Colorful Reactions of Red Cabbage Juice

Description

Cabbage juice is placed in a large flask. Vinegar is added to the point of color change. Aqueous ammonia is then added to the point of color change. Vinegar is then added again to show the reversibility of the color change.

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1. Add red cabbage juice to a large flask. Add vinegar until a dramatic change is observed.
2. Add ammonia until a dramatic change is observed.
3. Add vinegar until a dramatic change is observed.

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Name ___________________________ Class ________

Teacher__________________________

DoChem 098 Colorful Reactions of Red Cabbage Juice

Watch the movie.

1. Record the color of the red cabbage solution under the following conditions.

   Acidic

   Basic

   Neutral

2. Is the acid base reaction with red cabbage solution reversible?
3. Could this solution be used as an acid base indicator?

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Purpose

To illustrate the acid-indicating properties of a naturally occurring material.

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• 500 mL red cabbage juice (from boiling red cabbage)
• 500 mL vinegar
• 500 ml household ammonia
• 1-L Florence flask

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• Prepare the cabbage juice fresh. Use caution; boil red cabbage leaves in distilled water.
• Filter the juice through a glass wool plug. The solution may be filtered hot or cold.
• Keep the cabbage juice refrigerated or on ice until used.
• Other natural sources of indicators include: blueberries; cranberries; grapes; rose petals; iris petals; and tea.

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Advance Preparation (cabbage juice ): 40 minutes

Teacher preparation: 10 minutes

Presentation: 10 minutes

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Ammonia vapor is toxic.

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Perform the experiment in a well-ventilated area or use a hood.

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The materials used in this experiment may be disposed of safely at the sink.

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Swimming pool kits and tropical fish tank kits make use of indicators. Grape and cranberry juices are good indicators, as are the flower petals of red and purple flowers when soaked in hot water.

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• Indicators are weak acids that can be represented symbolically by the formula HIn. The dissociation for the indicator is:
  • HIn H⁺ + In^-
• The acid form of the indicator has one color, and the base (conjugate base) form another. Addition of acid leads to the formation of HIn. Addition of base leads to the formation of In^-.

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1. Acidic-red

   Basic-Green

   Neutral-blue or blue-violet

2. Yes, the reaction is reversible.
3. Yes, the reversibility and distinct color change makes the solution a good indicator.

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• pH indicator
• pH
• buffers
• acid
• base

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