General Safety Precautions.
    Always wear safety glasses.  Gases in syringes may be under pressure and could spray liquid chemicals.  Follow the instructions and only use the quantities suggested.

Toxicity.
    HCl(g) has an irritating and unpleasant odor and is toxic. Inhalation of the gas will cause coughing.

Suitability.
    All of these experiments are suited for use as classroom demonstrations.  These experiments are not advised for use as laboratory experiments conducted by typical high school students.  Advanced students or students with special laboratory skills could be allowed to generate HCl(g) by the large test tube method (Variant II).  In any case, all gas generation work should be performed under close supervision by the instructor.

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.)

• several 60-mL plastic syringes with a LuerLOK fitting
• Latex LuerLOK syringe cap fittings
• Small plastic weighing boats
• balance capable of measuring to 0.01 g
• two pieces, latex tubing, 1/8-inch (3.175 mm) ID, 5 cm lengths
• two 18 x 150 mm test tubes
• two-hole #1 stopper fitted with two short lengths (2 cm) of glass tubing
• pinch clamp or hemostat
• ring stand and three suitable clamps to hold test tube and syringes
• small Bunsen burner
• matches or a lighter
• ‘permanent’ marker pen

1.1 g anhydrous sodium hydrogen sulfate (sodium bisulfate) NaHSO4 (or 1.3 g sodium hydrogen sulfate (sodium bisulfate) monohydrate NaHSO4^.H2O)  Note: NaHSO4 gives better results but NaHSO4^.H2O gives adequate results and may be used if the anhydrate is not available

0.7 g sodium chloride, NaCl

Preparation of Hydrogen Chloride in the Microwave Oven.
     Samples of HCl(g) also can be prepared conveniently in a microwave oven.  (See Generating Gases in a Microwave Oven.)
 

Preparation of Solid Reagent Mixture.
     The solid reagent mixture is a 2:1 mass ratio of sodium hydrogen sulfate monohydrate, NaHSO4^.H2O, and sodium chloride, NaCl.  It is convenient to prepare a large quantity of the solid reagent mixture (20 g NaHSO4^.H2O + 10 g NaCl) in order to perform several gas preparations.  The mixture should be pulverized together with the aid of a mortar and pestle.  (During the pulverization, you may notice the faint smell of HCl(g).  This is normal.)  The mixture should be stored in a tightly sealed glass or plastic bottle.  We have also obtained very nice results with a 2:1 mixture of anhydrous potassium hydrogen sulfate and sodium chloride.
 
 

 Preparation of Hydrogen Chloride.
 

The gaseous hydrogen chloride (HCl) samples used in these experiments are generated by the thermal method.  The assembled apparatus is shown in Figure 1.  Every component of this apparatus must be dry!  Syringe plungers should move easily in the barrels.  This can be facilitated by applying a drop or two of silicone oil or spray to the groove in the plunger's rubber seal.  A small burner is also needed.  The left syringe, labeled 'HCl' is used to collect relatively pure HCl(g) and the syringe labeled 'Waste' is used to collect impure samples of gas and unwanted air.  A pinch clamp or hemostat is used to pinch closed one of the latex tubes.  Start by pinching closed the syringe labeled 'HCl'.  Place a boiling chip in the test tube.  Add the 2-g of reagent mixture to the test tube. Insert the stopper firmly in order to form an air-tight seal.  Caution: Do not crimp the latex tubing!        Hydrogen chloride is generated by the following 3-step maneuver described in Chapter 1 and summarized in Figure 2.

Figure 1

 
 
 
 
 
 

• Step 1. Gently heat the mixture in the test tube with the cool part of the flame.  Within a few seconds the solid mixture will begin to bubble and melt.  If NaHSO4.H2O is used, some water vapor will condense along the upper part of the test tube walls and turbulent clouds of HCl aerosol will be noticed in the upper part of the test tube. Continue to heat while gently assisting the plunger's movement.  Within a minute the volume of gas in the syringe will be 25 - 30 mL.  This gas is mostly air originally present in the test tube.
• Step 2. After 25 - 30 mL of air/HCl has been collected in the Waste syringe, switch the location of the pinch clamp to the other latex tube so that relatively pure HCl(g) can be accumulated in the syringe labeled 'HCl'.  Continue to collect HCl(g) until at least 50 - 55 mL has been collected in the HCl-syringe.
• Step 3.  Switch the pinch clamp back to the tubing connected to the HCl-syringe and remove the heat source.

