Water, probably the most important liquid on Earth, is also one
of its most unusual. While water has many special features, in this
writeup, I will discuss a rather basic one: its density. The density is the mass per unit of volume. This mass is on Earth strongly linked to its weight, to the extend that may non-physicists use them interchangably. Technically, the weight is the force exerted by the mass in a gravity field.
Almost all substances contract when they cool and expand when they heat
up. This effect is very well-known in gases, where the ideal gas law
predicts that the volume of a certain amount of gas increases linearly
in the temperature. The reason for this is that the speed and kinetic
energy of the gas molecules increases with increasing
temperature. Because the gas molecules knock harder to their surroundings,
the gas will expand, lowering the density. In fact, one could say that
the density of the kinetic energy remains constant.
For solids and liquids, a similar principle applies. If a
solid or liquid heats up, the molecules vibrate more, and this
expands the solid. Of course, the molecules in a solid or liquid
are packed closely, so the expansion is a lot smaller than in a gas.
In water, the same thing happens. However, there is also a competing
effect. Water has the ability to form so-called hydrogen
bond. A hydrogen bond, very simply, is a kind of "bond" between the
electrons of a strongly electronegative atom such as
fluorine, nitrogen or oxygen and a hydrogen atom of another
molecule 1. These bonds are very common in water, as
each water molecule has one oxygen atom with two electron pairs and two hydrogen atoms available for hydrogen bonds. It is the molecule with the most hydrogen bonds
compared to its size. Now, these hydrogen bonds are not quite as strong a
"normal" bonds, but still a lot stronger that the normal attractive forces
between molecules.
Great, but what does this have to do with density? Well, the
hydrogen bonds force ice into a crystal structure that is not
particularly dense. In fact, the density of ice is lower than that of
water. This means ice floats on water. This is very rare: for almost
all matter, the solid sinks in the liquid. This has important
consequences for the freezing of water. When the air above a
pond reaches 0 C, its top freezes, forming a solid layer. This
layer "isolates" the rest of the pond from the cool air. In almost any
other substance, the chunks of solid would drop to the bottom, and new
liquid would surface, also freezing, leading to much more efficient and
complete freezing. In the case of water, this means that
ponds will almost always have liquid water at the bottom in
which life can survive
It also has another consequence. The increased formation of hydrogen bonds
at lower temperature means that when water approaches the temperature at
which it becomes ice, it also starts to expand. Hence, water
reaches its maximum density of 999.9720 kg m -3 at a
temperature of 3.98 C 2, 3. This density is remarkably close to
1 gram per cubic centimeter of water - in fact, this is how it
historically was defined 4. However, this is a very
inconvenient definition if one wants do do accurate measurements - first of
all, the temperature has to be precisely 4 C and, secondly the water
has to be pure. Furthermore, the water has to be stored in a vessel, the
mass of which must be subtracted form the mass of the water.
Nowadays, the kilogram is defined using a block of metal in Paris,
which is a much more accurate way of defining it. So, now you know it - the
fact water almost has a mass of 1 gram per cubic centimeter is not a
coincidence, but it is not exact, either.
In summary, the weight, or rather, the density of water is a complex
function of temperature. In particular, water is most dense at around 4
C. This has important consequences for the way water behaves on Earth, and,
in particular, for life. For a while water was used to define the
gram; this is no longer done, as the result is inaccurate. However, this
does mean that, conveniently, 1 cubic centimeter of water weighs 1 gram.
Sources
- http://www.elmhurst.edu/~chm/vchembook/161Ahydrogenbond.html
- http://www.simetric.co.uk/si_water.htm
- http://en.wikipedia.org/wiki/Water_(molecule)
- http://en.wikipedia.org/wiki/Kilogram