water molecule is formed of one H atom and two O atoms linked by
covalent bonds. Water is an unusual liquid. It has a very high boiling
point and a high heat of vaporization. The maximum density is at
4ºC, and the water expands upon freezing. It has a very high surface
tension and is a very good solvent for salts and polar molecules.
These properties are a consequence of the dipolar character of H2O.
The electron cloud around H2O results from the
hybridization of s and p electrons, yielding two bonding orbitals
between the O and the two H atoms and two nonbonding sp3
orbitals on the O. The molecule has a high negative charge density
near 0 and high positive charge density near H.
the vapor phase the equivalent size of a water molecule is 3.3 Å,
or 0.33 nm (1 Å= 10-10 m). The molecules move at high
speed, and their translation energy is so high that during collisions
the van der Waals forces are inefficient for creating bonds. The
vapor expands when the temperature increases.
the liquid phase the molecules are close together and occupy a volume
of 29.7 Å3, indicating a porosity of 36.7%. Liquid water
forms a heterogeneous fluid structure of water molecules, clusters
of molecules, and H+ and OH- molecules. The
molecular structure influences the density and the viscosity. Liquid
water consists of an ice-tridymite structure, a quartzlike structure,
and a close-packed ammonia-like structure. In the ice structure
each 0 atom is bonded to four others by hydrogen bonds in a tetrahedral
configuration. The H atoms in the O-H ... O bond are no longer 0.96
Å from the O atom but may be either 0.99 Å or 1.77 Å away. The volume
per H2O molecule in ice is 32.3 Å3. The volume
per gram-molecule is 19.56 ml, and the resulting density is 0.92.
number of broken H bonds increases with temperature up to 50% at
40ºC. Broken hydrogen bonds are responsible to a considerable extent
for its high dielectric constant (D = 78.55 at 250ºC), contributing
to tire fact that water is the best solvent for polar compounds