Rainbow

Procedure

• Put some cabbage leaves in the pot with water and boil them for 10 minutes.
• Squeeze the orange and lemon juice into different beakers.
• Introduce a little amount of one of the products into a test tube. Do that with all the products.
• Add a pipet of red cabbage juice to each tube and mix it well.
• Observe how the colors appear.
• Arrange the glasses according to the pH scale, that is, we group all the reddish ones (acids), the purple ones (neutral), and the bluish ones (bases).

Theoretical explanation

This experiment is based on the use of a pH indicator, in this case, red cabbage. Acid-base indicators are substances that change color depending on the pH. They are very useful for detecting the equivalence point of a neutralization reaction. Mixtures of various indicators are also used to manufacture universal pH paper.

Acid-base indicators are usually weak organic acids or weak organic bases that have markedly different colors in their molecular and ionized forms. The presence of one or the other form in the solution depends on the pH, according to the laws of chemical equilibrium (Le Chatelier’s principle).

Anthocyanin is one of the most commonly used indicators. It constitutes a group of water-soluble pigments (soluble in water and alcohol but not in oil). It is responsible for the red, violet, and blue color of many vegetables and plants.

It is also used as a colorant in the food and pharmaceutical industries. It is obtained from strawberries, grapes, red cabbage, red onions, etc. Red cabbage is a vegetable similar to regular cabbage but purple in color because it contains anthocyanin in its leaves. To extract anthocyanin from its leaves, you simply need to blend the cabbage leaves or boil them for ten minutes and then filter the resulting broth. The liquid we have collected is a pH indicator with an intense purple color. Anthocyanin can also be extracted from cabbage with alcohol.

Did you know…? When tree leaves turn red and yellow in the fall, it’s because chlorophyll (the green pigment) disappears and anthocyanin (the reddish pigment) appears.