The reaction between carbon dioxide and water
When carbon dioxide reacts with water a weak acid is formed. Carbon dioxide present in exhaled air is blown into a flask containing an indicator sensitive to small changes of pH in the appropriate region of the pH scale, and the consequent colour changes observed and recorded. The equation for the reaction between carbon dioxide and water may be introduced for appropriate students.
This is a relatively brief and straightforward exploration of the reaction of carbon dioxide and water at a simple level, which should take no more than 15 minutes.
If students have not yet met the compositions of inhaled and exhaled air, this experiment can serve as part of the learning sequence for the topic of breathing and respiration in an introductory science course, using an appropriately elementary approach to the chemistry involved.
For students who have already covered the topic of breathing and respiration, and know that carbon dioxide is a significant component of exhaled air, the focus in this experiment can be transferred to the nature of the chemical reaction.
Phenol red indicator solution (HIGHLY FLAMMABLE), access to small bottle
Sodium hydroxide solution, 0.4 M (IRRITANT), small bottle
Refer to Health & Safety and Technical notes section below for additional information.
Each working group will require:
Conical flask (250 cm3)
Dropping pipettes, 2
Health & Safety and Technical notes
Wear eye protection.
Phenol red indicator - see CLEAPSS Hazcard. The indicator may be purchased as a solid reagent or as a ready-made solution in ethanol. The solution may be made from the solid reagents by preparing a 5% w/v solution in ethanol (IDA). If 30 cm3 or 60 cm3 dropping bottles with integral dropping pipettes are available, these are ideal for dispensing the indicator solutions. While phenol red itself is not flammable, its solution in ethanol is highly flammable.
Ethanol (IDA – Industrial Denatured Alcohol), CH3CH2OH(l), (HIGHLY FLAMMABLE, HARMFUL) - see CLEAPSS Hazcard.
Sodium hydroxide solution, NaOH(aq), (IRRITANT at concnetration used) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.
a Place about 125 cm3 of water (tap water will usually do) in the 250 cm3 conical flask.
b Add one or two drops of phenol red indicator solution to the water.
c Add two drops of sodium hydroxide solution to produce a red solution.
d Talk or blow gently into the flask until a colour change is observed. Swirling will speed up the reaction.
Q1: Why does the colour change not occur instantly? (Answer: the amount of carbon dioxide in each breath is small, so it takes a lot of breaths to react with the alkali.)
Q2: What is the reason for adding a few drops of sodium hydroxide solution (NaOH) before each experiment? (Answer: to ensure the solution is slightly alkaline at the beginning and to neutralise any CO2 or any other acid initially present).
Straws are not necessary for blowing exhaled air into the flask; simply breathing or speaking into the flask is sufficient to cause the indicator to change colour.
Phenol red indicator changes from yellow to red over the pH range 6.8 – 8.4. An alternative indicator is bromothymol blue, which may already be used in the department in biology experiments. The colour change is blue (alkaline) to yellow (acid) over the pH range 6.0 – 7.6 - see CLEAPSS Recipe Book, which also covers the long-established ‘bicarbonate-indicator’ solution.
Eventually sufficient carbon dioxide from the students’ breath dissolves and produces enough acid in the solution to change the colour of the indicator:
CO2(aq) + H2O(l) ⇌ H+(aq) + HCO3–(aq)
CO2 also reacts with NaOH. This reaction produces the less alkaline Na2CO3:
2NaOH(aq) + CO2(g) → Na2CO3(aq) + H2O(l)
The equilibrium between carbon dioxide and water can be reversed by heating the weakly acidic solution to just below boiling. The solubility of carbon dioxide in water decreases as the temperature is raised, and it is driven off into the atmosphere. The concentration of dissolved carbon dioxide therefore drops, causing the equilibrium to shift to the left and the indicator colour to change back to red. On cooling the solution and blowing exhaled breath into the flask again, the sequence can be repeated.
Another experiment on Practical Chemistry looks at the carbon dioxide-water equilibrium in more detail, investigating the effect of pressure and temperature in the context of Le Chatelier’s principle: Equillibria involving carbon dioxide in aqueous solution.
Health & Safety checked March 2009
Page last updated on 31 July 2012