Biology Reference
In-Depth Information
TABLE 3.1
Size of Water Clusters as a Function of Temperature.
Number of water
molecules in a cluster
Temp
o
C
0 (ice)
All (100)
0 (liquid)
91
40 (liquid)
38
100 (gas)
0
1. Most solids expand when they melt but water expands when it freezes. In fact water is
the only substance on Earth where the maximum density of its mass does not occur when
it becomes solidified. The density of liquid water (1.000 g/cm
3
) is greater than solid ice
(0.917 g/cm
3
) causing ice to float in water and resulting in an ~9% increase in volume
upon freezing. If ice were denser than liquid water, bodies of water would freeze from
the bottom up, vastly reducing the amount of liquid water on Earth. Also, it is expansion
during freeze/thaw cycles that turns inhospitable rock into soil.
2. Water has a boiling point almost 200
o
C higher than that expected on the basis of boiling
points of similar compounds. While the boiling point of H
2
O is 100
o
C, that of H
2
S is only
60
o
C. Therefore H
2
S is a gas at biologically important temperatures and could not serve
in place of water as 'life's solvent'.
3. Water's high latent heat of evaporation (2.270 kJ/kg) gives resistance to dehydration and
provides considerable evaporation cooling.
4. Water has a melting point (0
o
C) at least 100
o
C higher than expected on the basis of
melting points of similar compounds and also has a high heat of fusion (melting,
334 kJ/kg).
5. Water is an excellent solvent and is often referred to as a 'miracle liquid' or the 'universal
solvent'. Water is such a good solvent it almost always has some solute dissolved in it
(about 5 ppb impurities in distilled water). Almost every substance known has been
found dissolved in water to some extent. Water efficiently dissolves salts like NaCl that
have very limited solubility in most other liquids. Seawater for example is ~3.5% salt.
This property allows for facile transfer of nutrients necessary for life. Liquid water is an
excellent solvent due to its polarity, high dielectric constant, and small size. In contrast,
solid ice is a very poor solvent.
6. Although water has an unusually high viscosity, a measure of resistance to flow (0.001
Pa
.
sat20
o
C), it is still 10
4
to 10
5
less viscous than the interior of a membrane.
7. Water conducts heat more easily than any liquid except mercury.
8. Water has an unusually high heat capacity. It takes more heat to raise the temperature
of 1 g of water 1
o
C than any other liquid. Its specific heat is 1 cal/g
o
C or 4.186 J/g
o
C. As
a result water can store enormous amounts of thermal energy, minimizing the volume
of body coolant required to remove metabolic heat. Due to its high heat capacity, high
thermal conductivity, and abundance in cells, water is the major thermal regulator in life.
Liquid water is truly special as it has over twice the specific heat capacity of ice or steam.
Thermal properties of water are due to the ease of intra-molecular hydrogen-bonding.
