pH Indicator - Red Cabbage Indicator (Brie-anna Molina)

Author

Brie-anna Hope Molina

Principle(s) Illustrated

• pH indicators detect the presence of H+ and OH-. They do this by reacting with H+ and OH. Indicators are themselves weak acids and bases. If an indicator is a weak acid and is colored and its conjugate base has a different color, deprotonation causes a color change.¹

• Red cabbage contains an indicator pigment molecule called flavin, which is one type of molecule called an anthocyanin. This water-soluble pigment is also found in apple skin, red onion skin, plums, poppies, blueberries, cornflowers, and grapes. Very acidic solutions will turn anthocyanin a red color. Neutral solutions result in a purplish color. Basic solutions make a greenish-yellow or yellow color.

Standards

• NGSS Science & engineering standards

• NGSS Cross-cutting concept standards

• NGSS Disciplinary core idea standards

Questioning Script

Prior knowledge & experience:

A solution is a combination of two or more substances that fully dissolve together.

Some solutions are acidic and others are basic. We determine whether things are acids or bases by measuring their pH. We can find the pH or relative acidity by using an indicator.

Root question:

What liquid(s) are in the beakers? Are they all the same? How can you know?

What happens when you mix two liquids together?

What information can we learn about a liquid by observing its color?

Why did the liquids change colors?

What do the different colors tell us about the liquids?

Target response:

The color change is indicative of a chemical reaction.

Common Misconceptions:

A clear liquid and a colored liquid will result in a lighter shade of the same color liquid when mixed together.

Photographs and Movies

References

¹ How do pH indicators work?