Microscale Gas Chemistry:

Experiments with Hydrogen Chloride

 Link to HCl data page including physical properties.

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.

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

    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.)

  • 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
  •     This quantity of reagents will produce approximately 60 mL of HCl.  The production of HCl is relatively fast and it typically takes 15 seconds to fill a syringe.  Upon heating this mixture, HCl(g) is produced according to the reaction:

    NaHSO4(s) + NaCl(s)  HCl(g) + Na2SO4(s)
    NaHSO4.H2O(s) + NaCl(s)  HCl(g) + Na2SO4(s) + H2O(l)

    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


    Figure 2.

    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.

        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.

    Experiments with Hydrogen Chloride

    Experiment 1. HCl(g) Fountain.

    Chemicals:     Pour 100 mL of the Universal Indicator/pH 8 Solution into a 250 mL beaker or plastic cup.  Generate a syringeful of HCl(g) as described above.

        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. 
    • 100-mL graduated cylinder
    • 15 cm length of latex tubing
    • 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?

    Chemicals: Preparing saturated NaCl(aq)
        To prepare NaCl(sat'd), heat 40 g NaCl in 100 mL H2O until it comes to a boil.  Not all will dissolve.  Allow to cool to room temperature (takes > 1 hr).  Transfer 75 mL NaCl(sat'd) to a 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:

    NaCl(s)  Na+(aq) + Cl-(aq)       Ksp = 37

    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).
    • 250 mL beaker or plastic cup

    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.

    Chemicals:     The following reactions are conveniently performed in a 12-or 24-well plate.  Prepare the following reagents in separate wells before generating HCl(g).
    0.05 g AgNO3(aq) + 
    5 mL H2O
    Forms white precipitate:

    Ag+(aq) + Cl-(aq) ---> AgCl(s)


    0.5 g Pb(NO3)2
    5 mL H2
    Forms white precipitate: 

    Pb+2(aq) + 2 Cl-(aq) ---> PbCl2(s)


    4 or 5 solid magnesium turnings plus 5 mL water
    Forms bubbles of H2(g)

    Mg(s) + 2 H+(aq) ---> Mg+2(aq) + H2(g)


    0.25 g NaHCO3 +
    5 mL H2O
    Forms bubbles of CO2(g):

    HCO3-(aq) + H+(aq) ---> CO2(g) + H2O


    2 mL milk + 3 mL distilled water
    Milk curdles into a solid mass. 


        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.

    Chemicals:     One of the principle constituents of office paper is calcium carbonate (up to 30% for high-opacity paper).  Cut two 3 x 3 cm pieces of office paper that will lie flat on the bottom of a beaker or plastic cup. Round off the corners, if necessary.  One piece will serve as a control.  Place papers in separate beakers or plastic cup.  Cover both with 10 mL H2O.  Prepare a syringeful of HCl(g) as described above.  Using a 3-cm length of latex tubing, discharge the HCl(g) in 5-mL increments under the surface of the water in one of the beakers and in the vicinity of the paper.  Bubbles of CO2(g) will appear on the surface of the paper.  The reaction is:

    CaCO3(s) + 2 HCl(aq)  CaCl2(aq) + CO2(g) + H2O(l)

    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.
    • Additional clean syringe
    • 3-cm length of latex tubing
    • 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:

    CaCO3(s) + 2 HCl(g)  CaCl2(s, aq) + CO2(g) + H2O(l)

    Experiment 8. Grahams Law of Diffusion.

    Chemicals:     The demonstration of Graham's law is commonly done by using a long glass tube in which a cotton ball soaked with strong ammonia solution is placed at one end of the glass tube and simultaneously a second cotton ball soaked with concentrated HCl(aq) is placed at the other end of the glass tube.  Within minutes diffusion of the vapors produce white ammonium chloride crystals in the tube at approximately the place predicted by Graham's law calculations.  A significant improvement of this experiment is possible by using syringes containing NH3(g) and HCl(g) instead of cotton balls.  Hook the syringes to each end of the glass tube by running a plastic or glass tube through a rubber stopper on each end of the glass tube and attach the syringes to them.  Simultaneously inject the NH3(g) and HCl(g) into the tube.

    Clean-up and Storage.

       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.

    This article first appeared in Chem13 News in April, 1998.  The authors of the original Chem13 article are: 

    From the Department of Chemistry, Creighton University, Omaha, Nebraska 68178 USA:

    • Bruce Mattson, faculty member, principal investigator
    • Rebecca Catahan, now graduated from Creighton, (May, 2000); currently in PhD program in chemistry at University of North Carolina
    Rimantas Vaitkus, Department of Chemistry, Vilnius Pedagogical University, 2034 Vilnius, Lithuania

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    (This page last updated 29 January 2002)