Microscale Gas Chemistry

Experiments with Carbon Dioxide

Joseph Black CARBON DIOXIDE WAS discovered over 250 years ago by the 24-year old Englishman Joseph Black.  He prepared and characterized samples of CO2(g) which he called fixed air.  He found that the gas could be produced by heating chalk which lost mass during the heating process.  We now know this reaction is:

CaCO3(s) arrow   CaO(s) + CO2(g)
    Carbon dioxide is a colorless gas present in our atmosphere at very low levels.  The level of CO2 have been rising throughout the 20th century which is believed to cause the greenhouse effect by which the earth's atmosphere is slowly warming up.  Carbon dioxide is essentially odorless, however it causes a sharp sensation in one's nose when inhaled — such as from the bubbles of a carbonated beverage.  

    Carbon dioxide has many important uses.  It is used in fire extinguishers, the soft drink industry, and as a chemical reagent to make other compounds.  The major use of carbon dioxide is as a refrigerant (accounting for over 50%).  Dry ice, CO2(s) was first commercially introduced as a refrigerant in 1924.  Dry ice sublimes to a gas at -78.5 ºC at standard pressure.  By the 1960s dry ice was replaced by liquid CO
2 (commonly called liquid carbonic) as the most common CO2-refrigerant.  Carbon dioxide has a melting point of -56.6 ºC at 5.2 atmospheres.  Liquid CO2 is used to freeze materials such hamburger meat and metals and this improves the grindability of the material.  It is also used to rapidly cool loaded trucks and rail cars.  Another 25% of all CO2 produced is used in the soft drink industry.  Carbon dioxide is now widely used as the propellant in aerosol cans.

Carbon dioxide is manufactured by the combustion of hydrocarbons such as natural gas:

CH4(g) + 2 O2(g) arrow CO2(g) + 2 H2O(g)      ΔHºrxn = -803 kJ

The solubility of CO
2(g) in water is 3.48 g per L at 0 ºC and 1.45 g/L at 25 ºC.  This is equivalent to 1.77 mL CO2 per 1 mL water at 0 ºC and 0.74 mL CO2 per 1 mL water at 25 ºC.  When CO2(g) dissolves in water it produces CO2(aq) for the most part.  Solutions of CO2(aq) last longer if they are kept cool.  As the solution of CO2(aq) is warmed, CO2(g) is released as bubbles.

CO2(aq)  equi;ibrium arrow left   CO2(g)

The density of CO
2(g) is over 50% greater than that of air.  At 25 ºC and standard pressure, the density of carbon dioxide is 1.799 g/L, compared with 1.18 g/L for air.  

    The following experiments are included in this chapter. 

Experiment 1. Traditional limewater test for carbon dioxide
Experiment 2. Acidity of carbon dioxide
Experiment 3. Carbon dioxide extinguishes fires
Experiment 4. Carbon dioxide and aqueous sodium hydroxide react
Experiment 5. Equilibrium between carbon dioxide and carbonic acid

    The first three experiments are suitable as laboratory experiments for a wide variety of grade levels from middle school up through university-level.  Experiment 4 requires the use of a caustic solution (NaOH(aq)) and should be used with prudent caution (or as a demo).  It is also an example of a useful technique: Conducting a reaction with a gas inside a syringe.  Experiment 5 makes an excellent classroom demonstration or laboratory experiment.  However it requires drilling holes through the syringe plunger used. 

    For use in high school settings, this experiment can be conducted at about the time that chemical formulas and reactions are being introduced.  As a laboratory activity, these experiments are appropriate when discussing chemical compounds, chemical formulas, chemical reactions, the mole, as well as a variety of topics including physical and chemical changes.  Experiment 5 can be discussed on several levels ranging from a simple discussion of how carbonated beverages contain aqueous carbon dioxide to the details of the carbon dioxide/carbonic equilibrium that can be a challenge even for university students. 

Background skills required
    Students should be able to:

Time required
    Students should be able to perform all of these experiments in a single 45 minute laboratory period.

Preparation of carbon dioxide in a “gas bag
    Large samples of CO2(g) can be prepared conveniently in 1 L food storage bag.

Student Instructions
    For classroom use by teachers, one copy per student in the class may be made free of charge and without further permission.  Student instructions and questions only (without teaching tips, suitability information, etc.) can be downloaded free of charge as a Microsoft Word document from the website. Download now.

