Water in the Cosmos - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 10

structure that approximates to a tetrahedron, a pyramid with four
triangular faces. The oxygen and hydrogen atoms therefore effectively
lie at the apices of one of the faces of the water molecule pyramid.
Within the molecule, the oxygen atom grips the shared electrons
rather more tightly than do the hydrogen atoms, and so the oxygen
'apex' of the molecule has a relative negative charge, and the side with
the hydrogen atoms a positive one. This asymmetry (or polarity)
of charge is one of the key factors that help to explain a number of
water's literally life-giving properties. For instance, the positive end of
one water molecule is attracted to the negative end of another, and
this shared attraction, termed hydrogen bonding, helps explain why
water is liquid over a relatively large (and a relatively low, for its size)
temperature range. It is also a factor in the high surface tension of
water, which gives surface water a 'skin' that organisms such as pond
skaters can run across, and its strong capillary action (that makes it
'climb' up a narrow glass tube).
Crucially for life, the polarized electrical charge of a water molecule
is also a key factor in water's remarkable properties as a solvent. When
a substance that is made of ions of different charges is brought into
contact with liquid water, the negative ends of the water molecules
surround the positive ions and vice versa, forcing them apart and
keeping them surrounded by the (relatively small) water molecules.
Not everything dissolves in water (think of oils and fats) but many
substances do, to a greater or lesser extent—not least the kind of mole-
cules, such as proteins and sugars, that are biologically important.
Water bodies, hence, are chemical cauldrons of such diversity and
complexity that they can act, and have acted, as incubators of life.
Another feature of water relevant to planetary oceans is that, unlike
most substances, it expands slightly when it freezes, through a quirk
of the hydrogen bonding between water molecules. As a result, ice is
less dense than cold water and floats on the liquid water surface.