Universal indicator ranges from red (pH 4) to purple (pH10) as the pH of a substance changes. Adding NaOH to water starts the solution off as purple. When dry ice is added to water, some of it forms carbonic acid, and lowers the pH. This is why the solution changes from purple to blue to green to yellow. The “fog” that we see is condensed water vapor though, not carbon dioxide gas.
CO2 (s) + H2O (l) → H2CO3 (aq)
At the same time, the dry ice is subliming to gaseous carbon dioxide. The “fog” that we see is condensed water vapor though, not carbon dioxide.
- universal indicator
- large flask (any other glass container will work)
- dry ice
- dilute NaOH solution
1. Fill the flask ~3/4 full of the dilute NaOH solution
2. Add a few mL of universal indicator
3. Add dry ice
A solution that has 1 pellet of NaOH per 1000mL of water works well.
Universal indicator can stain, dry ice can cause severe burns, goggles should be worn at all times, and caution should be taken.
“Dry Ice “Titration”.” Chemistry Demos. Chemistry @ Indiana University, 07 Jun 2011. Accessed 14 Jun 2010. http://outreach.chem.indiana.edu/demos/5-4 Dry Ice Titration.doc
Foster, Laurence S.; Gruntfest, Irving J. Demonstration Experiments Using Universal Indicators. J. Chem. Educ. 1937, 14 (6), 247.
You can talk about how more carbon dioxide in our atmosphere effects the pH of aqueous environments, and how soda is so acidic because carbon dioxide is pressurized into it.