"Titrations" with Dry Ice

This is an illustration of an acid-base reaction. Solid CO₂ or dry ice (an acid, -78^° C) reacts with strong bases leading to neutralization.

            CO₂ + HO^- → HCO₃^-

The consumption of the base causes the solution pH to drop until all HO^- has reacted. The indicator changes color with the changing pH.

Materials

• Dry ice
• Acid-base indicator (bromothymol blue)
• Base solution (sodium hydroxide, NaOH 1M or ammonium hydroxide, NH₄OH)
• 4-L containers

Procedure

Each container is filled with the base solution and a small amount of acid-base indicator. Pellets of dry ice are added, resulting in strong bubbling and a white “fog” that develops at the top of the container. The “fog” is simply air containing water vapor cooled by the dry ice. It is white because it scatters light. After approximately 30 seconds, the entire container changes color abruptly, from blue to yellow (with a brief stage of green color).

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Other Demos: Chemiluminescence of Luminol, Photoresists, High Energy Emitters for Organic Light-emitting Diodes, The Spectrum and Complementary Colors, "Titrations" with Dry Ice, The Red-White-Blue Clock

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