Properties of Water


  1. Some extraordinary properties of water
    1. The cohesiveness of Liquid Water
      1. Cohesion = Phenomenon of a substance being held together by hydrogen bonds.
        1. Though hydrogen bonds are transient, enough water molecules are hydrogen bonded at any given time to give water more structure than other liquids.
        2. Contributes to upward water transport in plants.
      2. Surface tension = Measure of how difficult it is to stretch or break the surface of a liquid.
        1. Water has a greater surface tension than most liquids.
        2. Function of the fact that at the air/water interface, surface water molecules are hydrogen bonded to each other and to the water molecules below.
        3. Causes water to bead (shape with smallest area to volume ratio and allows maximum hydrogen bonding).
      3. Adhesion = Clinging of water to hydrophilic substances (e.g. glass).
      4. Hydrophilic = (Hydro=water; philo=loving) Property of having a affinity for water.
      5. Imbibition = Process of water soaking into a porous hydrophilic substance (e.g. a sponge).
        1. Imbibition by seeds, ruptures seed coat and allows germination.
    2. Water's High Specific Heat
      1. Heat and Temperature
        1. Kinetic energy = The energy of motion.
        2. Heat = Total kinetic energy due to molecular motion in a body of matter.
        3. Calorie (cal) = Amount of heat it takes to raise the temperature of one gram of water by one degree Celsius.
        4. Kilocalorie (kcal or Cal) = Amount of heat required to raise the temperature of one kilogram of water by one degree Celsius (1000 cal).
        5. Temperature = Measure of heat intensity due to the average kinetic energy of molecules in the body of matter.
        6. The Celsius Thermometer: (Figure 3.6) The Celsius scale at sea level:
          1. 100 degrees C (212 degrees F) = water boils
          2. 37 degrees C (98.6 degrees F) = human body temperature
          3. 23 degrees C (72 degrees F) = room temperature
          4. 0 degrees C (32 degrees F) = water freezes
        7. To convert temperature scales:
          1. degrees C = 5 (degrees F - 32) / 9
          2. degrees F = 9 degrees C / 5 + 32
          3. degrees K = degrees C + 273
        8. Specific heat = Amount of heat that must be absorbed or lost for one gram of a substance to change its temperature by one degree Celsius.
        9. Specific heat of water = One calorie per gram per degree Celsius (1 cal/g/degree C).
      2. How Water Stabilizes Temperature
        1. Water has a high specific heat, which means that it resists temperature changes when it absorbs or releases heat.
          1. As a result of hydrogen bonding among water molecules, it takes a relatively large heat loss or gain for each 1 degree C change in temperature.
          2. Hydrogen bonds must absorb heat to break, and they release heat when they form.
          3. Much absorbed heat energy is used to disrupt hydrogen bonds before water molecules can move faster (increase temperature).
      3. A large body of water can act as a heat sink -- absorbing heat from sunlight during the day and summer and releasing heat during the night and winter as the water gradually cools. As a result:
        1. Water, which covers three-fourths of the planet, keeps temperature fluctuations within a range suitable for life.
        2. Coastal areas have milder climates than inland.
        3. The marine environment has a relatively stable temperature.
    3. Water's High Heat of Vaporization
      1. Vaporization (evaporation) = Transformation from liquid to gas.
        1. Molecules with enough kinetic energy to overcome the mutual attraction of molecules in a liquid, can escape into the air.
      2. Heat of vaporization = Quantity of heat a liquid must absorb for 1 g to be converted to the gaseous state.
        1. For water molecules to evaporate, hydrogen bonds must be broken which requires heat energy.
        2. Water has a relatively high heat of vaporization (540 cal/g). (See Campbell, Table 3.2)
      3. Evaporative cooling = Cooling of a liquid's surface when a liquid evaporates.
        1. The surface molecules with the highest kinetic energy are most likely to escape into gaseous form; the average kinetic energy of the remaining surface molecules is thus lower.
      4. Water's high heat of vaporization:
        1. Moderates the earth's climate.
          1. Solar heat absorbed by tropical seas dissipates when surface water evaporates (evaporative cooling).
          2. As most tropical air moves poleward, water vapor releases heat as it condenses into rain.
        2. Stabilizes temperature in aquatic ecosystems (evaporative cooling).
        3. Helps organisms from overheating by evaporative cooling.
    4. Freezing and Expansion of Water
      1. Water is densest at 4 degrees C.
        1. As water cools from 4 degrees C to freezing (0 degrees C), it expands and becomes less dense than liquid water (ice floats).
        2. When water begins to freeze, the molecules do not have enough kinetic energy to break hydrogen bonds.
        3. As the crystalline lattice forms, each water molecule forms a maximum of 4 hydrogen bonds, which keeps water molecules further apart than they would be in the liquid state.
      2. Expansion of water contributes to the fitness of the environment for life:
        1. Prevents deep bodies of water from freezing solid from the bottom up.
        2. Since ice is less dense, it forms on the surface first. As water freezes it releases heat to the water below and insulates it.
        3. Makes the transitions between seasons less abrupt. As water freezes, hydrogen bonds form releasing heat. As ice melts, hydrogen bonds break absorbing heat.
    5. Water as a Versatile Solvent
      1. Solution = A liquid that is a homogenous mixture of two or more substances.
      2. Solvent = Dissolving agent of a solution.
      3. Solute = Substance dissolved in a solution.
      4. Aqueous solution = Solution in which water is the solvent.
      5. Water is a versatile solvent owing to the polarity of the water molecule.
        1. Ionic compounds dissolve in water.
          1. Charged regions of polar water molecules have an electrical attraction to charged ions.
          2. Water surrounds individual ions, separating and shielding them from one another.
        2. Polar compounds in general, are water-soluble.
          1. Charged regions of polar water molecules have an affinity for opposite charged regions of other polar molecules.
        3. Nonpolar compounds (which have symmetric distribution in charge) are NOT water-soluble.