|MadSci Network: Chemistry|
Thank you for your question. It is aimed right at the heart of all chemistry -- explaining why materials behave the way they do.
Red cabbage juice contains a pigment or coloring agent called anthocyanin. This pigment is found in many flowers, fruits and fall leaves, and is responsible for many of the reds, blues, and purples you see around you. It makes cornflowers blue, pumpkins orange, strawberries red, and cabbage purple. To explain why anthocyanins work as an indicator, I have to explain a bit about acids and bases.
Acids, in general, are compounds that release hydrogen ions when they are put in water. Hydrogen ions are hydrogen atoms that have picked up a positive electric charge. We symbolize them as H+. Strong acids (like the hydrochloric acid in your stomach) release lots of H+. Weak acids (like the citric acid in orange juice) release only a few H+. Bases, in general, are compounds that release hydroxide ions when they are put in water. Hydroxide ions are made of a hydrogen atom attached to an oxygen atom, with a negative charge. They can be symbolized as OH-. When you put equal amounts of H+ and OH- in water, they combine to make water (H2O). If there arenít enough OH- to combine with all the H+, the water will have extra H+ and be acidic. On the other hand, if there arenít enough H+ to combine with all the OH-, the water will have extra OH- and be basic.
Now back to your cabbage. Anthocyanins are complex molecules that can add and subtract hydroxide ions: very much like adding or subtracting beads to a necklace. If you put anthocyanin in acidic solutions, OH- ions will pop off the anthocyanin molecules. In basic solutions, OH- in the water will hop onto the anthocyanin molecules. So anthocyanin is really a name for a series of related compounds, each with a different number of hydroxide ions attached. There are lots of compounds in nature that form these kinds of series. What makes anthocyanin an indicator is that each form of the compound reacts with light differently, so that each form is a different color. All acid-base indicators, in fact, change color because they change their structure in some way. (What makes a particular compound show up as a particular color is a far more complicated topic, which I wonít go into here.)
I hope this answers your question. Red cabbage juice works as an indicator because it contains the pigment anthocyanin, which changes its structure and its reaction to light depending on the acidity of its environment.
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