Use a fume hood if available.
    The gas-generation and gas-washing steps should be carried out inside a working fume hood.

Reducing syringe pressure to prevent unintentional gas discharges.
The gases inside the syringe are possibly at a pressure slightly higher than the external room pressure.  If the syringe cap is removed under these circumstances, an unintentional discharge of the gas in the syringe will occur.  To prevent this, pull the plunger back by 5-mL or so before removing the syringe cap.  Then some air will rush into the syringe when the cap is removed rather than discharging some gas into the room.  The presence of small amounts of air does not affect the experiments described in this chapter.

Toxicity.
    Chlorine has an irritating odor and is a poisonous gas.  Concentrations of 4 ppm can be detected and 30 ppm will induce coughing.  To put this in perspective, if 30-mL of Cl2 were discharged into a volume of 1 m3, the concentration of Cl2 would be 30 ppm.  Exercise caution when working with poisonous gases and vacate areas that are contaminated with unintentional discharges of gas.

Suitability.
    All of these experiments are suited for use as classroom demonstrations.  These experiments are not advised for use as laboratory experiments conducted by high school students due to the toxicity of Cl2.

Syringe Lubrication.
    We recommend lubricating the black rubber diaphragm of the plunger with silicone spray (available from hardware stores) or medium-grade silicone oil (Educational Innovations, $5.95 Part #GAS-150; Fisher Catalog Number S159-500; $34/500 mL.)

Equipment. (This equipment can be ordered from a variety of vendors including Educational Innovations, Flinn Scientific (US sales only), Micro Mole, and Fisher Scientific.  Part numbers and links to their websites are provided.)

• 60-mL plastic syringes with a LuerLOK fitting
• Latex LuerLOK syringe cap fittings
• Plastic vial caps that fit within the barrel of the syringe
• Small plastic weighing boat
• Clear plastic beverage cup, 9 oz (260 mL)

4 g NaOH

1.0 mL 6 M HCl(aq)

3.0 mL household bleach (relatively new)

Preparation of Neutralization Solution.
    Prepare 100 mL of 1 M NaOH (4 g NaOH in H2O to make 100 mL) in a 250 mL flask.  Keep the flask stoppered when not in use.  Label the flask ‘1 M NaOH for neutralization.’  This solution will be used to neutralized excess excess reagents in the experiments.
 

Preparation of Chlorine.
    The Cl2 gas samples used in these experiments are generated by Method A.  If 1.0-mL 6 M HCl(aq) and 3.0 mL household bleach are used, about 55 mL Cl2(g) will be generated.  Unlike the reagents used to generate the other gases in this series, the reagent in the vial cap is (a) a liquid and (b) the reagent in excess.  This assures that the reaction product will be acidic.  If more than 3 mL bleach is used, the volume of gaseous Cl2(g) generated will decrease!  The Cl2(g) will effervesce from the solution.  If the plunger does not move easily in the barrel, gently pull the plunger outward every 10 seconds or so in order to accommodate the gas produced.  Stop the gas generation after the syringe is full by removing the latex syringe cap while it is directed upwards.  Rotate the syringe 180^o in order to discharge the reaction mixture into the Neutralization Tank and then recap the syringe.  Fit the latex syringe cap over the LuerLOK fitting.
 

Washing the gases.
    It is necessary to wash the Cl2-filled syringes for some of these experiments.  The gas-filled syringe is "washed" in order to remove traces of unwanted chemicals from the inside surfaces of the syringe before the gases can be used in experiments.  To do this, suction 5 mL distilled water into the syringe without discharging any gas, cap the syringe and gently shake the water to dissolve the contaminants on the inside of the syringe.  Remove the cap and discharge the water into the Neutralization Tank but not any of the gas.  Chlorine is partially soluble in water, especially at high pH values.  Some discoloration of the Cl2(g) present in the syringe should be expected from washing.

Disposal.
    Unwanted samples of Cl2(g) can be discarded in a Neutralization Tank.

Universal Indicator/pH 8 Solution.
    Experiments 2 and 4 require a slightly basic universal indicator solution.  Prepare a solution by mixing 50 mL distilled water plus 5 mL universal indicator solution.  Raise the pH to 8 by bubbling through the solution a pipetful of gaseous ammonia taken from the vapors above a solution of concentrated ammonium hydroxide solution.

