Specific Heat

            The ability of water to stabilize temperature depends on its relatively high specific heat.  The specific heat of a  substance is defined at the amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1º C.   The specific heat of water is               1.00 cal/g ºC.  Compared with most other substances, water has an unusually high specific heat.  For example, ethyl alcohol, the type in alcoholic beverages, has a specific heat of 0.6 cal/g ºC.

            Because of the high specific heat of water relative to other materials, water will change its temperature less when it absorbs or loses a given amount of heat.  The reason you can burn your finger by touching the metal handle of a pot on the stove when the water in the pot is still lukewarm is that the specific heat of water is ten times greater than that of iron.  In other words, it will take only 0.1 cal to raise the temperature of 1 g of iron 1ºC.  Specific heat can be thought of as a measure of how well a substance resists changing its temperature when it absorbs or releases heat.  Water resists changing its temperature; when it does change its temperature, it absorbs or loses a relatively large quantity of heat for each degree of change.

            We can trace water’s high specific heat, like many of its other properties, to hydrogen bonding.  Heat must be absorbed in order to break hydrogen bonds, and heat is released when hydrogen bonds form.  A calorie of heat causes a relatively small change in the temperature because must of the heat energy is used to disrupt hydrogen bonds before the water molecules can begin moving faster.  And when the temperature of water drops slightly, many additional hydrogen bonds form, releasing a considerable amount of energy in the form of heat.

            What is the relevance of water’s high specific heat to life on Earth?  By warming up only a few degrees, a large body of water can absorb and store a huge amount of heat from the sun in the daytime and during summer.  At night and during winter, the gradual cooling water can warm the air.  This is the reason coastal areas generally have milder climates than inland regions.  The high specific heat of water also makes ocean temperatures quite stable, creating a favorable environment for marine life.  Thus, because of its high specific heat, the water that covers most of planet Earth keeps temperature fluctuations within limits that permit life.  Also, because organisms are made primarily of water, they are more able to resist changes in their own temperatures than if they were made of a liquid with a lower specific heat.

  Water is one of the few substances that are less dense as a solid than as a liquid.  While other materials contract when they solidify, water expands.  The cause of this exotic behavior is, once again, hydrogen bonding.  At temperatures above 4º C, water behaves like other liquids, expanding as it warms and contracting as it cools.  Water begins to freeze when its molecules are no longer moving vigorously enough to break their hydrogen bonds.  As the temperature reaches 0º C, the water becomes locked into a crystalline lattice, each water molecule bonded to the maximum of four partners.  The hydrogen bonds keep the molecules far enough apart to make ice about 10% less dense than liquid water at 4º C.  When ice absorbs enough heat for its temperature to increase to above 0º C, hydrogen bonds between molecules are disrupted.  As the crystal collapses, the ice melts, and molecules are free to slip closer together.  Water reaches it greatest density at 4º C and then begins to expand as the molecules move faster. 

            The ability of ice to float because of the expansion of water as it solidifies is an important factor in the fitness of the environment.  If ice sank, then eventually all ponds, lakes, and even the oceans would freeze solid, making life as we know it impossible on Earth.  During summer, only the upper few inches of the ocean would thaw.  Instead, when a deep body of water cools, the floating ice insulates the liquid water below, preventing it from freezing and allowing life to exist under the frozen surface.

Page Last Updated: Saturday June 27, 2009           Webmaster: Larry Jones                 Pickens County School District