Red Cabbage as a pH Indicator

   2005-03-01      experiment

We learned last week that red cabbage is a general pH indicator. What is pH anyhow? The “potential” or “power” of Hydrogen: the pH scale measures the concentration of Hydrogen ions. If I had some phenolphthalein I would love to demonstrate turning water into wine. That page also has a great description of why pH indicators change colors when mixed with an acid or a base. “Different molecules absorb different colors of light, depending on their electronic structure.”

All very interesting, but in need of something a little more “first grade,” I found a nice description of acid and base:

ACIDS
- Weak acids like the natural acids in food give foods a sour, sharp flavor.
- Strong acids can burn your skin.
- Many acids are corrosive. They eat away metals and other substances.
- Some acids can be helpful. The acid in your stomach aids in digestion.
- Two acids (sulfuric acid and nitric acid) cause damage in acid rain.
- An acid releases hydrogen ions (H+) in water.
- Acids can neutralize bases.

BASES
- Bases taste bitter and feel slippery.
- Weak bases are used to settle upset stomachs.
- Detergents and many cleaning solutions are basic.
- Strong bases can burn the skin.
- A base releases hydroxide ions (OH-) in water.
- Bases can neutralize acids.

We then tested various liquids to find out if they were acid or base. We used lemon juice, laundry detergent, baking soda water and apple juice. We had a chance to talk about acid and base being a scale, that something could be more acidic or more basic. We also talked about different types of acids: acid rain and stomach acid. (First graders are always happy to discuss vomit and tell stories about their experiences.)