Experiment 1. Chlorine Reacts with Sodium Hydroxide Forming Aqueous Bleach.
Equipment:

• weighing boat or small beaker

• Cl2(g), 60-mL
• 5 mL 3 M NaOH

Experiment 2. Chlorine Disproportionates in Water to Form Acidic Species.
Equipment:

• pH meter
• plastic cup or small beaker, 250-mL (9 ounce)

• Cl2(g), 10-mL

Experiment 3. Chlorine Discolors the Natural Colors of Fruit Juices.
Equipment:

• weighing boats

• Cl2(g), 60-mL, several syringefuls
• 5-mL samples of various fruit juices such as cranberry juice, cherry drink, grape juice and tomato juice (ask students to bring samples)

Obtain a number of samples of fruit juices such as cranberry juice, cherry drink, grape juice and tomato juice.  Generate one syringeful of gas for each fruit drink to be tested.  Wash the Cl2(g) samples.  Suction about 5 mL of a fruit juice sample into a Cl2-filled syringe and shake.  The colors will change immediately or over a short period of time.  Cranberry juice turns yellow-orange; cherry drink turns yellow; grape juice becomes bleached and tomato juice goes from the familiar red to pale yellow.  Some food coloring dyes also react with Cl2(g).

Fruit juices: Middle row: normal; bottom row: after exposure to Cl2. (Top row empty.)

Experiment 4. Testing Colorfast Fabrics.
Equipment:

• pieces of fabric  or
• pieces of colored paper

• Cl2(g), 10-mL

Experiment 5. Activity Series.
Equipment:

• medium test tubes, two, with suitable corks or stoppers

• Cl2(g), 60-mL
• few crystals of NaBr or KBr
• few crystals of NaI or KI

Cl2(g) + 2 e^- 2 Cl^-(aq)   Eo = +1.36 V
Br2(g) + 2 e^- 2 Br^-(aq)   Eo = +1.07 V
I2(g) + 2 e^- 2 I^-(aq)   Eo = +0.54 V

    Repeat the above experiments with aqueous iodide solution.  In this case, the solution initially turns dark brown and then over the course of one minute turns pale yellow with chunks of dark I2(s) crystals.
 
 

Experiment 6. Chlorine and Sodium Form Sodium Chloride.
Equipment:

• medium test tube with suitable cork or stopper
• 15-cm length of latex tubing
• glass Pasteur pipet
• Bunsen burner
• Ring stand and clamp
• Matches or lighter

• Cl2(g), 60-mL
• Sodium, Na, 1/4 pea-sized

Use the syringe equipped with the latex tubing/glass pipet attachment to direct the Cl2(g) gas in 5 mL increments over the molten sodium from a distance of about 1 cm as shown in Figure 1.  A bright, vigorous and short-lived reaction will result.  The syringe helps control the direction and delivery of the chlorine gas.  The process can be repeated in 5 mL increments of Cl2(g) until all of the gas has been used.  The tip-up and vertical position of the syringe as shown in Figure 1 prevents residual liquids from being discharged.  The reaction is:
 
 

Figure 1

Molten sodium burns brightly under a jet of chlorine gas.

Clean-up.
    Allow the reaction to cool.  The reaction mixture may contain unreacted sodium metal.  To destroy this, half-fill the test tube or beaker with propanol.  After the bubbling stops (about 15 minutes), wash the solution down the drain with plenty of water.
 
 
 

Experiment 7. Hydrogen/Chlorine Rockets.
Equipment:

• medium test tubes, two, with suitable corks or stoppers
• piezoelectric rocket igniter device (See: Instructions for the construction of a piezo sparker)
• Beral pipet, large stem, large body

• Cl2(g), 40-mL
• H2(g), 40-mL (See: Instructions for the preparation of hydrogen)
• Universal Indicator/pH 8 Solution.

Fig. 2a Fig. 2b Fig. 2c

Launching in the dark displays the bright light as well as the loud bang of the reaction.

   Fill the pipet rockets completely with water and store them open-end down in test tubes filled with water (Figure 2a).  (See: General information on successfully filling and launching rockets.)  Remove the cap from the syringe and slip the water-filled pipet rocket over the LuerLOK fitting as shown in Figure 2b.  Bubble the H2(g) into the rocket until it is half-filled.  Fill the rest of the pipet with Cl2(g), leaving some water in the stem.  Store rockets in the water-filled test tube until ready for use.

