Question

\(\Theta\) The "Chemistry in Focus" segment Garden-Variety Acid-Base Indicators discusses acid-base indicators found in nature. What colors are exhibited by red cabbage juice under acid conditions? Under basic conditions?

Short answer

Under acidic conditions (pH < 7), red cabbage juice exhibits a red color. Under basic conditions (pH > 7), it shows a range of colors, such as green, blue, and purple, depending on the alkalinity of the solution. This color change is due to the presence of the anthocyanin pigment in red cabbage juice.

Step-by-step solution

(Step 1: Understanding Red Cabbage Juice as an Indicator)

Red cabbage juice is a natural substance which can be used as an acid-base indicator. It contains a pigment called anthocyanin which changes color depending on the acidic or basic nature of the solution. The anthocyanin in red cabbage juice will exhibit different colors when exposed to an acidic solution (pH < 7) in comparison to a basic solution (pH > 7).

(Step 2: Identifying the Color Under Acidic Conditions)

Under acidic conditions (pH < 7), the anthocyanin in red cabbage juice exhibits a red color. This means that if we expose red cabbage juice to an acidic environment, the resulting color change will turn it red.

(Step 3: Identifying the Color Under Basic Conditions)

Under basic conditions (pH > 7), the anthocyanin in red cabbage juice exhibits a range of colors, commonly going through green, blue, and then purple as the pH becomes more alkaline. If we were to expose red cabbage juice to a moderately basic environment, the resulting color change would be greenish-blue, and in a highly alkaline environment, purple. In summary, red cabbage juice changes its color under acidic and basic conditions due to the presence of anthocyanin pigment. Under acidic conditions, the color is red, and under basic conditions, it exhibits a range of colors from green to purple depending on the alkalinity of the solution.

Key concepts

Anthocyanin

Anthocyanins are natural pigments found in various plants, giving them vibrant colors like red, purple, and blue. These pigments belong to the flavonoid class and are water-soluble, which makes them great for use in solutions such as red cabbage juice. The fascinating property of anthocyanins is their ability to change color when exposed to different pH levels, serving as natural pH indicators.

How do they change color? Anthocyanins have molecular structures that can gain or lose hydrogen ions in response to the acidity or basicity of a solution, leading to alterations in their light absorption properties. As a result:

• In acidic environments (pH < 7), they often appear red.
• Neutral pH (around 7) can show them as purple.
• In basic conditions (pH > 7), they range from green to blue.

These unique properties make anthocyanins highly useful in educational settings to demonstrate the concept of pH through color changes.

Red Cabbage Juice

Red cabbage juice is derived from the leaves of red cabbage, and it's an excellent natural acid-base indicator. The juice is packed with anthocyanin pigments, which react visibly to pH changes by altering color. This makes the juice a popular choice for science experiments to demonstrate how pH affects different substances.

When making red cabbage juice, the cabbage leaves are chopped and boiled in water, releasing the anthocyanins into the liquid. Once prepared, this juice acts as a vivid indicator that can be used to test the pH of various solutions:

• In acidic solutions, the juice turns red.
• In neutral solutions, it remains purple.
• In basic solutions, it shifts from green to blue.

The straightforward preparation and the noticeable color changes make red cabbage juice a practical tool for exploring the chemistry of acids and bases.

pH Levels

The term "pH" stands for "potential of Hydrogen" and is a scale used to specify the acidity or basicity of an aqueous solution. The pH scale ranges from 0 to 14, with 7 representing a neutral point where the concentration of hydrogen ions ( ⁢ H⁺ ⁢ ) is equal to that of hydroxide ions ( OH⁻ ).

• A pH less than 7 indicates an acidic solution, characterized by a higher concentration of hydrogen ions.
• A pH greater than 7 signifies a basic or alkaline solution, with a higher concentration of hydroxide ions.
• Substances such as anthocyanins in red cabbage juice help visualize these changes by altering their color based on the pH.

Understanding pH is fundamental in fields like chemistry, biology, and environmental science, where controlling acidity and basicity is crucial for experiments, chemical reactions, and ecological balance.