Students add laundry detergent powder (a base) and cream of tartar (an acid) to a red cabbage indicator to investigate the question: What can the color of an indicator tell you about the substances added to it?

Objective

Students will be able to explain that if two substances cause turn a pH indicator different colors, they must be different substances. Students will be able to explain that the color that a substance turns a pH indicator is a characteristic property of that substance. Students will also be able to explain that different substances react in characteristic ways that can be used to identify a substance.

Key Concepts

• A pH or acid-base indicator is a compound that changes color depending on the pH of the substance that is added to it.
• Acids and bases make pH indicators turn different colors.
• Two substances that turn the same pH indicator different colors must be different substances.
• The color that a substance turns a pH indicator is a characteristic property of that substance.
• An acid can neutralize a base, and a base can neutralize an acid

NGSS Alignment

• NGSS 5-PS1-3: Make observations and measurements to identify materials based on their properties.
• NGSS 5-PS1-4: Conduct an investigation to determine whether the mixing of two or more substances results in new substances.

Summary

• Students squeeze red cabbage leaves in water to make an acid-base indicator solution.
• Students add laundry detergent powder (a base) and cream of tartar (an acid) to the indicator and see that the two solids cause different color changes.
• Students recognize that a particular color change is a characteristic property of a substance and that a color change can also be used as evidence that a chemical reaction has occurred.
• Students will explore what happens during neutralization by using a base to neutralize an acid, and an acid to neutralize a base.

Evaluation

Download the student activity sheet (PDF) and distribute one per student when specified in the activity. The activity sheet will serve as the Evaluate component of the 5-E lesson plan.

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