 Acid-based indicators are compounds that change color at different pHs. I've added some water to the graduated cylinder. I then added some universal indicator, turning it green. Universal indicator is a mixture of acid-based indicators so that it changes color at different pHs. I'm going to make this solution basic by adding some 6-molar sodium hydroxide. I'm then going to add some carbon dioxide in the form of dry ice. As the carbon dioxide dissolves in the water, it will produce carbonic acid. The pH will go from a high pH to a low pH and we'll see the color range for universal indicator.