Antibiotics in Action

    Demonstration
    Molecules in Motion
    Seven Demonstrations

    Molecular Motion in Gases
    1. Odor
    2. HCl-Ammonia
    Molecular Motion in Liquids

    1. Food Coloring in Water
    2. Mercury Tube Demonstration
    3. Slow Copper Diffusion
      Molecular Motion in Solids

      1. Salol Crystals
      2. Molecular Motion Simulators

      General Safety Guidelines
      Relevant National Science Education
      Standards
      Relevant New Jersey State Science
      Education Standards

    The motion of molecules is central to understanding chemical changes. Chemists believe that all molecules are in motion in a constant and random way. This motion is part of “the way things are.” In addition, in living cells, chemicals move in and out of the cells continuously. This passive (nonchemical) method of transporting substances through the cell wall depends on the motion of molecules. In the activity Cell Membranes you examine passive transport. The demonstrations below illustrate the underlying concept of molecular motion. Since the molecules in a gas are free to move over distances relatively larger than those in solids or liquids, it is somewhat easier to observe the result of the motion. However, we have included demonstrations of molecular motion in gas, liquid, and solid state matter.

    Molecular Motion in Gases

    1. Odor

    At some distance from students in the classroom, open a bottle containing a substance with an odor. Ammonia works well, provided you have checked first that no student in your class has ammonia allergies or chronic lung conditions. Ask students to signal when they detect the odor. You can continue with another part of the lesson as the odor diffuses throughout the room. It is not necessary that all students detect the odor. Leaving the bottle open long enough for all students in the room to detect the odor may produce concentrations of the gas that are unpleasant or harmful to students nearer the source. The focus of the discussion should be on the question, “How does the odor get from the bottle to your noses?” The fact is that substances with odor give off a gas, the particles of which are in constant random motion, according to the kinetic-molecular theory.

    2. HCl-Ammonia

    In advance of the demonstration, obtain two watch glasses or petri dishes (or other similar shallow glass containers. You will need concentrated HCl and concentrated ammonia solutions. Prior to observing the demonstration, students need to know that when NH3 and HCl react chemically, a white solid, NH4Cl, is produced. The reaction is a simple combination reaction, and the equation

    Neutralization of hydrochloric acid and ammonia

    is easily understood by students. Prepare for the demonstration in view of the class. Be sure to wear safety glasses as you perform the demonstration.

    1. Place a small volume of the HCl solution in one watch glass.

    2. Keeping the second watch glass at least 1 meter away from the HCl, place a small volume of concentrated ammonia solution in that watch glass.

    3. Slowly move one of the watch glasses along the table top until the finely divided ammonium chloride begins to form in the space between the two watch glasses.

    4. The demonstration works best if there are no ambient air currents in the room from open windows or heat vents, etc. The question to be discussed is, “Where did the ammonium chloride form and why did it form in between the two watch glasses?” Students easily see that in order for the white solid to form, molecules of both reactants must have diffused from their respective watch glasses into the surrounding air space where they collided and reacted chemically.

    A variation of this demonstration can be done as a laboratory investigation by students. It appears in many standard chemistry lab manuals as “Graham's Law.” Omitting the calculation steps in this procedure gives students a chance to see the diffusion of two gases in a narrow chamber. Briefly, the procedure requires a buret tube (open at both ends), two one-hole rubber stoppers to fit, two cotton swabs, and concentrated solutions of HCl and NH3. The cotton swabs are inserted through the holes of the rubber stoppers. One cotton swab is soaked in HCl and the other cotton swab is soaked in ammonia and the two stoppers are inserted simultaneously in the ends of the buret tube. After several minutes students will observe the formation of ammonium chloride as a “white ring” in the tube. One of the advantages of this procedure is that students observe the NH4Cl form closer to the ammonia end of the tube. This allows some discussion not only of molecular motion, but also of the fact that lighter molecules move more rapidly.

