Environmental Engineering Reference
In-Depth Information
TABLE 1.1
Water Parameters
Molar mass
18.015 g/mol
Density max
1 g/cm
3
at 4°C
Melting point
0°C at 1 atm
Boiling point
100°C at 1 atm
Refractive index
1.333
Viscosity
0.001 Poise at 20°C
Dipole moment
1.85 D
Molecular structure
Hexagonal
Molecular shape
Bent
Angle between H atoms
104.45°
Distance between O and H atoms
0.1 nM
Dissociation constant
10-14
K
w
at 25°C
Vapor pressure
2.3 kPa at 20°C
Compressibility
5.1 × 10
−10
Pa
−1
at 0°C at 0 Pa
Compressibility min
4.4 × 10
−10
Pa
−1
at 45°C at 0 Pa
Compressibility absolute min
3.9 × 10
−10
Pa
−1
at 0°C at 100 MPa
Bulk modulus
2.2 GPa
Heat capacity
4.2176 (J/g)·K at 100 kPa
Heat of vaporization
45.054 k J/mol at 0°C
Heat of fusion
333.55 k J/kg at 0°C
Electrical resistivity max
182 kΩ·m at 25°C
Electrical conductivity
0.055 μS/cm at 25°C
Surface tension
72.8 mN/m at 25°C
his name to the great ocean current found on the Paciic Ocean. The mass in water was
thus determined to be composed of 11.1% hydrogen and 88.9% oxygen.
We now know that individual water molecules have two hydrogen atoms that are cova-
lently bonded to a single oxygen atom and the solitary oxygen molecule attracts electrons
much more strongly than hydrogen atoms, giving a net positive charge on the hydrogen
atoms and a net negative charge on the oxygen atom. The presence of a charge on each of
these atoms gives each water molecule a net dipole moment.
A molecule with such a charge difference is known as dipolar, and the direction of the
dipole moment points from the oxygen toward the center of the hydrogens. Associated
water molecules are not planar in two dimensions, but instead are three-dimensional (3D)
and form speciic angles, with hydrogen atoms at the vertices and oxygen in between them
at its head. Since oxygen has a higher electronegativity than hydrogen, the head of the
molecule with the oxygen atom has a partial negative charge with which the lone electron
pairs of hydrogen tend to repel the oxygen atom. Owing to this dipole moment, the boiling
point of water is above the expected result. The charge differences cause individual water
molecules to be attracted to each other and to other polar molecules.
Hydrogen bonding and the resulting electrical attraction between individual water mol-
ecules make it rather dificult to separate molecules from each other, giving it a very high
speciic heat capacity and a high heat of vaporization. Hydrogen bonding is a relatively
weak attraction compared with the covalent bonds within the water molecule itself; how-
ever, it is the reason for the high melting and high boiling point of water.
