Environmental Engineering Reference
In-Depth Information
A water molecule forms a maximum of four hydrogen bonds because it can accept two
and donate two hydrogen atoms. In water, the four hydrogen bonds give rise to an open
structure and a 3D bonding network, resulting in the decrease of density when it is cooled
below 4°C.
Water molecules experience cohesion, due to the collective action of hydrogen bonds
between water molecules. However, water also has high adhesion properties because of its
polar nature. On extremely smooth surfaces such as glass, the water may form a thin ilm
because the adhesive molecular forces between glass and water molecules are stronger
than the cohesive forces.
The molecules of water are not statically bound but instead are constantly moving in
relation to each other, with hydrogen bonds continually breaking and recombining very
quickly. However, these hydrogen bonds are suficiently strong to create the unique prop-
erties we observe in water. However, not all hydrogen or oxygen atoms that constitute
water as we know it are the same. The hydrogen isotope protium (no neutron) is the most
abundantly found in natural water, accounting for >99.98% of its instances; however, water
can also be found having deuterium hydrogen isotopes (3 × 10
−3
%), or even tritium hydro-
gen isotopes, but at far lower concentrations (3 × 10
−6
%). Oxygen also has three stable iso-
topes, with which it is found in water, the most common resulting in 99.76%; however,
isotopes comprising 0.04% and 0.2% of carbon in water molecules are also found.
Pure water has a pH of 7 and is deemed neutral; however, water is also deemed ampho-
teric, meaning it can become an acid or a base depending on what is mixed within it. Pure
water has a balanced concentration between hydroxide ions, and hydronium or hydrogen
ions. However, water, being a polar liquid, can dissociate into the hydronium ion and a
hydroxide ion.
Water in nature is dificult to ind as pure, and to produce what is known as ultrapure
water is indeed very dificult. Instead water is normally found in nature with many con-
taminants, and with dissolved gases such as CO
2
, nitrogen, or sulfur oxides, compounds
making it slightly acidic and dropping the pH value to just below 6.
Water is mostly transparent to parts of the visible spectrum, and to a part of ultraviolet
light, and near red light; however, it readily absorbs most of the ultraviolet light, infrared
light, and the microwave regions of the electromagnetic spectrum. The relected part of
the visible spectrum from the sun gives it a faint blue color. Interestingly, there are win-
dows within the visible spectrum, such as 498 nm or blue-green light, that many of the
biological luminescent creatures use to manifest their position or excited states.
Water and ice are poor heat conductors, and it is this speciic property of water that has
profound consequences for our ecosystem. Water is densest in the liquid state and at a
temperature of 4°C and will sink to the bottom of deep lakes and oceans, regardless of the
temperature in the atmosphere. This cold water sinking produces the downward convec-
tion of colder water, and also an expansion of water as it becomes colder than the freezing
point. This denser saltwater sinks, producing convection, and the great conveyer belts of
ocean currents forming to transport water away from the Arctic and Antarctic poles, lead-
ing to a global system of thermohaline circulation.
When the temperature of freshwater reaches 4°C, the layers of water near the top in
contact with cold air continue to lose heat energy and their temperature falls below 4°C.
On cooling below 4°C, these layers may rise up as freshwater. However, because ice is
less dense than 4°C water, ice loats, and this unusual negative thermal expansion is due
to intermolecular interactions within the water molecules. The intermolecular vibrations
decrease, allowing the molecules to form steady hydrogen bonds with their neighbors and
thereby locking into hexagonal packing.
