properties of 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) H30+(aq) + OH-(aq),
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:
= 1.0 x 10-14,
[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
+ OH- = H20,
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: ¡@
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.
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:
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.
Pressure and Relative Humidity
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.
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.