RHYS LEWIS & MINH TAN PHAM, AHS, DECD, UNISA / SCIENCE PHOTO LIBRARY RHYS LEWIS & MINH TAN PHAM, AHS, DECD, UNISA / SCIENCE PHOTO LIBRARY
Neutralisation reaction changing the colour of universal indicator (UI). Initially, the measuring cylinder contains alkaline sodium hydroxide, and the UI is purple. 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 cylinder. As the carbon dioxide dissolves in the water it produces carbonic acid. This reacts with and neutralises the sodium hydroxide, forming water and sodium carbonate. As this occurs, the colour of the solution changes to blue and then green, indicating that the solution is neutral. With an excess of dry ice, the end point is an acidic solution of carbonic acid, and the UI turns yellow.
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