Environmental Engineering Reference
In-Depth Information
The density of water is dependent on the dissolved salt and gas contents as well as the
temperature. Ice also loats in the sea; however, the salt content of oceans lowers the freez-
ing point by about 2°C, lowering the temperature of the density maximum of water to
just above 0°C. As the surface of salt water begins to freeze at 1.9°C, the ice that forms is
salt free with a density approximately equal to that of freshwater ice. Upon freezing, the
density of water decreases by about 9%. This ice loats on the surface, and the salinity and
density of the seawater just below it increases.
The hydrogen bonds become shorter in ice than in the liquid phase; this locking effect
reduces the average coordination number of molecules as the liquid approaches nucleation.
However, not all ice is less dense than the liquid form; only ordinary ice appears to be so.
The melting point of ordinary hexagonal ice falls slightly under moderately high pres-
sures. Under increasing pressure, ice undergoes a number of transitions to other allotropic
forms, each of which has higher density than liquid water—ice II, ice III, HD amorphous
ice, and VHD amorphous ice are examples of such. As ice transforms into its allotropes just
below 210 MPa, the melting point increases, reaching 82°C at 2.216 GPa.
Water also expands signiicantly as the temperature increases. Water near the boiling
point is about 96% as dense as water at 4°C. The melting point of ice is 0°C at standard pres-
sure; however, pure liquid water can be supercooled well below that temperature without
freezing, and it can remain in a luid state down to −42°C.
Under special circumstances, liquid water can remain so even at 42°C below zero and
ice can remain solid at temperatures of up to +82°C. Luckily this is not the case in nature's
own manifestations.
Starting with the energy from the sun, and the heating of the water at the surface cou-
pled with the density maximum of water at the liquid phase just above freezing, it is suf-
icient and necessary to drive the thermal cycles that drive the huge ocean currents such
as the Gulf Stream in the Atlantic or the Humboldt Current, and these determine our
global weather patterns, producing hurricanes, or ice ages, and the formation of new des-
erts where there once existed lush forests. The energy from the sun also drives the water
cycle from the seas, through evaporation, to become the clouds that precipitate on land as
freshwater, which simultaneously allows natural desalination.
Seawater moderates Earth's climate by buffering luctuations in ambient temperature,
allowing the oceans to absorb more heat than the atmosphere and buffering the heat
resulting from global warming. The potential consequence of global warming is that the
loss of ice in the poles will result in the alteration of these currents, which will have con-
sequences on weather patterns we may just be beginning to see. In addition, the speciic
enthalpy of fusion of water is also very high, adding further resistance to melting of the ice
on the poles, making water a good heat storage medium and heat shield.
In extreme conditions such as the abyssal ocean depths and near deep-water chimney
stacks, liquid and gas phases merge into one homogeneous luid phase exhibiting proper-
ties of both gas and liquid, or an indeterminate phase form, when the water's supercritical
point is reached at a temperature of 375°C and pressure of 221 atm. The work done by Baret
in producing ultrapure water uses these conditions to develop techniques in supercritical
luids that can also occur in these extreme conditions in nature.
If the vapor partial pressure is 2% of atmospheric pressure and the air is cooled from
25°C, starting at about 22°C water will start to condense, deining the dew point, and creat-
ing fog or dew. If the humidity is increased at room temperature and the temperature stays
about the same, the vapor soon reaches the pressure for a phase change and then condenses
out as steam. Water vapor pressure above 100% relative humidity is called supersaturated
and can occur if air is rapidly cooled, for example, by rising suddenly in an updraft.
