Water has important effects on all biological systems. What makes water so unique are two very important properties.

Water is a polar molecule

A water molecule is formed when two atoms of hydrogen bond covalently with an atom of oxygen. In a covalent bond electrons are shared between atoms. In water the sharing is not equal. The oxygen atom attracts the electrons more strongly than the hydrogen. This gives water an asymmetrical distribution of charge. Molecules that have ends with partial negative and positive charges are known as polar molecules. It is this polar property that allows water to separate polar solute molecules and explains why water can dissolve so many substances.

Water is highly cohesive .

  The positive regions in one water will attract the negatively charged regions in other waters. The dashes show the hydrogen bond. In a hydrogen bond a hydrogen atom is shared by two other atoms. The donor is the atom to which the hydrogen is more tightly linked. The acceptor (having a partial negative charge) is the atom which attracts the hydrogen atom.

Hydrogen bonds are much weaker than covalent bonds. However, when a large number of hydrogen bonds act in unison they will make a strong contributory effect. This is the case in water.

Liquid water has a partially ordered structure in which hydrogen bonds are constantly being formed and breaking up.

Movie  http://www.nyu.edu/pages/mathmol/textbook/water_dynamics.mpg

On the other hand ice has a rigid lattice structure.

In liquid water each molecule is hydrogen bonded to approximately 3.4 other water molecules. In ice each each molecule is hydrogen bonded to 4 other molecules. Notice the hole in the middle of each ring above!

Compare the two structures below. Notice the empty spaces within the ice structure.

The density of ice is 0.931 gm/cubic cm. This compares with a density of 1.00 gm/cubic cm. for water.

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