The physical and chemical properties of water
The properties of water: generalities and curiosities
The properties of water: isotopes and molecular structure
Water was considered by the Ancients as one of the 4 fundamental elements: the world was made up of a mixture of these 4 essential principles in varying proportions. It was considered a simple body until the 1774th century. Then several chemists discovered that water was not a simple body by carrying out the synthesis and then the analysis. Let us cite the precursors, Priestley who produced water from the combustion of hydrogen (1783), Watts (1783) who hypothesized that water was not a simple body, Monge who realized the synthesis under the action of an electric spark from a mixture of oxygen and hydrogen. But the decisive synthesis experiment was that of Lavoisier and Laplace (1800) who synthesized water from hydrogen and oxygen in a memorable public experiment. The decomposition of water took place later, after the discovery of the electric cell by Volta in 2. The electrolysis of water made it possible to measure the respective ratio of oxygen and hydrogen to finally arrive at the well known chemical formula H1800O. The first practical (and spectacular) electrolysis was carried out in 1803 in Paris by Robertson; the chemical formula was clarified by the theoretical work of Dalton (1811) and Avogadro (XNUMX).
Physical properties of water
Water has quite special physical properties compared to other liquids. It appears as a “structured” liquid, and not disordered like other liquids, by the fact that its elementary constituents are associated.
The properties of water serve as a reference for the international standardization of numerical scales: temperature, density, mass, viscosity, specific heat. The specific heat is exceptionally high (18 mole calories per degree), it explains the great thermal inertia of water and its role in regulating the temperature of the earth's surface. The oceans store an enormous quantity of heat which it redistributes by sea currents; evaporation of water absorbs energy in the aquatic environment and lowers its temperature, the condensation of vapor into droplets in clouds releases this heat to the atmosphere. The masses of water on the surface of the globe are real thermal flywheels for climates.
The density of water varies with its temperature; it increases when the temperature drops, but the maximum density is at 4 ° C (0,997 g / cm3) and not at 0 ° as one might expect. Thus, seas and lakes freeze from the surface and not from the bottom where the densest water accumulates by stratification. Water in the solid state is lighter than liquid water (density of ice: 0,920 g / cm3).
The viscosity of water depends on its isotopic composition: heavy water is 30% more viscous than ordinary water. Viscosity first decreases with pressure and then increases thereafter.
The isothermal compressibility coefficient of water is small (4,9 10-5 per bar) and as a first approximation we can consider water as incompressible. Nevertheless, the great atmospheric depressions act on the sea level which rises during storms. The surface tension is high: water is a good wetting agent (72 dyne / cm); it creeps and penetrates into all interstices and pores of rocks as well as into soils by capillarity phenomenon. This property is fundamental for the storage of water in aquifers, for the surface erosion of rocks (bursting under the effect of frost: the water-ice passage develops a pressure of up to 207 KPa). The high surface tension also explains the spherical shape of the water drops.
The physical state of water depends on temperature and pressure. The liquid-gas passage is conventionally done at 100 ° C at normal pressure but at 72 ° C only at the top of Everest (8 m). The melting temperature of the ice decreases with the pressure: under the effect of pressure the ice becomes liquid again: thus, the skaters actually slide on a thin film of liquid water formed under the effect of the pressure of the skate . The triple point of water is at 848 ° C under 0,01 mbar.
Water can remain liquid below the melting point of ice: this supercooling phenomenon can be maintained down to a temperature of -40 ° C. This is explained by the absence of seeds to initiate solid crystallization. In nature, the germ is provided by a common bacteria, Pseudomonas syringae. The genetic manipulation of this bacterium makes it possible either to delay the freezing of fruit trees, or to accelerate the frost to make artificial snow more easily.
Finally, water is an excellent solvent which serves as a vehicle for most of the ions on the surface of the globe.
Chemical properties of water
Water is an excellent solvent which dissolves a large number of salts, gases and organic molecules. The chemical reactions of life take place in an aqueous medium; organisms are very rich in water (up to over 90%). It has long been considered a neutral solvent intervening little or not in chemical reactions. The dilution in water made it possible in particular to slow down the activity of the reagents. In fact, water is a very aggressive chemical agent which risks attacking the walls of the container which contains it: in a glass bottle, silicon ions pass through the water. Pure water can exist from a regulatory point of view, i.e. water without bacterial and chemical contaminants, but it practically does not exist from a chemical point of view: even distilled water contains traces of ions or organic molecules taken from pipes and vessels.
In chemical reactions, water first intervenes by its dissociation into H + protons, often associated with H2O to form hydrated protons H3O +, and into OH- hydroxyl ions. It is the ratio between these 2 types of ions which determines the pH of the solution (pH: logarithm of the inverse of the molar concentration of H +). Many metals can decompose water, giving off hydrogen and a metal hydroxide.
The dissolution of ions (salts, acids, bases) is a consequence of the polar nature of water. The ion concentration of a salt characterizes the solubility product. Salts have product values of different solubility, which explains the phenomenon of fractional crystallization during the evaporation of a saline solution.In salt marshes, sea water first deposits calcium carbonate, calcium sulphate, then sodium chloride and finally very soluble salts such as potassium, iodides and bromides.
An important property on the Earth's surface is the dissolution of CO2 which produces a weak acid, carbonic acid, responsible for the chemical weathering of many rocks, in particular limestone. The amount of dissolved CO2 is a function of pressure and an inverse function of temperature. Calcium carbonate can be dissolved in the form of acid carbonate and then reprecipitated according to variations in temperature and pressure, as in the case of karst networks.