    Answers to the questions, lists of materials and chemicals, and additional reference information.
    This page is fairly similar in content to Chapter 2 in our book Microscale Gas Chemistry.   Our 500-page book includes all of the information included at this website and much more!  Answers to all of the questions, chapter-by-chapter lists of the equipment and chemicals needed to conduct the experiments as classroom demonstrations or laboratory activities for the entire class, construction instructions for various pieces of equipment, information for the preparation of stock solutions, ordering information, and a complete index — are all available in the book, but not at the website.  The book,
Microscale Gas Chemistry, can be ordered from Educational Innovations (Part Number BK-590) from  their website.


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.  CAUTION! Hydrogen forms explosive mixtures with air.

    Oxygen is non-toxic in normal quantities.  Pure oxygen can be toxic if inhaled in large quantities as the pure gas, but this is not a concern with these experiments.  Do not intentionally inhale oxygen samples produced in these experiments.

Equipment (Vendors and Part Numbers)
    Microscale Gas Chemistry Kit:

two 60 mL plastic syringes with a LuerLOK fitting
two Latex LuerLOK syringe caps
two plastic vial caps
one 15 cm length of Latex tubing
one 3 cm length of Latex tubing
one small bottle of silicone oil
one plastic pipet
one clear plastic beverage cup (250 mL/9 oz)
one small plastic weighing dish
one small test tube (12 x 100 mm)
one medium test tube (18 x 150 mm)
one birthday candle

Chemicals (needed for each syringe full of hydrogen produced)

    This quantity of sodium bicarbonate will produce approximately 60 mL of CO2(g).  The production of CO2 is relatively fast and it typically takes 15 seconds to fill a syringe.  The reaction is:

NaHCO3(s) + CH3COOH(aq) arrow CO2(g) + H2O(l) + NaC2H3O2(aq)
NaHCO3(s) + HCl(aq) arrow CO2(g) + H2O(l) + NaCl(aq)

The production of CO2 is fairly ripid and it typically takes 15 - 30 seconds to fill a syringe. 

Generating carbon dioxide gas samples

    Samples of carbon dioxide are generated by the In-Syringe Method. and, carbon dioxide was the example used when the In-Syringe method was presented.  A summary of these steps is provided here:

1. Wear your safety glasses!

2. Lubricate the seal.  

    Lubricate the black rubber seal of the plunger with silicone oil. 
3. Measure out 0.22 g sodium bicarbonate, 
    Place the 
NaHCO3(s) directly into the vial cap.

measure out solid reagent      

4. Fill the syringe barrel with water.
    Fill the barrel completely with water.  Place your finger over the hole to form a seal.    

filling syringe with water  

5. Float the vial cap
    Float the vial cap containing the solid reagent on the water surface.   

float the vial cap    

6. Lower the cap by flotation
    Release the seal made by finger to lower the cap into the syringe barrel without spilling its contents.    

lower the vial cap by flotation  

7. Install the plunger
    Install the plunger while maintaining the syringe in a vertical position.  The plunger has a plastic “rib” near the rubber seal that snaps past the “catch” — a small ridge just inside the mouth of the syringe.  Usually it takes a firm push to move the rib past the catch.  After that, the plunger should move smoothly.

insert plunger

8. Draw 5 mL 2 M HCl(aq) into syringe
    Pour the 2 M HCl(aq) into a small weighing dish.  Draw 3 – 5 mL of the solution into the syringe. 

draw up agueous reagent      

9. Install syringe cap
    Push the syringe into the syringe cap. 

push syringe into cap      

10. Generate the gas
    Shake the device up and down in order to mix the reagents.  Gently help the plunger move up the barrel.   

shake reagents to generate gas     

11. Remove cap to stop gas collection
    Remove the syringe cap with the syringe held “cap-up” as shown.  Assume contents are under positive pressure. 

remove the syringe cap

12. Discharge reagents
    Discharge the liquid reagent into the plastic cup.  Immediately cap the syringe to prevent loss of gas.   

drain spent reagents     

Wash away contaminants
    Carbon dioxide-filled syringes must be washed in order to remove traces of unwanted chemicals from the inside surfaces of the syringe before the gases can be used in experiments.  Follow the procedure summarized here.

1. Remove the syringe cap,
2. draw 5 mL water into the syringe,
3. cap the syringe,
4. shake syringe to wash inside surfaces,
5. remove cap,
6. discharge water only, and finally
7. recap the syringe.
8. Repeat?

Repeat these Washing Steps if necessary.
(All traces of the reactants should be washed away.)

Disposal of carbon dioxide samples
    Unwanted carbon dioxide samples can be safely discharged into the room.

