Because purple cabbage is rich in anthocyanins, it can be used as a self-made acid-base indicator, while white vinegar, water and baking soda are acidic, neutral and alkaline substances respectively. According to the principle that acid-base indicator is red in acid and blue or light green in alkali, purple leaf cabbage juice will show different colors when added to these solutions respectively.

Extended data:

Acid-base indicator is a kind of organic weak acid or organic weak base with complex structure, which can be partially ionized into indicator ions and hydrogen ions (or hydroxide ions) in solution. Its molecules and ions show different colors due to structural changes, so they show different colors in solutions with different pH values.

References:

China's "Magic Acid-base Indicator"