Water (hydrogen oxide) is a binary inorganic compound with the chemical formula H2O: a water molecule consists of two hydrogen atoms and one oxygen atom, which are connected by a covalent bond. Under normal conditions, it is a transparent liquid without color (with a small layer thickness), smell and taste. In the solid state it is called ice (ice crystals can form snow or frost), and in the gaseous state – water vapor. Water may also exist as liquid crystals (on hydrophilic surfaces).
Water is a good highly polar solvent. In natural conditions it always contains dissolved substances (salts, gases).
The role of water in the global circulation of matter and energy, the emergence and maintenance of life on Earth, in the chemical structure of living organisms, in the formation of climate and weather is extremely important. Water is the most important substance for all living beings on Earth.
In total, there are about 1,400 million cubic kilometres of water on Earth. Water covers 71% of the world’s surface (oceans, seas, lakes, rivers, ice – 361.13 million square kilometers). Most of the earth’s water (97.54%) belongs to the world’s oceans – salt water, unsuitable for agriculture and drinking. Freshwater is mainly found in glaciers (1.81%) and groundwater (about 0.63%), and only a small part (0.009%) in rivers and lakes. Mainland salt water is 0.007 % and the atmosphere contains 0.001 % of the total water of our planet.
Water is in a liquid state under normal conditions, while similar hydrogen compounds of other elements are gases (H2S, CH4, HF). Hydrogen atoms are attached to the oxygen atom, forming an angle of 104.45° (104°27′). Because of the large difference in the electrical negativity of hydrogen and oxygen atoms, the electronic clouds are strongly shifted towards oxygen. For this reason, the water molecule has a large dipole moment (p = 1.84 D, inferior only to hydrocyanic acid and dimethyl sulfoxide). Each water molecule forms up to four hydrogen bonds – two of them form an oxygen atom and two form hydrogen atoms. The number of hydrogen bonds and their branched structure determine the high boiling point of water and its specific heat of vaporization. If there were no hydrogen bonds, the water, based on the place of oxygen in the Mendeleev table and the boiling point of hydrides similar to oxygen elements (sulfur, selenium, tellurium), would boil at -80 ° C, and freeze at -100 ° C.