![]() Water absorbs the red and orange parts of the visible spectrum to a high extent, which is the reason for the blue colour of the light that has passed through a thick layer of it. While ultraviolet passes through it well, infrared, which is so useful from a physical and biological perspective, hardly penetrates it. Water’s transparency depends on the wavelength of the light that passes through it. It increases when salts are dissolved in the water (see characteristics solution constants) and it varies depending on the temperature. The conductivity of the purest water ever obtained is 4.2 micro siemens per metre at 20☌ (for a resistivity of 23.8 mega-ohm centimetre). Water’s dielectric constant, which is approximately 80 Farads Steradian per metre, is one of the highest known, which is why water has major ionising power. Surface tension of water according to the temperature electrical properties dielectric constant Other substances decrease it, and these are called “surfactants”(detergents, for example).Ĭlick here to create your account in order to view the illustrations Table 4. Surface tension decreases when the temperature increases (table 4).Īdding dissolved salts generally increases the surface tension (γ = 74.6 mN.m -1 for an aqueous solution of NaCℓ at 1 mol Its action is such that it causes a capillary rise of 15 cm at 18☌ in a 0.1 mm-diameter tube. It is defined as a tensile force which acts on the liquid’s surface by always tending to reduce the extent of the surface to the greatest possible extent (figure 3). This characterises a property of interfaces (surfaces that limit two phases). When the pressure continues increasing, water assumes a liquid structure free of internal stresses, and follows the general rule - in other words, the viscosity increases with the pressure level. Unlike other liquids, moderate pressure makes water less viscous at low temperatures: to an extent it crushes its molecular organisational structure. The pressure has a quite special effect on water’s absolute viscosity. To the contrary, it increases with the content of dissolved salts sea water is therefore more viscous than river water (table 3). Water’s viscosity decreases when the temperature increases (table 2). dynamic (or absolute) viscosity µ, which is defined by the relationship between tangential shearing stress T (in Pa) and speed gradient V (in m/s) in a direction y (in m) that is perpendicular to the slide plane:.It is the reason for losses of kinetic energy (head loss) and therefore plays a major role in water treatment.Ī distinction is drawn between two types of viscosity: This resistance is due to the moving molecules’ reciprocal friction. This is the ability of a fluid (liquid or gas) to withstand various internal movements (turbulence for instance), or overall movements (flow for example). This is also why water is used as a heat-transfer fluid. The importance of specific heat and the vaporisation enthalpy ensures that the large expanses of water on the Earth’s surface constitute real heat accumulators. mol -1) at normal pressure and at a temperature of 100☌.mol -1) while for vaporisation the value is 2,259 kJ.transformation enthalpies (or latent heats) It varies depending on the temperature, with a minimum of 4.1784 kJ ☌) -1 in old units) at a temperature of 20☌.This is very high for a liquid, being 4.18 kJ thermal properties specific heat (or specific thermal capacity) Note: as pure water is the reference substance for measuring the density of liquids and solids, by definition its density equals 1.000. L -1 causes a variation in mass of 0.0008 kg.L -1at 0☌ a salinity variation of 1 g.L -1, has an average density of 1.0281 kg.As a matter of fact, it is a slightly elastic fluid: its volume decreases by approximately 0.048 % each time the pressure increases by one atmosphere. This property of water has various consequences, both in the natural world (stratification phenomena in lakes) and at treatment plants (for example: rising sludge in clarifiers).Īt the pressure levels encountered in hydraulic systems, water is considered to be an incompressible fluid. Density of water according to the temperature industrial processes and effluent treatmentĬlick here to create your account in order to view the illustrations Table 1.treatment and conditioning of industrial water.degasification, odour control, evaporation.flocculators – settling tanks – flotation units.fundamental biological engineering processes applicable to water treatment.fundamental physical-chemical engineering processes applicable to water treatment.
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