Sequence | Background | Objectives | Materials | Activity | Extensions | Evaluations

TOPIC-TITLE Physical Parameters - The Water Molecule AUTHOR Eileen Gray Ann Alexander Tina Darling Nelda Sharkey GRADE SUITABILITY Middle School SCOPE Chemical Oceanography

Sequence Review the previously covered concept of density (the effects and relationships of mass, volume, temperature, and pressure). Introduce water as a molecule that occasionally likes to "bend" these rules and obeys the rules only when you are dealing with differences in pressure. Construct a molecular diagram of a water molecule (containing one atom of oxygen and two atoms of hydrogen). Explain why the molecule is dipolar. The dipolar aspect gives water great dissolving and cleaning powers. Introduce salt and the concept of salinity. < < go top

Background Summary Purpose: To demonstrate the various chemical properties of water and how these properties influence the oceans of the Earth. Water is one of the most unusual compounds on Earth. It has a variety of properties not found in any other liquid. A water molecule is formed from two atoms of hydrogen and one atom of oxygen. An oxygen atom has eight protons and eight neutrons in its nucleus and eight electrons surrounding the nucleus. Hydrogen has one proton and no neutrons in its nucleus and one electron surrounding the nucleus. The oxygen atom has more electrons than both hydrogen atoms and, therefore, is more negative than the hydrogen ion. The hydrogen portion of the water molecule is more positive than the oxygen portion. These opposite charges on each portion (side) of the water molecule result in an attraction that forms a weak hydrogen bond between the oxygen atom and each hydrogen atom (Greene, 1998). Because of the two opposite charges of the water molecule, it is known as a dipolar molecule and acts as a magnet. The negative portion attracts positively charged sodium ions, while the positive end attracts negatively charged chloride ions. These sodium and chloride ions separate and are surrounded by water molecules, which is why salt is dissolved easily in water (Greene, 1998). < < go top Objectives Students will be able to do the following: Recognize how the temperature of water affects density. Define salinity, explain how it is determined, and describe some conditions that cause it to vary. Explain where and how salinity influences the density of oceans. Recognize the manner in which the properties of temperature, density, and salinity interact to create currents. < < go top Materials 500 ml graduated cylinder (one/group) Karo® syrup Oil Candel Cotton ball Freshwater Ice cubes Heat lamp Journal notebook (one/student) Glycerine Colored water Rock (1/group) Cork Penny Saltwater 100 ml beakers (2/group) Plastic wine glass < < go top Activity Density Cocktail: Divide students into cooperative learning groups of three or four. Provide each group with a graduated cylinder and 2 liters of saltwater (salinity 30-35 ppt). Give each group the following items -- glycerine, Karo® syrup, colored salt water (salinity 40 ppt), oil, a rock, one candle, one cork, one cotton ball, and one penny. Have the students test to see which of these items float and which sink. Students should record observations after each item has been added. The students will need a different volume of saltwater after testing the glycerine, oil, Karo® syrup, and colored water. Fill the graduated cylinder with 300 ml of saltwater each time. Salinity Lab: Place students into groups of three or four. Provide each group with two beakers. Have each group weigh beaker one and two and record weights. Place 50 ml of freshwater into beaker one. Place 50 ml of saltwater (35 ppt) into beaker two; weigh each beaker again and record weights. Place each beaker under a heat lamp overnight until all the water has evaporated. The following day weigh each beaker and record results. There should be little to no difference in weight for beaker one; determine the weight fraction of salt (salinity) of seawater based on weight differences in beaker two. Temperature and Density: Provide each group with a plastic wine glass. Have students place as many ice cubes as possible in a glass full of water until the water level is at the edge of the glass (water looks as if it is about to overflow). Have students hypothesize whether or not the water will overflow if the ice melts completely (note: it should not). Explain that when water freezes, a lattice is formed and the molecules are not as close together as in fluid water; therefore, ice is less dense than water and floats. < < go top Possible Extension Create water color paintings. Show designs created by applying different volumes of water and salt to the water color paint to the class. Determine how varying densities (specifically salinity) affect individual cells and entire organisms. Discovery-Method Lab -- Experimenting to determine the best salinity to grow brine-shrimp (sea monkey) eggs. Complete a creative writing assignment: "You are a salesman trying to sell water to a planet where the inhabitants have never seen water but could benefit greatly from it." < < go top Teacher Evaluation Administer and grade a test on the chemical properties of water. Explain how ice floats. If one swimming pool is filled with freshwater, and another pool is filled with saltwater, in which pool would you be the "better" swimmer? Explain. Student Performance: Observe student performance in the laboratory setting and review science journals. Observe student responses during class discussions. Test students on concepts. < < go top