Washing the gases.
    Hydrogen chloride is extremely soluble in water and cannot be washed.  However, it is unnecessary to wash gases collected by the thermal method because the gas sample is collected in a clean, dry syringe.

Disposal.
    Unwanted samples of HCl(g) can be discarded by dissolving in water.  The solid remaining in the test tube is Na2SO4 can be dissolved in water and discarded down the drain.  The test tubes can be reused unless they have been damaged.
 

Universal Indicator/pH 8 Solution.
    Several of the experiments require a slightly basic universal indicator solution.  Prepare a solution by mixing 200 mL distilled water plus 20 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. HCl(g) Fountain.
Equipment:

• 250 mL beaker or 9-ounce plastic cup

• HCl(g), 60-mL
• Universal Indicator/pH 8 Solution, 100-mL

    Remove the syringe cap from the HCl(g)-filled syringe and place it in the cup of water.  Keep the syringe’s LuerLOK fitting under the surface of the water.  Because HCl(g) is so soluble in water, water is rapidly suctioned into the syringe producing a small fountain.  Be sure to hold the plunger in place to prevent it from being pulled into the syringe.  There is a considerable force pulling the plunger inward!
 
 
 

Experiment 2. HCl(g) is an Acid.   Equipment: 100-mL graduated cylinder 15 cm length of latex tubing Chemicals: HCl(g), 40-mL 5 mL 1 M NaOH(aq) universal indicator solution corn syrup, 50 mL  Experiment 2A.      Prepare a very dilute NaOH(aq) solution by adding 1 drop of 1 M NaOH(aq) to 75 mL distilled H2O.  Add several mL of universal indicator solution.  If the pH is <7, add one drop of 1 M NaOH(aq) until the pH is > 7.  Pour the solution into a 100-mL graduated cylinder.  Next, generate HCl(g) as described above.  Remove the syringe cap and attach a 15 cm length of latex tubing to the syringe.  Dispense some of the HCl(g) near the surface of the water (Figure 11.3) and notice the production of an acidic solution at the surface.  You should also notice two other phenomena.  An aerosol cloud of HCl/H2O is formed above the surface.  This is caused by HCl(g) condensing H2O(g) into an aerosol because of the great affinity of HCl for water.  You should also notice that the acidic solution sinks through the column of water and soon the entire contents of the graduated cylinder are the same color (acidic).  The 'sinking' occurs because the HCl(aq) solution that is produced near the surface is more dense than water and sinks.

Figure 3.

 Experiment 2B.
     Repeat Experiment 2A but instead of using 75 mL distilled water, use 75 mL of 50% corn syrup and 50% distilled water.  You may find that it takes 2 or 3 drops of 1 M NaOH(aq) to make the solution pH > 7.  This time the HCl(aq) layer is less dense than the corn syrup solution and layers of various colors are produced along the pH gradient.
 
 

Experiment 3. Acid Snow?
Equipment:

• flashlight
• 250 mL beaker
• hot plate or Bunsen burner (some method for heating 100 mL water to its boiling point in a 250 mL beaker)
• 15 cm length of latex tubing

• HCl(g), 20-mL
• 40 g NaCl
• 100 mL graduated cylinder

    In terms of laboratory technique, this experiment is similar to Experiment 2.  You will need a flashlight.  Generate HCl(g) as described above.  Remove the syringe cap and attach a 15 cm length of latex tubing to the syringe.  Dispense some of the HCl(g) near the surface of the NaCl(sat'd) (Figure 11.3) and notice the production of NaCl(s) at the surface.  The HCl(g) dissolves in the NaCl(sat'd) which causes the Cl-(aq) concentration to exceed that allowed by the solubility of NaCl.  LeChatelier's principle predicts that the equilibrium will shift left and NaCl(s) will form:
 
 

It does and slowly falls through the NaCl(sat'd) solution.  By darkening the room and holding a flashlight up to the graduated cylinder, the beautiful glittering crystals of NaCl can be observed as they fall through the solution giving the appearance of snow falling.
 
