Acidification changing the colour of universal indicator (UI). Initially, the water bath contains neutral water, and UI is a green colour. Dry ice is then added. Dry ice is the solid form of carbon dioxide, produced by cooling it below -78.5 degrees Celsius. The rapid warming first produces bubbles and billowing clouds of carbon dioxide, which pour from the vessel. As the carbon dioxide dissolves in the water it produces carbonic acid. As this occurs, the colour of the solution changes to yellow and orange, indicating that the solution is becoming more acidic.