Teaching tips

1. The generation of carbon dioxide is a great way to introduce students to the technique. 

2. CAUTION!  Using more than 0.22 g sodium bicarbonate will generate more than 60 mL CO2(g) and syringes left unattended will “pop” their plungers.

3. The amount of reagents recommended will produce more than 60 mL gas if working at high elevations (Mexico City, Denver, Quito, etc.).  Plan to scale back by by some amount in order to prevent producing more than 60 mL gas in a 60 mL syringe!

Introductory Questions (repeated from "In-Syringe Method"
1. Write the formulas for (a) baking soda; (b) vinegar; and (c) carbon dioxide.

2. Gases usually have “(g)” as the last part of their formula.  For example, oxygen gas would be written as O2(g).  In a similar way, solids have “(s)” and liquids have “(l)”.  Aqueous solutions, substances dissolved in water, have “(aq)” as the last part of their formula.  For example, salt water would written as NaCl(aq).  Add these endings to the three formulas in Question 1.

3. Why should the gas be “washed”?

4. Why is it important to use only the specified amounts of reagents?  

5. Why must the syringe be upright when removing the cap?
6. What was the purpose of vigorously shaking the syringe?  

7. Why must one start over if some of the solid spills out while the vial cap of sodium bicarbonate is being lowered?

8. What is the molar mass of carbon dioxide?  
Advanced Questions
9. Write the balanced chemical equation for the reaction occurring in your syringe.

10. Using 0.22 g of NaHCO3 and 5.0 mL of 1.0 M HCl, which reactant is the limiting reactant?

11. Use the ideal gas law and your answer to the previous question to determine the volume of gas is predicted.  Assume 25 ºC and standard pressure.

12. Use the ideal gas law to determine the density of carbon dioxide at 25 
ºC and standard pressure.  

13. Use the ideal gas law to determine the density of air at 25 
ºC and 1 atm pressure.  You can use the “average molar mass of air” in your calculations; its value is 28.964 g/mol.  Compare the density of carbon dioxide (Question 7)  with the density of air.  Calculate the ratio of densities, dcarbon dioxide/dair



transfer gas to test tube Equipment

Microscale Gas Chemistry Kit
match or lighter
wooden splint


CO2(g), 20
limewater, 3 – 5 mL

    middle school lab, high school lab, university lab, and classroom demonstration

Applications, Topics, Purpose
    chemical formulas, chemical properties of gases, types of chemical reactions, precipitation reactions, characterization of gases

    Prepare a syringe full of CO2. Pour 1 – 2 mL of limewater, Ca(OH)
2(aq), into a test tube.  Remove the syringe cap and attach a 15 cm length of tubing to the syringe.  Discharge 10 – 20 mL CO2 over the limewater surface as shown in the figure.  Stopper the test tube with your thumb or finger.  These chemicals are not dangerous if contacted to skin.  Shake the gas and liquid. Notice the production of precipitated CaCO3, which makes the solution cloudy. The reaction is: 

Ca(OH)2(aq) + CO2(g)  arrow CaCO3(s) + H2O(l)

    Save the syringe of unused carbon dioxide for the next experiment.

Teaching tips

1. Limewater is saturated Ca(OH)2.  Limewater should be clear — not cloudy.  See appendix for construction of a limewater dispenser from a plastic beverage bottle.

Introductory Questions

1. What is a precipitate? 

2. What is limewater?

3. What is the formula of carbon dioxide?

4. What do the symbols (aq), (g), (s) and (l) stand for in the equation for the reaction given above?


5. Would the precipitate settle if allowed to stand for a period of time?

6. What does calcium carbonate look like?

7. What is the formula of the carbonate ion?

8. What makes this a good test for carbon dioxide?

9. What is the traditional test for the carbonate ion?

Advanced Questions

10. Carbon dioxide is a covalent molecular compound.  What class of compound is calcium carbonate?

11. Write the chemical reaction that took place in the form of a sentence: “Aqueous calcium hydroxide and ….”



Microscale Gas Chemistry Kit
100 mL graduate cylinder
plastic square, 5 cm x 5 cm (for Part 2; cut from a sandwich bag or food wrap, etc.)
rubber band (Part 2)

CO2(g), 20 - 40 mL
universal indicator solution, 10 mL (or red cabbage juice solution)
concentrated ammonium hydroxide solution, (only the NH3 fumes will be used)

    middle school lab, high school lab, university lab, and classroom demonstration

Applications, Topics, Purpose

    acid-base properties of gases, chemical properties of gases, solubility of gases, acid-base reactions, indicators, acidic nature of non-metal oxides


