Besides oxygen, water is essential to sustain life on earth. This molecule, which we often take for granted, is quite phenomenal (see Chapter 18). H2O can change its physical state from ice to liquid (water) to steam within a temperature range of 100 °C. The important feature of water is its polar nature. Since the oxygen atom in the water molecule has a higher electronegativity than hydrogen, the electrons are not homogeneously distributed but are, rather, closer to the oxygen than the hydrogen atom. This creates a weak electrical field, with a negative pole at the oxygen and a positive pole at the hydrogen (Fig. 10.1).