, ice, floats on its liquid form. This solid phase is not as dense as liquid water because of the geometry of the strong hydrogen bonds which are formed only at lower temperatures. For almost all other substances and for all other 11 uncommon phases, the solid form is denser than the liquid form. Fresh water at standard atmospheric pressure is most dense at 3.98 °C, and will sink by convection as it cools to that temperature, and if it becomes colder it will rise instead. This reversal will cause deep water to remain warmer than shallower freezing water, so that ice in a body of water will form first at the surface and progress downward, while the majority of the water underneath will hold a constant 4 °C. This effectively insulates a lake floor from the cold. Almost all other chemicals are denser as solids than they are as liquids and freeze from the bottom up.
Position of the Earth relating to water
Scientists theorize that most of the universe's water is produced as a byproduct of star formation. Gary Melnick, a scientist at the Harvard-Smithsonian Center for Astrophysics, explains: "For reasons that aren't entirely understood, when stars are born, their birth is accompanied by a strong outward wind of gas and dust. When this outflowing material eventually impacts the surrounding gas, the shock waves that are created compress and heat the gas. The water we observe is rapidly produced in this warm dense gas." [3]
The coexistence of the solid, liquid, and gaseous phases of water on Earth is vital to existence of life on Earth. However, if the Earth's location in the solar system were even marginally closer to or further from the Sun (a million miles or so), the conditions which allow the three forms to be present simultaneously would be far less likely to exist.
Earth's mass allows gravity to hold an atmosphere. Water vapor and carbon dioxide in the atmosphere provide a greenhouse effect which helps maintain a relatively steady surface temperature. If Earth were less massive, a thinner atmosphere would cause temperature extremes preventing the accumulation of water except in polar ice caps (as on Mars).
It has been proposed that life itself may maintain the conditions that have allowed its continued existence. The surface temperature of Earth has been relatively constant through geologic time despite varying levels of incoming solar radiation (insolation), indicating that a dynamic process governs Earth's temperature via a combination of greenhouse gases and surface or atmospheric albedo. This proposal is known as the Gaia hypothesis.
Effects on human civilization
Civilization has historically flourished around rivers and major waterways; Mesopotamia, the so-called cradle of civilization, was situated between the major rivers Tigris and Euphrates. Large metropolises like London, Montreal, Paris, New York, and Tokyo owe their success in part to their easy accessibility via water and the resultant expansion of trade. Islands with safe water ports, like Singapore and Hong Kong, have flourished for the same reason. In places such as North Africa and the Middle East, where water is more scarce, access to clean drinking water was and is a major factor in human development.
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