The Importance of Water to Life on Earth a look at the unique structure and behavior of the water molecule by Steven Luscher, Student Water is the most important substance in our evolution and our daily lives. Without water, life as we know it would not have been possible. This essay will examine the water molecule in order to ascertain how it brought about Earth's thriving ecosystem and how important it is to us today. ============================== ______________________________ ============================== Each water molecule consists of one oxygen atom and two hydrogen atoms. The oxygen atom (or the apex of the water molecule) bears a slight electronegative charge while hydrogen possesses a more positive one1 (figure a). Because opposite charges attract, the water molecules are drawn together. When an oxygen atom is linked to a neighboring molecule's hydrogen atom, a bond called a hydrogen bond is formed2. In an ice crystal the hydrogen bonds govern the shape of the crystal so that the grid of molecules surrounds relatively large spaces (imagine figure b in three dimensions). In a liquid form, water has no such spaces; thus ice is less dense and will float on liquid water. If not for this, great bodies of water would freeze from the bottom up without the insulation of a top layer of ice and all life in the water would die. The water molecule is a very small one but because of its unique properties it behaves like a larger one. The bonds between water molecules are so strong that water resists changes in its state (Solid, liquid, gas); thus water has a higher melting point and a higher boiling point than another molecule of similar size. If water followed the example of other molecules its size it would have a boiling point of -75?C and a freezing point of -125?C4. This would mean that, on Earth, water would be a gas all of the time and life would not be possible. When heat is applied to solid water, some hydrogen bonds get so much kinetic energy that they break and the ice melts. Liquid water does not necessarily have all four hydrogen bonds present at all times but it must retain some of them5. For any object to penetrate water, it must be able to break the hydrogen bonds on the surface of the water. These bonds resist breaking thus forming a "skin" that allows small insects to walk on the surface of the water. Without the cohesiveness of water, those insects would not have survived. All plant life on Earth benefits from the ability of water to make a hydrogen bond with another substance of similar electronegative charge. Cellulose, the substance that makes up cell walls and paper products, is a hydrophilic substance ("water-loving")6. It interacts with water but, unlike other hydrophilic substances, it will not dissolve in it. Cellulose can form strong hydrogen bonds with water molecules7. This explains why a paper towel will "wick" water upwards when it comes in contact with it. Each water molecule will make a hydrogen bond with cellulose and pull another water molecule up from down below and so on. Without this feature (capillary action8), plants would find it more difficult to transport water up their stems to the leaves in order to make food through photosynthesis. Water has a very high heat capacity. Most of the heat introduced to water is used not to set water molecules in motion (giving them kinetic energy and causing their temperature to rise), but to move hydrogen atoms around between neighboring oxygen atoms9. If all of this heat was used solely to warm the water, living cells would boil in their own heat. Every action in a living cell releases some heat. If the heat was not dissipated by the water, all living things would cook themselves. In order for water to evaporate from the surface of liquid water, a certain amount of energy must be expended to break its hydrogen bonds. Because these hydrogen bonds are so strong, water requires a lot of heat to boil (100?C). When water vaporizes, it takes along all of the heat energy required to break its bonds thus having a powerful cooling effect on the original body of water 10. It takes very little water loss to