Water, hydrogen oxide, is the simplest durable chemical compound of hydrogen and oxygen. Its molecule consists of two hydrogen atoms bonded to one oxygen atom, H2O. Pure water is transparent and colorless, odorless, and tasteless. The range of surface temperatures and pressures on the Earth permit water (as the only substance) to exist naturally in all three states on our planet. Water in liquid state is predominant, but occurrence of water in the solid and gaseous states is also common. Water molecules undergo state changes: from liquid to gaseous phase - by evaporation (evapotranspiration); from gaseous to liquid phase - by condensation; from liquid to solid state - by freezing; and from solid to liquid state - by melting. Direct phase change between the solid and gaseous phase is also possible, in the process of sublimation.
Due to its molecular structure, water is a unique substance in that no other substance has similar physical and electrochemical properties. Understanding the physical properties of water is indispensable to interpret the functions of the hydrosphere. A summary of basic physical characteristics of water is compiled in Table 1.
The molecular polarity and the dipole structure of a water particle are responsible for the high surface tension and the solvent properties. Liquid water has a tetrahedral structure, which breaks down in the process of evaporation. There is a change in the arrangement of molecules corresponding to phase changes. When freezing, water molecules arrange themselves in such a way that water expands its volume (by about 9% for rapid freezing), becoming lighter than liquid water. Water is the only known substance where the maximum density does not occur in the solid state. The water density attains its maximum in the liquid phase, at 4 °C. Water becomes lighter when warming up above 4 °C or cooling down below this temperature.
There are further unique properties of water, making it an astonishing substance, which behaves in an anomalous way. These features play a crucial role in many processes in the geosphere and biosphere. The liquid water is an excellent, and universal, solvent, able to dissolve many chemical compounds, for example, mineral salts. Having a neutral pH (i.e., being neither acidic nor basic) in a pure state, water changes its pH when dissolving substances, being slightly acidic in rain (due to dissolution of carbon dioxide and sulfur dioxide, present in the air). Some 97.5% of all water on Earth is salty oceanic waters, containing dissolved natrium chloride (NaCl), with concentration of 33-37gkg~\ Water on the move carries dissolved and particulate substances (e.g., in hydrological processes of precipitation, runoff and river flow, infiltration and groundwater flow). The liquid water is adhesive and elastic due to high surface tension, which counteracts the downward pull of the gravity force. It aggregates in drops rather than spreading out as a thin film over a surface. Water conducts heat easier than any liquid, with the exception of mercury. Water has a high specific heat (higher than other liquids, except liquid ammonia), and a high latent heat of freezing/melting and evaporation/condensation. All these features play a significant role in the heat exchange processes in the Earth's system.
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