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Color changes with acids and bases

How can you tell if a substance is an acid, a base, or neutral?

In this activity, students will use the color changes of red cabbage indicator to classify substances as either acid or base.

Materials needed for each group

  • Red cabbage leaves
  • Cream of tartar
  • Vinegar
  • Powdered laundry detergent
  • Water
  • Zip-closing plastic bag, storage-grade, quart-size
  • 5 Clear plastic cups
  • 3 Small cups
  • 2 Flat toothpicks
  • 1 Dropper
  • 1 Tablespoon
  • Permanent marker
  • White piece of paper

Notes about the materials

Preparing materials

  • Label 3 small cups vinegar, detergent, and cream of tartar for each group.
  • Place about 1 teaspoon of vinegar in its labeled cup.
  • Place about ¼ teaspoon of detergent and ¼ teaspoon of cream of tartar in their labeled cups.

Activity sheet

Download the student activity sheet, and distribute one per student when specified in the activity.

Assessment

An assessment rubric for evaluating student progress during this activity is via download on this page. For this formative assessment, check a box beside each aspect of the activity to indicate the level of student progress. Evaluate overall progress for the activity by circling either “Good”, “Satisfactory”, or “Needs Improvement”.

Question to investigate: How can you tell if a substance is an acid, a base, or neutral?

  1. Introduce this activity.

    In this activity, students will make their own indicator solution and experiment with these powders. Tell students that red cabbage indicator changes color when certain chemicals are added to it. It turns pinkish when acids are mixed with it. It turns green when bases are mixed with it. And it remains blue when neutral substances are mixed with it. A neutral substance is neither an acid nor a base.

    Distribute the student activity sheet.

  2. Have students prepare for the activity.

    Procedure
    1. Tear 2 red cabbage leaves into small pieces and place them in a storage grade zip-closing plastic bag.
    2. Add about 1 cup of room temperature water. Get as much air out of the bag as possible and seal the bag securely.
    3. While holding the bag, repeatedly squeeze the water and cabbage leaves until the water turns a medium to dark blue. This is your indicator solution.

      A student squeezes a zip closing plastic bag containing water and red cabbage leaves to preapre an indicator solution
    4. Open a corner of the bag and carefully pour the red cabbage indicator into an empty clear plastic cup, leaving the cabbage pieces behind in the bag.
    5. Label four empty clear plastic cups indicator + detergent, indicator + cream of tartar, indicator + vinegar, and control.
    6. Carefully pour 2 tablespoons of indicator solution into each cup.
    7. Place the four labeled cups on a white piece of paper.
  3. Have students add acids and a base to the indicator.

    Procedure
    1. Use the flat end of a toothpick to scoop up a small amount of cream of tartar. Add the cream of tartar to the indicator + cream of tartar cup.
    2. Gently swirl to mix. Observe any color change in the cup and record this in the chart on the activity sheet.
    3. Use the flat end of a toothpick to scoop up a small amount of laundry detergent. Add the detergent to the indicator + detergent cup.

      A student uses a toothpick to add a small amount of detergent to a cup containing indicator solution 3
    4. Gently swirl to mix. Observe any color change in the cup and record this in the chart.
    5. Use a dropper to add 1 drop of vinegar to the indicator + vinegar cup.
    6. Gently swirl to mix. Observe any color change in the cup and record this in the chart.

    Expected results: Cream of tartar and vinegar turn the indicator a pinkish color. Laundry detergent turns the indicator a greenish-blue color.

  4. Discuss student observations.

    Remind students that red cabbage indicator turns pink when acids are added to it and greenish-blue when bases are added to it. Ask students which substances they tested are acids and which are bases. They should conclude that vinegar and cream of tartar are both acids and that laundry detergent is a base.