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Dry ice is added to a basic solution of dish detergent and universal indicator.
Commercially, dry ice is used as a refrigerant in the food industry. Your students may be familiar with seeing a piece of dry ice, the solid form of carbon dioxide (CO2), bubble in water to create an eerie fog. Carbon dioxide dissolves in water to form a weak solution of carbonic acid:
CO2(g) + H2O(l) → H2CO3(aq) ↔ 2H+(aq) + CO32–(aq)
In this demonstration, dry ice is added to a basic solution of dish detergent and universal indicator. As the dry ice dissolves, carbonic acid is formed which causes the solution’s pH to decrease and the universal indicator to change colors. As carbon dioxide is released from the solution, it forms bubbles with the dish detergent, producing a mass of foam.
Bogen universal indicator is used to create the rainbow effect. The indicator is blue at a pH above 10.0, green at a pH of 8.0, yellow at a pH of 6.0, orange at a pH of 5.0, and cherry red at a pH of 1.0 or lower. You can use other acid-base indicators to create other color changes to match school colors for a pep rally or for a holiday. This pH indicator chart will help you find the best indicator for your demonstration.
If possible, purchase the dry ice (which is available at many grocery stores) on the same day you perform the demonstrations. You will only need a small amount of dry ice, but will lose some due to sublimation during transport and storage. Purchase about 1 pound of dry ice to allow for loss. Use a foam cooler to transport it to your classroom. If you are looking for activities that can be done with dry ice, CO2OL Demonstrations with Dry Ice Kit includes instructions for 5 dry ice activities as well as most of the materials needed.
Foam Cooler with Loose-Fitting Lid
Pillowcase or Large Cloth
Stir Bar (appropriate size for the graduated cylinder)
Pipet or Dropper
Dry ice is very cold, –78.4° C (–109° F). Do not allow it to contact bare skin. Store dry ice in a polystyrene foam cooler with a loose-fitting lid to prevent the pressure inside the cooler from increasing to unsafe levels. Never store dry ice in a sealed cooler, refrigerator, or freezer. Carbon dioxide gas is odorless and colorless. Use, store, and transport dry ice only in well-ventilated areas.
Make sure that the soapy foam does not create a slipping hazard.
Put on PPE.
Place the stir bar inside the graduated cylinder and fill the cylinder about half-full of water.
Place the cylinder on a stir plate. Turn on the stir plate.
Add 5 drops of dish detergent and 1 mL of indicator.
Add sodium hydroxide drop by drop until the solution is a deep shade of blue.
Remove the cylinder from the stir plate. Set aside until ready to use.
Put on PPE.
Place the graduated cylinder containing the solution inside the terrarium.
Drop a piece of dry ice in the graduated cylinder. As the dry ice sublimes the solution will become more acidic changing from blue to green to yellow and eventually to red.
Add additional pieces of dry ice as needed until the solution in the graduated cylinder turns red.