A simple classic demo that is a good phenomenon for demonstrating the connection of CO2 to coral reefs and shelled ocean life. Its also very doable in virtual settings.
***Disclaimer- The connection is nuanced. Ocean Acidification has increased the acids going into the ocean but the ocean is still basic overall, with a decrease in Ph from 8.3 to 8.2 since the industrial age. The carbonic acid has does not actually dissolve shells rather it neutralizes the calcium carbonate and hinders the development of shells.