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Classroom activity Grade 6 Art & Music

Cabbage-Juice Reactive Painting for the Birchwater Creek Stream Club

Grade 6 students draw with vinegar and baking soda on paper dyed with red cabbage indicator, planning a composition whose pink and green marks show acid and base.

Objectives

  1. Students will plan a composition that uses acid and base marks on indicator paper to create deliberate color contrast.
  2. Students will control the size and spread of their marks by adjusting how much liquid they load on a swab or brush.
  3. Students will explain in a booth label what the pink and green colors on their painting show about acids and bases.

Vocabulary

pH
pH is a number that tells whether water is acidic, like lemon juice, or basic, like baking soda mixed in water. Pure water sits in the middle of the scale at a pH of 7.

Also: indicator, acid, base, complementary colors, contrast, negative space

Teacher overview

Red cabbage contains natural pigments that change color with pH, so juice made from it works as an indicator. It turns pink or red in an acid, stays purple or blue-violet near neutral, and turns blue-green or green in a base. Stream monitors and water utilities measure pH because it affects which organisms can live in a stream and how water acts on the pipes it passes through, and EPA's secondary standard for drinking water sets a pH range of 6.5 to 8.5. In this 50-minute studio activity, students sit in table groups of four but each makes an individual painting. They work on paper that was soaked in cabbage indicator and dried, and they draw with vinegar or lemon juice (acids) and with baking soda solution (a base). The Birchwater Creek Stream Club, a fictional volunteer group, wants small artworks for its booth at a community fair that show visitors how testers see the difference between acids and bases, so each composition must be planned so that the color changes carry the message.

Because the paper itself is the indicator, every mark a student makes is also a small pH test, so the painting and the chemistry happen in the same step. A common misconception is that the pink and green marks are dyes carried onto the paper by the liquids. The color is already in the paper, and the acid or base only changes it, which is why a green mark can be pulled back toward pink by layering an acid over it. The debrief returns to that point directly.

Activity Directions

  1. Before class (the day before): Chop about half a red cabbage, cover it with hot tap water, let it steep for 30 minutes, and strain out the purple liquid. Dip or brush sheets of heavy white paper with the juice and let them dry flat overnight; the dried sheets will be a pale purple or blue-violet. Stir a few spoonfuls of baking soda into a cup of water for the base solution. Set each table with two cups of vinegar or lemon juice, two cups of baking soda solution, swabs, brushes, and paper towels. Students wear goggles, because vinegar and lemon juice sting if they splash into eyes, and nobody tastes any of the liquids. Use vinegar instead of lemon juice for any student with a citrus allergy.
  2. Introduce the indicator paper (5 min): Tell students that the paper is the indicator and the liquids are the acid and the base. Every mark they make is a small pH test, and the finished painting is a record of where acid and base touched the paper. Demonstrate one swab of vinegar and one swab of baking soda solution on a scrap so students see the pink and the green appear.
  3. Test strip (8 min): Each student tears a strip from the edge of their sheet and makes test marks: a light dab with a nearly dry swab and a heavily loaded swab of each liquid, one brush line of each, and one spot where an acid is layered over a green base mark after it has sat for a minute. Students answer questions 1 through 5 on the Student Handout from what they see on their strip.
  4. Plan the composition (8 min): On sketch paper, students draw a thumbnail for a small booth artwork with a water theme, such as a creek bank, a trout, or a test tube held up to the light. They mark which areas will be pink, which will be green, and which will stay the untouched purple of the paper as negative space. Point out that pink is a light red, and red and green sit on opposite sides of the color wheel, so placing them side by side creates strong contrast.
  5. Make the painting (15 min): Students lightly transfer their plan in pencil and apply the liquids. Large areas get a brush, and fine lines get a nearly dry swab. Marks spread on damp paper, so students blot with a paper towel to keep edges crisp. They may layer acid over a green area to pull it back toward pink, and they may notice a little fizzing where the two liquids meet, because the acid and base react.
  6. Write the booth label (7 min): Each student writes a three-to-four-sentence label to display beside the artwork. The label explains what the pink and green areas show, uses the words indicator and pH correctly, and gives one reason stream testers care about pH.
  7. Gallery check (5 min): Students set out their paintings and labels. Each student reads two neighbors' labels and checks that each label matches the colors in its painting. Students then answer questions 6 and 7 on the handout.
  8. End and debrief (2 min): The activity ends when every student has one finished painting and one label on the table. Ask two questions: Why could you turn a green mark back toward pink, and what does that tell you about where the color comes from? How did the untouched purple areas help your composition?
Cabbage-Juice Reactive Painting for the Birchwater Creek Stream Club — Grade 6 Art & Music
Name: ____________________________ Date: ______________

Student Handout

Studio record: reactive painting for the Birchwater Creek Stream Club

Answer questions 1 to 5 from your test strip. Answer questions 6 and 7 after the gallery check.

  1. What color did the indicator paper turn where you dabbed vinegar or lemon juice?
  2. What color did the indicator paper turn where you dabbed baking soda solution?
  3. Based on your test strip, is baking soda solution an acid or a base?
  4. What happened when you put an acid on top of a green mark?
  5. Which gave you a sharper edge on your test strip, a heavily loaded swab or a nearly dry swab?
  6. What do artists call two colors that sit on opposite sides of the color wheel, such as red and green?
  7. Describe one area of your painting that you left untouched on purpose, and tell what it does for the picture.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: Pink or red. The acid lowers the pH where it touches the paper, and the cabbage pigment turns pink in an acid. Accept pink, red, or magenta.
  2. Answer: Green or blue-green. The base raises the pH where it touches the paper, and the pigment turns green in a base. Accept green, blue-green, or teal.
  3. Answer: A base. It turned the indicator green, the color the pigment shows in a base.
  4. Answer: The green area turned back toward pink or purple, often with some fizzing. The acid reacted with the base and lowered the pH again, which shows that the color comes from the pigment already in the paper and that the change can be reversed.
  5. Answer: The nearly dry swab. Less liquid spreads less through the damp paper fibers, so the edge stays crisp. Accept an answer drawn from the student's own strip if it explains the amount of liquid.
  6. Answer: Complementary colors. When they are placed side by side they create strong contrast, and each one looks brighter.
  7. Answer: Answers vary. Accept any untouched area the student names if the explanation connects it to negative space, contrast, or giving the eye a place to rest, such as a purple sky that makes a pink trout stand out.

Differentiation

Support

Give students a simple printed outline, such as a trout or a creek bank, to transfer onto their indicator paper so they can concentrate on where each liquid goes. Post a color key card at each table showing pink for acid, green for base, and purple for untouched paper. Offer label sentence frames: "The pink areas show where an acid touched the indicator." and "Stream testers check pH because..."

Extension

With teacher approval, students test one more safe household liquid, such as club soda or a clear soap solution, on a spare strip. They place its color on a line running from pink to green, decide whether it is an acid or a base, and work that color into their composition as a planned middle value. Their label then explains where the new liquid falls between the two ends.

ELL supports

Pre-teach acid, base, indicator, and pH with the test strip as the visual, pointing to each color as the word is said. Post a bilingual word bank beside the materials. Allow the booth label to use short phrases with arrows drawn to the matching colors, and let students rehearse their label aloud with a partner before writing it.

Extensions

Cabbage indicator colors can fade or shift as the paper ages, so photograph each painting at the end of class. After a week, students compare the photograph with the painting, describe any change, and discuss how an artist or a museum would record work made with a material that keeps changing.