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Water has many unusual properties that are critical for life: it is a good solvent and has high surface tension. Fresh water has its greatest density at 4° C: it becomes less dense as it freezes or heats up. As a stable, polar molecule prevalent in the atmosphere, it plays an important atmospheric role as an absorber of infrared radiation, crucial in the atmospheric greenhouse effect. Water also has an unusually high specific heat, which plays many roles in regulating global climate.
Water is a very good solvent and dissolves many types of substances, such as various salts and sugar, and facilitates their chemical interaction, which aids complex metabolisms.
Some substances, however, do not mix well with water, including oils and other hydrophobic substances. Cell membranes, composed of lipids and proteins, take advantage of this property to carefully control interactions between their contents and external chemicals. This is facilitated somewhat by the surface tension of water.
Water drops are stable due to the high surface tension of water. This can be seen when small quantities of water are put onto a nonsoluble surface such as glass: the water stays together as drops. This property plays a key role in plant transpiration.
A simple but environmentally important and unique property of water that its solid form, ice, floats on the liquid. Its solid phase, is less dense than liquid water, due to the geometry of the strong hydrogen bonds which are formed only at lower temperatures. For almost all other substances, the solid form is more dense than the liquid form. Fresh water is most dense at 4°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.
Life on earth has evolved with and fine tuned itself to the important features of water. The existence of abundant liquid, vapor and solid forms of water on Earth has no doubt been an important factor in the abundant colonization of Earth's various environments by life-forms adapted to those varying and often extreme conditions.
The coexistence of the solid, liquid, and gaseous phases of water on Earth is vital to the development of life. However, if not for a coincidence of Earth's location in the solar system, the conditions which allow the three forms to be present simultaneously could not have occurred.
Earth's mass allows it the gravity to hold an atmosphere, which provides a steady surface temperature. If Earth was less massive, a thinner atmosphere would cause temperature extremes preventing the accumulation of water except in polar ice caps (as on Mars). According to the solar nebula model of the solar system's formation, Earth's mass is mostly due to its distance from the Sun.
The distance between Earth and the Sun ensures that its surface is neither too cold nor too hot for liquid water. If Earth was more distant, most water would be frozen. If Earth was nearer to the Sun, its higher surface temperature would limit the formation of ice caps, or cause water to exist only as vapor. In the former case, the low albedo of oceans would cause Earth to absorb more solar energy. Either could lead to a greenhouse effect and inhospitable conditions similar to Venus.
Life itself may maintain the conditions that allowed its existence. The surface temperature of Earth is essentially constant across geologic time despite varying solar flux, indicating that a dynamic process governs Earth's temperature. See Gaia hypothesis.
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