Water is important to all of us.

We use and depend on water every day.  We drink water and cook and wash with it.  We may enjoy swimming in water.  And we hope for rain to fall when crops are in the ground.  Water appears in numerous religious practices.  And water covers about 71% of the earth’s surface.  The chemistry of water is a source of wonder. 

A water molecule (H₂O) consists of two hydrogen atoms and one oxygen atom with the two hydrogens bonded to the oxygen atom in the center.  The H-O-H bonds occur not in a straight line, but at an angle of approximately 104.5 degrees. 

So the water molecule is “bent” and this fact leads to a quality called polarity.  Polarity means that one end of the molecule has a little more positive electrical charge and the other has a little more negative electrical charge.  In the case of water, the hydrogen ends become a little more positive and the oxygen a little more negative.  Because the water molecule has this polarity, water has unusual and marvelous characteristics that are important for life on earth.

• Water as a substance has a very cohesive structure – water molecules are attracted to each other and stick together through hydrogen bonding.  That is, the positive and negative ends of the bent water molecule are attracted to the opposite charge ends of other water molecules.  The molecules want to stick together so it takes a lot of energy to interrupt the attraction of the hydrogen bonds. 

As a result of hydrogen bonds, water remains liquid at a wide range of common earth temperatures.  Water can be a solid in the form of ice (below 0^o Centigrade or 32^o Fahrenheit) or a gas in the form of steam. (above 100^o Centigrade or 212^o Fahrenheit).  Clouds are simply water vapor in the sky.

• Water is a near universal solvent because the polarity of the water molecule makes it possible for many, many other substances to dissolve in water.  Dissolving means that other compounds break up into positively and negatively charged particles called ions that are attracted to the opposite charges of the polarized water molecule. As a result, water carries nutrients in soil, plants, and the bodies of animals including us.

• Water tends to adhere to other substances leading to capillary action.  When water is in a narrow tube it moves upwards around the edges of the tube against gravity! This is capillary action and makes it possible for watery sap to rise in plants and trees against gravity!

• Because water molecules are cohesive, liquid water has surface tension that supports some small things that would sink without surface tension.  Surface tension lets Water Striders walk along the surface.

• As a liquid, water is transparent. This makes it possible for light to penetrate lakes, rivers, and oceans. Plants need to receive the light of the sun for photosynthesis anywhere they live. The light that penetrates water supports plant life, which benefits all the other aquatic creatures. 

• Most substances contract and get smaller as they get colder and freeze.  Water, however, is different – it forms crystals and expands as it freezes! The polarity of the water molecule is the reason for this amazing difference. Like charges of the molecules are trying to avoid proximity to each other.  Because water expands on freezing, ice floats on top of the unfrozen water of a pond or lake, and even an ocean.  The ice protects all the aquatic life below from more severe cold. 

More importantly, if ice contracted when it freezes, it would not float but sink to the bottom.  And if ice sank, lakes and oceans would probably be frozen solid from bottom to top.  Only a shallow area of the surface water would thaw during warm periods.  Frozen solid, icy lakes and oceans would make it impossible for living creatures to develop on our planet!

• Water has a high heat capacity, and bodies of water function as a heat sink.  Heat capacity means that a substance can absorb and hold heat.  Because of its high heat capacity, large bodies of water, especially the oceans, absorb and hold a great deal of the earth’s heat.  We know that some areas of the oceans are warming noticeably as the planet is warming – and for now our huge oceans are protecting us from more severe effects of the changing climate.

The chemistry of water is a wonder that makes possible our lives – and all life — on our beautiful and fragile planet!