Biology Reference
In-Depth Information
9. Water's pKa is 15.74 making the pH of unmodified water ~7.0, neither acidic nor basic.
However, absolutely pure water is very hard to produce. CO
2
is rapidly absorbed
producing carbonic acid and dropping the pH of freshly distilled water to ~5.7.
10. Water is highly non-compressible. Even in a deep ocean at 4,000m, under a pressure of
4
10
7
PA, there is only a 1.8% change in water's volume.
11. Pure water is an excellent electrical insulator and in fact does not conduct electricity at
all! But water is such a good solvent it almost always has some solute dissolved in it,
meaning that in a living cell where the salt concentration is high, water can readily
conduct electricity.
D. SURFACE TENSION
Another important property of water stemming from its structure is surface tension
[9]
.
Surface tension is of tremendous importance in the historical study of membranes (Chapter
2) and is essential in understanding membrane structure and stability. Water has the largest
surface tension (72.8 dynes/cm at 25
o
C) of any common liquid except mercury. For example,
ethanol, a much weaker H-bonding molecule than water, has a surface tension of only
22 dynes/cm. Surface tension is responsible for holding raindrops together, for making ocean
waves possible and for capillary action that brings water up a tree through the xylem. Water's
surface tension is even high enough to support the weight of some insects like the water
strider. An extreme example is the Basilisk lizard that can actually 'walk on water' for 10
to 20 minutes and so is more commonly referred to as the 'Jesus lizard' (
Figure 3.4
).
Surface tension is a measure of the strength of the water surface film that reflects the strong
intra-molecular H-bond-driven cohesive forces of water. The force of cohesion (sticking
together) between water molecules is the same in all directions (
Figure 3.3
). In bulk solution
every water molecule can be H-bonded to 4 other water molecules, but waters that are unfor-
tunate enough to find themselves at the air
water interface can only form
2 H-bonds (see
Figure 3.5
). The increase in number of H-bonds in the bulk solution compared
water or oil
e
e
FIGURE 3.4
The Basilisk or 'Jesus lizard' can 'walk on water' due to water's high surface tension. Reproduced
with permission
[13]
