The Reaction of Carbon Dioxide with Water

Straws are unnecessary 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. Thymolphthalein (the alternative bromothymol blue could also be used) changes from blue (alkaline) to colourless (acid) over the pH range 9.3–10.5. See CLEAPSS Recipe Book RB000, which also covers 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. When the solution is cooled and exhaled, breath is blown into the flask again; the sequence can be repeated.