Wednesday, February 16, 2011

General Science and Biology: Purple cabbage broth...more than just just smelly and blue

This week we boiled up some purple cabbage. Why did we do this? Well, it wasn't simply to overwhelm our senses with its powerful smell and uncanny blue color. Cabbage broth has the interesting property of indicating for acids and bases.
The kids added cabbage juice to 7 different liquids and observed the results. They recorded the color changes and using their data from last week, ordered the cups according the pH values. They observed that cabbage juice indicates strong acids with pinks, weak acids with purples, neutrals with blues, weak bases with greens, and strong bases with yellows. Finally they were given an unknown liquid, and using their cabbage-pH scale they guessed its pH.

In the Biology group we took the analysis one step further. After measuring each liquid's pH with the cabbage juice indicator, we explored what happens to the pH of an acid as we add increasing amounts of a base. The kids started off with vinegar + cabbage juice, giving them a nice bright pink, indicating a pH of around 2. As they added more and more baking soda they were able to observe the vinegar change colors as its pH gradually moved closer to a neutral 7.

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