    Molecular Motion in Liquids

    3. Food Coloring in Water

    A simple demonstration of molecular motion in water can be shown by introducing a drop of food coloring into a container of water. Fill a large test tube three-fourths full of water. Draw a small volume of red or blue food coloring into a plastic micropipet. Wipe off any excess food coloring with a paper towel. Lower the tip of the micropipet near the bottom of the test tube containing the water. Squeeze out one drop of food coloring into the water. Allow the test tube to stand. A variation of this can be done using a petri dish filled with water and placed on the stage of an overhead projector. Place a small drop of food coloring in the center of the petri dish and allow students to observe.

    The molecular motion is slower in liquids than in gases. This requires students to observe over a longer period of time. If you use three test tubes of water, each water sample at a different temperature, you can increase student interest considerably and reduce observing time. Fill one test tube with water at 8-10ºC. Fill the second with room temperature water and fill the third with water that has been heated to 50-60ºC. Place the drop of food coloring in the colder water first, then room temperature water, and finally the heated water

    4. Mercury Tube Demonstration

    Before doing this demonstration, check with local policies on using mercury in your classroom, and be sure to wear safety glasses as you perform the demonstration. A small volume of mercury is sealed in a long glass tube from which air has been removed. Resting on the surface of the mercury are low-density glass chips. The chips are typically red or blue in color to improve their visibility. As the mercury is heated, the glass chips begin to move inside the tube in a random way, mirroring the motion of the mercury atoms. The heating boils the mercury and as the mercury atoms move into the space above the liquid, they collide with the glass chips, which then exhibit the random motion. The motion will continue for a brief period even when the heat is removed.

    5. Slow Copper Diffusion

    This demonstration requires several days of observations, but involves some interesting chemistry in addition to showing the movement of molecules (actually, ions in this case). The procedure is as follows:

    1. In a large test tube, add copper (II) sulfate crystals to a depth of 2-3 cm.

    2. Cut 2 circles from filter paper. Each circle should have the same diameter as the test tube. Place one paper circle on top of the CuSO4 crystals.

    3. Add sodium chloride to twice the depth of the copper sulfate.

    4. Place the second paper circle on top of the NaCl.

    5. Add water so it reaches close to the top of the test tube.

    6. Add 2 iron nails to the water.

    7. Observe for several days.

    As the copper ions migrate slowly into the salt layer, the complex ion CuCl4-2forms. This complex ion is green in color. The reaction is

    Reaction of Copper ion with chloride ion

    As the copper ions continue to migrate upward, they come in contact with iron atoms in the nails. A redox reaction occurs and metallic copper appears on the surface of the nails.

    The color change in the salt layer and the appearance of solid copper on the nails are indicators of how the copper ions have moved.

    Molecular Motion in Solids

    6. Salol Crystals

    Place a sample of phenyl salicylate (salol) crystals about the size of a quarter on a watch glass. Very gently warm the crystals until they JUST melt. Place the watch glass on a dark surface and observe the liquid with a hand lens until solid reappears. Students can watch the crystallization process as the molecules of the liquid slow down and reform as solid.

    7. Molecular Motion Simulators

    There are a number of mechanical models that simulate molecular motion for students. Among the most common is a vertical tube made of clear plastic. A piston driven by a small motor is inserted in the bottom of the tube. Small plastic or metal beads are placed in the tube and then a small plastic disk traps the beads. When the motor is turned on, the beads exhibit the random motion in a sample of gas.

    A two-dimensional model also is available for use on the stage of an overhead projection. Beads are placed in a shallow well, the bottom of which is glass. The walls of the well can be vibrated by a small motor. The beads then exhibit the random motion of molecules in a gas.

    Relevant National Science Education Standards

      Unifying Concepts and Processes — The demonstrations involve the use of the kinetic-molecular model of matter to explain the observed phenomena.

      Physical Science — The demonstrations involve systems moving from lesser to greater states of disorder. The molecular nature of matter also is central to the demonstrations.

    Relevant New Jersey State Science Education Standards

      5.1 The demonstration involves understanding the interaction of molecules in a heterogenous systems.
      5.8 The behavior of matter at the molecular level is central to the demonstrations.

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