Carbon dioxide (CO2) in its solid form finds application in several areas of life (as a cooling agent, for instance). Solid carbon dioxide sublimates, yielding gaseous carbon dioxide (or carbonic acid gas).
First, solid carbon dioxide was placed in an open container - a beaker, the container was filled with carbon dioxide gas. As carbon dioxide is heavier than air, it remains in the container for a while. To test carbon dioxide, a burning lath was taken to the beaker.
In the two following experiments, granules of solid carbon dioxide were poured into weakly basic solutions (pH∼9). When placed into water, solid carbon dioxide starts to evaporate vigorously, bubbles of CO2 escape from the solution.
The released carbon dioxide is also dissolved in water to some extent, yielding carbonic acid:
CO2(g) + H2O(l) ↔ H2CO3(aq)
Carbonic acid is a weak acid, only dissociating to a small extent:
H2CO3(aq) ↔ H+(aq) + HCO3–(aq)
The addition of carbon dioxide to a weakly basic solution first neutralized the OH--ions present in the solution:
H+(aq) + OH–(aq) → H2O(l)
After that the medium already became weakly acidic.
1) In the first experiment, an universal indicator solution was used. In the weakly basic solution (pH∼9), the color of the indicator was blue, the action of solid carbon dioxide turned the solution yellow, indicating that the medium had become weakly acidic (pH∼5).
2) Phenolphthalein was used as the indicator in the other case. In the weakly basic solution, the indicator had turned pink. If pH of the solution is less than 8, phenolphthalein is colorless.
1) CARBON DIOXIDE: “Change of pH upon dissolution of CO2”