    Part 1. Prepare a syringe full of carbon dioxide or use the leftover carbon dioxide from Experiment 1.  Add 2-mL water to the test tube. Add 10 drops of Universal Indicator solution. Use a plastic pipet to transfer some ammonia vapors to the indicator solution. Stopper the test tube with your thumb or finger.  Shake the contents of the test tube to assure mixing.  Next, transfer 20 mL CO2 above the surface of the solution using the long tubing.  Notice the correct way to dispense gas:  Grasp the plunger and pull the barrel towards you.  By holding the syringe opening upward, no liquid is accidentally discharged.
incorrect way correct way
INCORRECT WAY                       CORRECT WAY

    Shake the contents of the test tube to assure mixing.  

Part 2. Prepare 75 mL of the NH3-vapors/indicator solution and transfer it to a graduated cylinder.  Discharge the CO2 above the surface of the solution and cover the graduated cylinder with the plastic square and rubber band.  Swirl gently to agitate the surface a small amount.  Layers of color will develop.  

universal indicator + CO2

Teaching tips
1. Red cabbage juice solution works well instead of universal indicator solution. See Appendix D for instructions.

2. You may wish to prepare a large quantity of the NH3-vapors/indicator solution for all to use.

2. Chart of indicator color vs. the corresponding pH:

    Indicator Colors    
pH Universal RedCabbage
4.0 Red Red
5.0 Orange-Red Purple
6.0 Yellow-Orange Purple
7.0 Dark Green Purple
8.0 Light Green Blue
9.0 Blue Blue-Green
10.0 Reddish-Violet Green
11.0 Violet Green
12.0 Violet Green
13.0 Violet Green-Yellow
14.0 Violet Yellow

Introductory Questions
1. Explain why carbonated beverages are slightly acidic.  

2. Would vinegar, known to contain acetic acid, cause universal indicator to be violet?

3. Suppose your friend tested the pH of carbonated water as per this experiment.  Suppose also that your friend did not remember whether he/she used universal indicator or red cabbage indicator, however, the solution is purple.  Which indicator did he/she use and why do you know?

4. Does ammonia seem more soluble than carbon dioxide?  

5. Why does the indicator solution eventually turn the color associated with acid?

Advanced Questions
6. What is the pH of the distilled water in your laboratory?  Explain why.

7. What does carbon dioxide form when it dissolves in water?  


extinguishing fire Equipment

Microscale Gas Chemistry Kit
Matches or lighter
CO2(g), 50 mL     
    middle school lab, high school lab, university lab, and classroom demonstration

Applications, Topics, Purpose
   gas density, physical properties of gases, chemical properties of gases, how fire extinguishers work, combustion

    If necessary, prepare a syringe full of carbon dioxide.  Affix a short candle to a coin with a drop of molten wax.  Place the candle/coin into the cup as shown.  Ignite the candle.  Equip the syringe with the tubing and transfer the CO2 to the bottom of the cup.  Discharge the entire contents of the syringe in a quick manner — within a second or two.  The flame will go out.  (The picture shows the gas being discharged in the “incorrect way” according to the discussion in the previous experiment.  We almost never discharge gas in this manner, except in this experiment where rapid discharge is crucial to the success of the experiment.)

flame out

Teaching tips
1. Prepare the coin/candle devices for your students: Using a scissors, cut a candle to a length of 1.5 cm including the wick.  By partially bearing down on the scissors, the wax portion of the candle will be cut, but not the wick.  Light the candle and allow a drop of hot candle wax to fall on the coin.  Push the base of the candle into the molten wax.
candle on coin  

2. Variation:  Demonstrate that CO2(g) is denser than air by pouring it from a syringe over a lighted candle from about 3 cm away.  Candle immediately goes out.

3. Explain that combustion is the reaction of oxygen with another substance, often organic.  Combustion is simply a rapid oxidation of the organic compound and reduction of oxygen.  Combustion is stopped because the oxygen is replaced by the carbon dioxide and carbon dioxide is not a combustible gas.

Introductory Questions
1. Can a candle burn in carbon dioxide?  Does carbon dioxide burn?

2. Can a candle burn in oxygen? 

3. What happened to the burning candle?  Could carbon dioxide be used as a fire extinguisher?

4. Why should you release the carbon dioxide in the bottom of the cup? 

5. Why was it important to use a short candle? 

6. Which gas has a greater density, carbon dioxide or air?  How could you tell? Hint: Compare the molar masses of oxygen and nitrogen with carbon dioxide.

