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Bubbling Cylinder Activity

By Bruce Wilson
Product Manager, Chemistry

Introduction

Use this demonstration as you discuss changes in states of matter, acid-base chemistry, equilibrium, reversibility, or the sensitivity of acid-base indicators to changes in pH. In the experiment, solid carbon dioxide (dry ice) reacts with water to produce carbonic acid. As the pH drops, the color of the solution changes. Adding sodium hydroxide increases the pH and recharges the solution for another run. The formation of bubbles and color changes make this demonstration memorable and entertaining.

National Science Education Standards

  • Grades 9–12: Chemical Reactions

Materials

Safety

Wear safety goggles. Wear insulated gloves to handle dry ice.


Preparation and procedure

Store the dry ice in a cooler. Break the ice into golf ball-sized pieces using a hammer and a screwdriver.

  1. Add 500 mL of water to a 1-L graduated cylinder.
  2. Add 10 mL of Bogen universal indicator to the cylinder. If the solution is not neutral or basic (green, blue, or violet), add just enough 1 M sodium hydroxide to turn it green.
  3. Tilt the cylinder at a 45° angle and slide a golf ball-sized piece of dry ice into it. Set the cylinder upright. Bubbles form, and a wispy fog escapes and flows down the side of the cylinder. After several seconds, the solution turns yellow or red.

Extension

To increase the duration of the color change, begin with a more basic solution before adding the dry ice. Add 50 mL of 1 M sodium hydroxide to the cylinder before proceeding with step 3. The solution will be violet initially and slowly change to blue, then green, then yellow.

To recharge the cylinder, add another 50 mL of 1 M sodium hydroxide.

Conclusion

Dry ice is solid carbon dioxide. At room temperature and pressure, carbon dioxide sublimes rather than melts. When dry ice is added to water, bubbles of carbon dioxide form as the solid sublimes. This method is an easy way to show students the gas evolved through sublimation.

As the carbon dioxide gas bubbles through the water, only some of the gas dissolves. The gas that does not dissolve in the water bubbles out. Because carbon dioxide gas is denser than air, it falls to the floor, where it produces a wispy fog. The fog forms as the cold carbon dioxide causes gaseous water in the air to condense into visible droplets. The carbon dioxide gas therefore reveals its own path as it flows.

The carbon dioxide gas that dissolves in the solution reacts with water to form carbonic acid, H2CO3, by this chemical equation:

CO2(g) + H2O(l) H2CO3(aq)

In the experiment, the initial pH of the solution containing water and Bogen universal indicator is neutral, and the color is green. Once the solid carbon dioxide is added, the pH drops and the indicator turns yellow or red, indicating an acidic pH.

The process can be repeated by neutralizing the carbonic acid with sodium hydroxide as shown:

2NaOH(aq) + H2CO3(aq) Na2CO3(aq) + 2H2O(l)

The solution turns basic again, and its color becomes violet. Adding more dry ice yields more carbonic acid and reverses the color change as the pH drops.

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