The Nature of Water


Discover the properties of water, H2O, which make this molecule and compound so important to life on earth. Click here for an outline of Campbell's Chapter on Water.


Review this topic by answering some multiple choice questions

  1. Ubiquity - Water is plentiful - about 75% of earth is covered with it. Water makes up any where from 70 to 90+% of the body weight of living things. At most temperatures on the surface of the earth water is a liquid. In this state water is an excellent solvent and because there is so much of it available on the earth's surface water is home (oceans, lakes and rivers) to much of life. The water cycle is one of the most important biogeochemical processes. You may also what to review some general facts about water.

 

  1. Structure of water. H2O as a liquid
    1. Water is a polar molecule and can bond both to itself and to other water molecules by weak attractions called hydrogen bonds. Each water molecule can bond with as many as 4 others (See Figure 1).
Figure 1

Note that only three of the possible four hydrogen bonds are shown. 

 

  1. hydrogen bonds make water an excellent solvent. The hydration shells of water molecules which form around both positive and negative ions as they dissolve, keep these ions in solution by eliminating their ionic attraction. See Figure 2 below.

 

Figure 2

  1. hydrogen bonding is responsible for the unusual thermal properties of water including:

    Water's high specific heat capacity. Specific heat is defined as the amount of heat energy needed to raise the temperature of one gram of a substance 1°C. Since it takes much more energy that normal to break all the hydrogen bonds in liquid water, water resists rapid temperature fluctuations, adding stability to earth's environments where liquid water is plentiful.

    Water has a very high heat of vaporization. The heat of vaporization is defined as the energy needed to change the phase of a liquid to a gas. Again, because of the number and relative strength of water's hydrogen bonds, it takes a great deal of energy to break a molecule free of its liquid partners. Heat of vaporization causes a cooling effect because as the warmer molecules evaporate from your skin they take the heat energy with them, leaving you cooler.

    Water also has a high heat of fusion. Look this one up if you are interested.

 

  1. Capillary action involves two properties of water, cohesion and adhesion.

    In cohesion water's hydrogen bonds make liquid water self-sticky. This stickiness makes water bead up more on a surface than other substances. See Figure 3.

Figure 3
Water is also highly adhesive. This property of water gives it the ability to literally climb the wall of any container it is in. The top of the water column assumes a u-shape called a meniscus. See Figure 4.
Figure 4
When the container happens to be the woody walls of xylem in a plant, both adhesion and cohesion of water molecules produce a force called capillary action. As water evaporates (Transpiration) from the air sacs within the spongy layer of a plant leaf, the meniscuses in these air spaces become more concave increasing the tension on the water columns in the xylem. Along with capillary action this force, described below, helps move water (against the force of gravity) from the root up to the leaves of a mighty tree. (See TACT forces in Wallace)

 

  1. Surface Tension, the force produced by the difference in hydrogen bonding at water's surface verses its interior, is able to create the illusion that a body of water has a skin. Insects are light enough that they can literally walk on water. Without the natural surfactant (soapy material) produced in our lungs water's high surface tension could actually collapse them, cutting off our air supply. Learn more about complications resulting from the lack of surfactants in premature babies.
    Quickly find the reference to surfactant by typing the word into the find field (under the Edit menu) of your Browser.

 

  1. Structure of Ice. Ice (solid water) has a regular bonding arrangement between the molecules of water which actually increases the distance between molecules in certain directions. The result is that ice is not as dense as liquid water at 4°C. Therefore ice FLOATS. This is beneficial to bottom dwellers in lakes, rivers and oceans. You figure out why.


Multiple Choice Questions to help you learn the vital properties of water.


Links to Other lectures about water:


Modified Nov 2, 2013