Advanced Question
7. If the carbon dioxide is discharged slowly rather than quickly, this experiment will not work.  Explain why.  Sketch the flow of gases around a heat source. 

8. Design an experiment to determine whether carbon dioxide or air has the greater density.



Microscale Gas Chemistry Kit


CO2(g), 50 mL
sodium hydroxide, NaOH(aq), 6 M, 10 mL; See Precautions

    high school lab, university lab, and classroom demonstration

    Aqueous sodium hydroxide, NaOH(aq) (6 M) is a caustic substance that can cause serious damage to skin and eyes.  Use care when handling this chemical.

Applications, Topics, Purpose
    chemical formulas, chemical properties of gases, types of chemical reactions, balancing chemical reactions, household chemicals, to illustrate that the carbon dioxide is no longer in the gas phase after it reacts with the sodium hydroxide

    Prepare a syringe full of carbon dioxide.  Fill a small weighing dish with 6 M NaOH(aq).  Draw 5 mL NaOH(aq) into a CO2-filled syringe.  Fit the syringe with a syringe cap.  Shake the syringe.  The plunger will move inward as the CO
2(g) reacts with aqueous NaOH(aq) forming NaHCO3(aq) and/or Na2CO3(aq).  The reaction is:

2 NaOH(aq) + CO2(g) arrow   Na2CO3(aq) + H2O(l)

Teaching tips

As CO2(g) reacts with NaOH(aq), the pressure in the syringe decreases forcing the plunger inward. 

Introductory Questions

1. What are the formulas for sodium hydroxide and sodium carbonate? 

2. Did the carbon dioxide dissolve in the solution or react with the solution?

3. Why does shaking the syringe speed up the reaction?


4. Suggest an explanation for what you observed in this experiment.

5. Solutions of bases such as sodium hydroxide or calcium hydroxide are not "stable" if they sit in the air for an extended period of time.  Based on your experiments with CO2(g) suggest a reason for this.

Advanced Questions

6. Carbon dioxide is a covalent molecular compound.  What class of compound is sodium carbonate?

7. Write the chemical reaction that took place in sentence form: “Aqueous sodium hydroxide and …”



Plastic cup, 9-ounce (250 mL)
special purpose syringe with nail through plunger


CO2(g), 40 mL
ice water

    high school lab, university lab, and classroom demonstration
Applications, Topics, Purpose
    carbonation and soft drinks, chemical formulas, chemical properties of gases, gas solubility, household chemicals

    Transfer 40 mL CO2(g) into the special purpose syringe. Pull 10 mL water into the syringe and install the syringe cap.  Push the plunger inward until the nail can be inserted into the middle hole in the plunger as shown in the figure.  The volume of the gas should be compressed to 20or less.  Place the syringe into a large container of crushed ice and water.  Allow the system to come to equilibrium over the next hour.  Remove from the ice and allow to warm to room temperature for 15 minutes.  Next pull the plunger up to the 50 mL mark and insert the nail in the hole near the rubber seal.  Tap the syringe on the countertop.  You should see bubbles of CO2 swirling out of solution.  The process can be repeated.  The equilibrium involves CO
2 as the primary aqueous species.  Approximately 1 CO2(aq) in 600 exists as H2CO3(aq):

CO2(aq)  arrows CO2(aq)  equi;ibrium arrow left H2CO3(aq)

Teaching tips

1. The special purpose syringe is constructed by drilling two holes through the plunger, one is drilled in such a position that the syringe will hold about 30 mL when the nail is flush with the rim of the barrel (left figure).  Use a drill bit that is somewhat larger in diameter than the nail that is to be used. Drill the second hole in such a position as shown in the right figure below: the syringe will hold about 55 mL when the nail is flush with the rim of the barrel (left figure).
euipment equipment

2. One can tap the syringe with knuckles as well.  Avoid hitting the syringe cap.


1. Why did we use ice in this experiment?  Is carbon dioxide more soluble in cold or warm water?

2. Why was pressure used? 

3. What familiar product is based on the solubility of carbon dioxide at low temperature and high pressure?

4. What happens to an opened bottle or bottle of soda pop if it is allowed to warm up?

5. When the syringe was tapped, carbon dioxide bubbles were formed.  What analogy exists with the properties of soda pop? 

Advanced Questions

6. It is known that if the syringe cap is left off, carbon dioxide will diffuse at a rate of about 0.3 mL per minute.  Reviewing the experiments performed in this chapter, suggest an experiment that could be used to verify this statement. 

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 November, 1996. 

(This page last updated on 11 June 2003)