Pure water is colorless, odorless, and tasteless and so common that you probably never think about how unique it is and how essential to life. Most plants and animals contain more than 60% water by volume. Without water, life would not have evolved on Earth, and it is the presence of water on Mars and some moons of Jupiter and Saturn that causes us to speculate about past or present life there as well.
Water has a number of unique chemical and physical properties that make it essential for life. One such property is familiar to everyone: solid water floats on liquid water. Almost all liquids contract when they get colder and reach a maximum density when they solidify. Water is different. As water cools, it contracts until it reaches 4 C, then it expands until it freezes at 0 C. Ice is less dense than water which allows ice cubes to float in a soft drink, icebergs to float in the ocean, and ponds and lakes to freeze from the top down so that aquatic plants and animals can survive in the unfrozen liquid below.
Water molecules have a simple structure: two hydrogen atoms bonded to one oxygen atom – H2O. This simple structure is responsible for water’s unique properties. The bond between each hydrogen atom and the oxygen atom results from a pair of electrons shared between the two atoms. In water, the electrons in the shared pair are not shared equally between the hydrogen and oxygen atoms. The oxygen atom has a greater affinity for electrons than does the hydrogen atom, and the electrons in the O–H bond are more attracted to oxygen. Because electrons have a negative charge, the unequal sharing in the O–H bond results in oxygen acquiring a partial negative charge
( −) and hydrogen a partial positive charge ( +). The H–O–H bond angle in water is 104.5 , which means that the molecule has a bent shape.
This bent geometry and the accumulation of electrons on the oxygen side of the molecule cause the water molecule to have a negative charge on one side, the oxygen side, and a positive charge on the other side, the hydrogen side. Molecules with negative regions and positive regions are called polar molecules. Water molecules are polar molecules.
Polar molecules are attracted to each other. The attraction results from the negative region of one molecule, the oxygen atom, being drawn to the positive region of another molecule, the hydrogen atom. Opposites attract! The attractions between water molecules are particularly strong. Oxygen atoms have a very great affinity for electrons, and so the hydrogen atoms bonded to an oxygen atom acquire a significant positive charge. These hydrogen atoms are very tiny, so the positive charge is quite concentrated. This concentrated positive charge enhances the attraction of the hydrogen atoms in one molecule for the oxygen atom in another molecule. These attractions are represented by the green lines (highlighted by arrows) in the figure. This attraction is so strong that it has been given a particular name:
hydrogen bonding. The energy associated with hydrogen bonds in water is about 20 kJ•mol–1, which is about 1/10 the strength of a typical shared-electron bond within a molecule.