Under normal conditions, water exists in one of three phases, the solid phase (ice), the liquid phase (water), and the gaseous phase (steam).  Liquid Water is an amazing substance.  At one atmosphere of pressure, it exists at temperatures between 0 ^oC and 100 ^oC.  Over most of this temperature range (4 ^oC and 100 ^oC) its density increases slightly as the water cools, but for temperatures between 0 ^oC and 4 ^oC its density decreases as it cools.

Every water molecule (H₂O) consists of one oxygen atom and two hydrogen atoms.  A water molecule has no net charge because the number of positively charged protons equals the number of negatively charged electrons in each atom.  In the water molecule, each hydrogen atom is bound to the oxygen atom by a covalent bond.  The hydrogen atom and the oxygen atom share an electron, which has a slightly higher probability to be closer to the oxygen atom than to the hydrogen atom.  This results in a polar molecule, in which the oxygen "side" of the molecule has a slight negative charge, while the hydrogen "sides" have a slight positive charge.  The negative and the positive sides attract each other, forming hydrogen bonds.  These hydrogen bonds are about twenty times weaker than the covalent bonds between hydrogen and oxygen atoms.  Because water is a polar molecule it dissolves many inorganic and organic materials.  Organisms can build macromolecules to attract or repel water as needed simply by varying the charge on side chains.

Strong inter-molecular forces are the reason that water molecules are hard to pull apart.  The melting point of ice is higher than for other substances of similar molecular weight, and the boiling point of water is higher than expected for similar compounds.  Each transition, melting or boiling, requires an input of thermal energy to break the bonds.

Water freezes when the molecules have slowed down enough to develop bonds upon collision.  The rate at which freezing occurs is governed by nucleation and growth.  Nucleation is the formation of small solids in a liquid.  Once these "nuclei" have formed, they become the landing sites for other molecules to attach onto.  The growth rate is the rate at which the radius of a nucleus grows after formation.  It is determined by how many molecules bind and how many bonds are broken per unit time.  Adding a foreign substance such as salt, sugar, or ethylene glycol to water lowers the freezing temperature.  The foreign substance replaces water molecules in the liquid and there are fewer water molecules to bind to the nuclei.  Fast nucleation rates and/or slow growth rates result in the formation of many small crystals.  Slow nucleation rates result in the formation of large crystals.

The average distance between the water molecules in ice at 0^o C is greater than it is between water molecules at 4^o C.  Ice is less dense than water and therefore floats.  Water, like most things in nature, expands when heated and contracts when cooled except in the temperature range between 4^o Cs and 0^o C.  The hydrogen bond gives water this unique property.  In ice, a water molecule has four nearest neighbors to which it is bonded via hydrogen bonds.  The geometry leads to a rigid, rather open hexagonal structure.  When the temperature is raised above the melting point, jostling begins to destroy this open hexagonal structure.  Liquid water has a partially ordered structure in which hydrogen bonds are constantly being formed and broken.  In liquid water each molecule is hydrogen bonded on the average to 3.4 other water molecules.  Molecules are now allowed into the open spaces of the hexagonal structure.  This allows the molecules to move closer to each other and increases the density.  As the temperature increases towards 4^o C, molecules move through the liquid so fast that fewer bonds are formed at any one time, shortening the average time each bond exists and increasing the density even further.  At atmospheric pressure the temperature of greatest density is 4^o C.

External link:  States of matter simulation from the University of Colorado PhET group