 
 

Experiment 4. HCl(g) Reacts with NH3(g). Equipment: 250 mL beaker or plastic cup Chemicals: HCl(g), 60-mL NH3(g) (See: Preparation of NH3(g))

Figure 4.

       Prepare a syringeful of NH3(g) and a syringeful of HCl(g) as described above.  Connect the HCl(g)-filled syringe to the NH3(g)-filled syringe with a 15-cm length of latex tubing as shown in Figure 4.  Slowly push on the plunger of the ammonia syringe to produce plumes of white NH4Cl(s) in the HCl-filled syringe.  Variant:  Tap firmly on the plunger of the NH3-filled syringe so that the plunger moves inward 1 - 2 mL at a time.  This will produce interesting "smoke rings" in the HCl-filled syringe.  Note that the syringe barrel becomes warm from this exothermic reaction.  Also notice that the plungers are being drawn inward as the reaction proceeds.

Experiment 5.  Well-plate Reactions Between HCl(g) and Various Aqueous Solutions.
Equipment:

• 12-or 24-well plate
• 3-cm length of latex tubing

• HCl(g), 60-mL
• 0.05 g AgNO3(aq)
• 0.5 g Pb(NO3)2
• 4 or 5 solid magnesium turnings
• 0.25 g NaHCO3
• 2 mL milk

    Next, prepare a syringeful of HCl(g) as described above. One syringeful of HCl(g) is adequate for all five of these experiments.  Equip the syringe with a 3-cm length of latex tubing.  In the case of Wells 1 - 3, add five 1-mL 'puffs' of HCl(g) just above the surface.  For Well 4, slowly discharge 5 mL HCl(g) just at the surface (latex tubing just touching the surface).  For Well 5, bubble 5 mL HCl(g) just below the surface.
 
 

Experiment 6. HCl(g) Reacts with Office Paper.
Equipment:

• two 250 mL beakers or plastic cups
• 3-cm length of latex tubing

• HCl(g), 60-mL
• Office paper

After 15 minutes, remove the office papers from the solutions and allow them to dry on a clean surface or paper towel.  Compare the treated paper with the 'control' sample side-by-side on a dark surface.  You should notice that the treated paper has lost some of its opacity.  It has also lost some of its mass.  Calcium carbonate is added to paper make it whiter and more opaque.
 
 
 
 
 

Experiment 7. HCl(g) Reacts with Antacids. Equipment: Additional clean syringe 3-cm length of latex tubing Chemicals: HCl(g), 60-mL antacids tablet such as Tums

Figure 5.

    Traditional antacids contain either magnesium hydroxide, Mg(OH)2(s) (for magnesia tablets) or calcium carbonate, CaCO3(s) (for example Tums).  The latter have gained popularity in recent years because they are a good source of dietary calcium.  In this experiment tablets of CaCO3(s)-style antacids will be reacted with HCl(g).  Place a few drops of water on an antacid tablet and allow the drop to soak into the surface for a few minutes.  Next place the tablet into a 60-mL syringe.  Install the plunger most of the way as shown in the left part of Figure 5.  Connect a 15-cm length of latex tubing to the syringe.  Generate HCl(g) as described above.  Transfer the HCl(g) to the syringe containing the antacid tablet by pushing on the plunger of the HCl(g)-filled syringe while gently pulling on the plunger of the syringe with the tablet.  The drop of water on the tablet greatly enhances the rate of the reaction due to the water solubility of HCl(g).  Upon contact, effervescent bubbles of CO2(g) will be observed where the tablet was moistened:

Experiment 8. Grahams Law of Diffusion.
Equipment:

• long glass tube (typically 1 m x 3 cm diameter)
• two one-hole rubber stoppers, suitable for use with the glass tube
• two short length of class or plastic tubing that will fit inside the hole in the rubber stopper
• two 3-cm lengths of latex tubing

• HCl(g), 60-mL
• NH3(g) (See: Preparation of NH3(g))

   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.