    Position the rocket over the wire leads of the igniter.  Water must remain in the stem because this serves as the propellant, however the ends of the wire leads must be above the water in the gas-filled region of the rocket.  If the water leaks out of the stem while positioning the rocket over the wires, immediately fill the stem again by holding the wires plus rocket assembly in a cup of water and suctioning a very small amount of water into the stem.  Trigger the igniter and the rocket will fly 5 m or more.  A small fireball is noticed in a darkened room as the gas mixture ignites. The reaction is:

Immediately retrieve the rocket and suction the slightly basic indicator solution into the rocket.  The indicator will turn its acid color confirming the presence of HCl(g).

     It is also possible to photochemically initiate the H2/Cl2 mixture with the flash from a camera, although it works best if the uv-filter is first removed.  To do this, use a water-filled test tube as a launcher as shown in Figure 2c.

Note: NEVER scale the H2/Cl2 reaction up or mix large quantities of these gases!  Serious explosions can occur which can be photochemically triggered by room light or sunlight.
 
 

Experiment 8. Chemiluminescence and Singlet Oxygen.
Equipment:

• large test tube
• 15 cm length of latex tubing
• sintered glass gas dispersion tube or an 'air-stone' from an aquarium supply store (or simply a glass pipet)

• Cl2(g), 60-mL
• 10 mL of 30% hydrogen peroxide
• 10 mL 6 M NaOH(aq)

    Equip a 15 cm length of latex tubing with a sintered glass gas dispersion tube or an 'air-stone' from an aquarium supply store.  Clamp a large test tube (15 x 180 mm) in a ring stand and fill with 10 mL of 30% hydrogen peroxide.  Generate a syringeful of Cl2(g) and set it aside.  Add 10 mL 6 M NaOH(aq) to the test tube containing the H2O2(aq).  Hydrogen peroxide may start to decompose to O2(g) before you add the chlorine.  If the reaction starts to proceed at too fast of a rate, it can be quenched by adding water from a squeeze bottle.  Remove the latex cap from the syringe and equip the syringe with the latex tubing/gas dispersion apparatus as shown in Figure 3.

A completely darkened room is required for best results.

Figure 3

    To perform the demonstration, carefully bubble chlorine gas slowly through the solution.  With the lights off (completely darkened room), you can observe the spectacular red chemiluminescent glow near the gas dispersion tube.  The reactions are:

Experiment 9. Chlorine Reacts with Aqueous Sodium Sulfite.
 Equipment:

• small beaker

• Cl2(g), 60-mL
• 60 mL 1 M Na2SO3(aq)

     Prepare 60 mL 1 M Na2SO3(aq) and pour it into a small beaker.  Generate a syringeful of chlorine according to the procedure.  It is unnecessary to wash the gas.  Remove the latex cap from the syringe and suction 5 mL of the 1 M Na2SO3(aq) into the syringe.  Keep the syringe positioned in the beaker of sulfite solution while gently rocking/swirling the syringe around to mix the solution with the gas.  The level of solution will rise in the syringe as the gas reacts with the sulfite according to the equation:

An analogous reaction can be performed with bisulfite in place of sulfite, however the products are acidic enough to form SO2(g) odors from unreacted sulfite.
 
 

Experiment 10. Liquid and Solid Chlorine. Equipment: cotton swab Chemicals: Cl2(g), 60-mL liquid nitrogen       This experiment requires liquid nitrogen.  Place a Q-tip cotton swab into liquid nitrogen and then immediately place the swab against the side of a Cl2(g)-filled syringe near the end as shown in Figure 4.  It will be necessary to repeat the process several times.  Soon chlorine will condense to a liquid near the point of contact and will stream down the side of the syringe barrel and vaporize.  With continued application of liquid nitrogen to the same spot, solid chlorine will be formed and the plunger will be drawn inward.  Avoid condensing so much of the chlorine that the plunger moves past the frozen spot.

Figure 4

 

Cross-reference. Chlorine and Acetylene Make Spectacular Underwater Fireworks!      This experiment is described in the  Ethyne Chapter.  In that experiment, acetylene and chlorine were simultaneously bubbled through water.  As they combined, they produced underwater bursts of light.  The reaction is: C2H2(g) + Cl2(g)  2 C(s) + HCl(aq)

  At the end of the experiments, wipe excess lubricant off of rubber diaphragm. Clean all syringe parts (including the diaphragm), caps and tubing with soap and water.  Use plenty of soap to remove oil from the rubber seal.  This extends the life of the plunger.  It may be necessary to use a 3-cm diameter brush to clean the inside of the barrel.  Rinse all parts with distilled water.  Be careful with the small parts because they can easily be lost down the drain. Important: Store plunger out of barrel.