Advanced knowledge

The properties of water

Water undergoes various types of chemical reactions. One of the most important chemical properties of water is its ability to behave both as an acid (a proton donor) and a base (a proton acceptor), the characteristic property of amphoteric substances. This behaviour is most clearly seen in the autoionization of water:

H₂0(l) + H₂0(l) H₃0⁺(aq) + OH^-(aq),

where the (l) represents the liquid state, the (aq) indicates that the species are dissolved in water, and the double arrows indicate that the reaction can occur in either direction and an equilibrium condition exists. At 25¢XC the concentration of hydrated H⁺ (i.e., H₃O⁺ , known as the hydronium ion) in water is 1.0 x 10^-7 M, where M represents motes per litre. Since one OH- ion is produced for each H30+ ion, the concentration of OH^- at 25ºC is also 1.0 x 10^-7 M. In water at 25ºX ÅnÅbÅ` must always be 1.0 x 10^-14:

[H⁺][OH^-] = 1.0 x 10^-14,

where [H⁺] represents the concentration of hydrated H⁺ ions in motes per litre and [OH^-] represents the concentration of OH^- ions in motes per litre. When an acid (a substance that can produce H⁺ ions) is dissolved in water, both the acid and the water contribute H⁺ ions to the solution. This leads to a situation where the H⁺ concentration is greater than 1.0 x 10^-7 M. Since it must always be true that [H⁺][OH^-] = 1.0 x 10^-14 at 25¢XC, the [OH^-] must be lowered to some value below 1.0 x 10^-7. The mechanism for reducing the concentration of OH^- involves the reaction

H⁺ + OH^- = H₂0,

which occurs to the extent needed to restore the product of [H⁺] and [OH^-] to 1.0 x 10^-14 M. Thus, when an acid is added to water, the resulting solution contains more H⁺ than OH^-; that is, [H⁺] > [OH^-]. Such a solution (in which [H⁺] > [OH^-]) is said to be acidic. The most common method for specifying the acidity of a solution is its pH, which is defined in terms of the hydrogen ion concentration: pH = -log [H⁺], where the symbol tog stands for a base-10 logarithm. In pure water, in which [H⁺] = 1.0 x 10^-7 M, the pH = 7.0. For an acidic solution, the pH is less than 7. When a base (a substance that behaves as a proton acceptor) is dissolved in water, the H' concentration is decreased so that [OH^-] > [H⁺]. A basic solution is characterized by having a pH > 7. In summary, in aqueous solutions at 25ºC: ¡@

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Density and Elasticity

The density of water at 4ºC is I g per ml. Table I gives density values at different temperatures. Water is assumed to be an incompressible fluid. Nevertheless it has a modulus of elasticity of about 300,000 psi, meaning a volumetric decrease of about 0.000048 for each added atmosphere of pressure.

Viscosity

The viscosity of a fluid is the proportionality factor in the expression for the intensity of viscous shear at a point in the moving fluid:

where Ån is the shear per unit area of surface normal to the s-direction, dv /ds is the maximum velocity gradient at the point, with the s-direction representing the direction in which the maximum occurs, v is the kinematic viscosity (=Åg/p), and Åg is the absolute viscosity (force x time)/length ¡.

The unit of viscosity is the poise (dyne = s/cm¡).

The viscosity of pure water at atmospheric pressure, as a function of the temperature, is presented in Table 2. The intensity of viscous shear corresponds to the internal energy loss. The velocity gradient and the shear intensity are important in flocculation, settling, and filtration processes.

Vapor Pressure and Relative Humidity

The vapor pressure of a liquid is the pressure of the liquid vapor in contact with the liquid at which vapor molecules condense as fast as they evaporate from it. Vapor pressure is a function of temperature.

Surface Tension

Water molecules are held together by attractive forces. Beyond a certain radius, Rcritical, the attractive forces become negligible. Molecules closer than Rcritical, to a free surface are attracted to the interior of the liquid by the resultant force. The potential energy per unit surface area is the surface energy. The numerical value of the surface energy is equal to the surface tension of the liquid. The surface tension decreases with increasing temperature. The interfacial tension between water and another liquid that is immiscible with water is approximately equal to the difference between their surface tensions. Gibbs' rule shows that the addition of a solute to a solvent leads to different behaviors, depending on the surface tension. If the solute at a low concentration has a weak surface tension it will be concentrated at the surface of the solvent and lower the surface tension of the solution. On the contrary, large amounts of a solute of high surface tension will concentrate away from the surface and will not increase the surface tension of the solution. This phenomenon is of great interest in the treatment of surface water and wastewater.