Carbon Dioxide Equilibrium in Water

This demonstration uses carbonated water in a sealed syringe with bromcresol green indicator to show how dissolved carbon dioxide affects acidity and how pressure influences solubility. The indicator changes color as CO2 is released, revealing shifts in pH.

1. Draw carbonated water into a large syringe and add a few drops of bromcresol green indicator.
2. Seal the syringe tightly with a cap to trap the liquid and gas.
3. Observe the initial color of the solution; under pressure, CO2 remains dissolved and the solution is more acidic.
4. Gently pull back on the plunger to reduce pressure inside the syringe.
5. Watch for bubble formation as CO2 comes out of solution and note the color change of the indicator as the acidity decreases.
6. Push the plunger back in to increase pressure again; observe that some CO2 redissolves, shifting the indicator color back toward acidic.

Le Chatelier and Pressure Changes using carbonated water - Linda Hanson:

LeChatelier's Principle and the Solubility of Carbon Dioxide - FlinnScientific:

• Repeat with different indicators (e.g., universal indicator or phenol red) to compare color ranges.
• Try using sparkling water at different temperatures; colder samples retain more CO2 and show slower color change.

• Do not pull the plunger out completely; carbonated liquid may spray.
• Keep the syringe upright to minimize spillage during pressure changes.
• Wear safety glasses and protect work surfaces from staining by the indicator.

• Why does the indicator show a color change when the syringe pressure is lowered? (Less CO2 dissolves, lowering H+ concentration and increasing pH.)
• How does Henry’s Law explain the behavior of CO2 in the syringe? (Gas solubility is proportional to its partial pressure above the liquid.)
• Why does the solution become acidic when CO2 is dissolved? (CO2 reacts with water to form carbonic acid, releasing H+ ions.)
• What differences would you expect if you repeated the demonstration with warm carbonated water? (Less CO2 would dissolve, so the solution would be less acidic and lose its fizz faster.)